...
Run Format

Package fmt

import "fmt"
Overview
Index
Examples

Overview ▾

Package fmt implements formatted I/O with functions analogous to C's printf and scanf. The format 'verbs' are derived from C's but are simpler.

Printing

The verbs:

General:

%v	the value in a default format
	when printing structs, the plus flag (%+v) adds field names
%#v	a Go-syntax representation of the value
%T	a Go-syntax representation of the type of the value
%%	a literal percent sign; consumes no value

Boolean:

%t	the word true or false

Integer:

%b	base 2
%c	the character represented by the corresponding Unicode code point
%d	base 10
%o	base 8
%q	a single-quoted character literal safely escaped with Go syntax.
%x	base 16, with lower-case letters for a-f
%X	base 16, with upper-case letters for A-F
%U	Unicode format: U+1234; same as "U+%04X"

Floating-point and complex constituents:

%b	decimalless scientific notation with exponent a power of two,
	in the manner of strconv.FormatFloat with the 'b' format,
	e.g. -123456p-78
%e	scientific notation, e.g. -1.234456e+78
%E	scientific notation, e.g. -1.234456E+78
%f	decimal point but no exponent, e.g. 123.456
%F	synonym for %f
%g	%e for large exponents, %f otherwise. Precision is discussed below.
%G	%E for large exponents, %F otherwise

String and slice of bytes (treated equivalently with these verbs):

%s	the uninterpreted bytes of the string or slice
%q	a double-quoted string safely escaped with Go syntax
%x	base 16, lower-case, two characters per byte
%X	base 16, upper-case, two characters per byte

Slice:

%p	address of 0th element in base 16 notation, with leading 0x

Pointer:

%p	base 16 notation, with leading 0x
The %b, %d, %o, %x and %X verbs also work with pointers,
formatting the value exactly as if it were an integer.

The default format for %v is:

bool:                    %t
int, int8 etc.:          %d
uint, uint8 etc.:        %d, %#x if printed with %#v
float32, complex64, etc: %g
string:                  %s
chan:                    %p
pointer:                 %p

For compound objects, the elements are printed using these rules, recursively, laid out like this:

struct:             {field0 field1 ...}
array, slice:       [elem0 elem1 ...]
maps:               map[key1:value1 key2:value2 ...]
pointer to above:   &{}, &[], &map[]

Width is specified by an optional decimal number immediately preceding the verb. If absent, the width is whatever is necessary to represent the value. Precision is specified after the (optional) width by a period followed by a decimal number. If no period is present, a default precision is used. A period with no following number specifies a precision of zero. Examples:

%f     default width, default precision
%9f    width 9, default precision
%.2f   default width, precision 2
%9.2f  width 9, precision 2
%9.f   width 9, precision 0

Width and precision are measured in units of Unicode code points, that is, runes. (This differs from C's printf where the units are always measured in bytes.) Either or both of the flags may be replaced with the character '*', causing their values to be obtained from the next operand (preceding the one to format), which must be of type int.

For most values, width is the minimum number of runes to output, padding the formatted form with spaces if necessary.

For strings, byte slices and byte arrays, however, precision limits the length of the input to be formatted (not the size of the output), truncating if necessary. Normally it is measured in runes, but for these types when formatted with the %x or %X format it is measured in bytes.

For floating-point values, width sets the minimum width of the field and precision sets the number of places after the decimal, if appropriate, except that for %g/%G precision sets the maximum number of significant digits (trailing zeros are removed). For example, given 12.345 the format %6.3f prints 12.345 while %.3g prints 12.3. The default precision for %e, %f and %#g is 6; for %g it is the smallest number of digits necessary to identify the value uniquely.

For complex numbers, the width and precision apply to the two components independently and the result is parenthesized, so %f applied to 1.2+3.4i produces (1.200000+3.400000i).

Other flags:

+	always print a sign for numeric values;
	guarantee ASCII-only output for %q (%+q)
-	pad with spaces on the right rather than the left (left-justify the field)
#	alternate format: add leading 0 for octal (%#o), 0x for hex (%#x);
	0X for hex (%#X); suppress 0x for %p (%#p);
	for %q, print a raw (backquoted) string if strconv.CanBackquote
	returns true;
	always print a decimal point for %e, %E, %f, %F, %g and %G;
	do not remove trailing zeros for %g and %G;
	write e.g. U+0078 'x' if the character is printable for %U (%#U).
' '	(space) leave a space for elided sign in numbers (% d);
	put spaces between bytes printing strings or slices in hex (% x, % X)
0	pad with leading zeros rather than spaces;
	for numbers, this moves the padding after the sign

Flags are ignored by verbs that do not expect them. For example there is no alternate decimal format, so %#d and %d behave identically.

For each Printf-like function, there is also a Print function that takes no format and is equivalent to saying %v for every operand. Another variant Println inserts blanks between operands and appends a newline.

Regardless of the verb, if an operand is an interface value, the internal concrete value is used, not the interface itself. Thus:

var i interface{} = 23
fmt.Printf("%v\n", i)

will print 23.

Except when printed using the verbs %T and %p, special formatting considerations apply for operands that implement certain interfaces. In order of application:

1. If the operand is a reflect.Value, the operand is replaced by the concrete value that it holds, and printing continues with the next rule.

2. If an operand implements the Formatter interface, it will be invoked. Formatter provides fine control of formatting.

3. If the %v verb is used with the # flag (%#v) and the operand implements the GoStringer interface, that will be invoked.

If the format (which is implicitly %v for Println etc.) is valid for a string (%s %q %v %x %X), the following two rules apply:

4. If an operand implements the error interface, the Error method will be invoked to convert the object to a string, which will then be formatted as required by the verb (if any).

5. If an operand implements method String() string, that method will be invoked to convert the object to a string, which will then be formatted as required by the verb (if any).

For compound operands such as slices and structs, the format applies to the elements of each operand, recursively, not to the operand as a whole. Thus %q will quote each element of a slice of strings, and %6.2f will control formatting for each element of a floating-point array.

However, when printing a byte slice with a string-like verb (%s %q %x %X), it is treated identically to a string, as a single item.

To avoid recursion in cases such as

type X string
func (x X) String() string { return Sprintf("<%s>", x) }

convert the value before recurring:

func (x X) String() string { return Sprintf("<%s>", string(x)) }

Infinite recursion can also be triggered by self-referential data structures, such as a slice that contains itself as an element, if that type has a String method. Such pathologies are rare, however, and the package does not protect against them.

When printing a struct, fmt cannot and therefore does not invoke formatting methods such as Error or String on unexported fields.

Explicit argument indexes:

In Printf, Sprintf, and Fprintf, the default behavior is for each formatting verb to format successive arguments passed in the call. However, the notation [n] immediately before the verb indicates that the nth one-indexed argument is to be formatted instead. The same notation before a '*' for a width or precision selects the argument index holding the value. After processing a bracketed expression [n], subsequent verbs will use arguments n+1, n+2, etc. unless otherwise directed.

For example,

fmt.Sprintf("%[2]d %[1]d\n", 11, 22)

will yield "22 11", while

fmt.Sprintf("%[3]*.[2]*[1]f", 12.0, 2, 6)

equivalent to

fmt.Sprintf("%6.2f", 12.0)

will yield " 12.00". Because an explicit index affects subsequent verbs, this notation can be used to print the same values multiple times by resetting the index for the first argument to be repeated:

fmt.Sprintf("%d %d %#[1]x %#x", 16, 17)

will yield "16 17 0x10 0x11".

Format errors:

If an invalid argument is given for a verb, such as providing a string to %d, the generated string will contain a description of the problem, as in these examples:

Wrong type or unknown verb: %!verb(type=value)
	Printf("%d", hi):          %!d(string=hi)
Too many arguments: %!(EXTRA type=value)
	Printf("hi", "guys"):      hi%!(EXTRA string=guys)
Too few arguments: %!verb(MISSING)
	Printf("hi%d"):            hi%!d(MISSING)
Non-int for width or precision: %!(BADWIDTH) or %!(BADPREC)
	Printf("%*s", 4.5, "hi"):  %!(BADWIDTH)hi
	Printf("%.*s", 4.5, "hi"): %!(BADPREC)hi
Invalid or invalid use of argument index: %!(BADINDEX)
	Printf("%*[2]d", 7):       %!d(BADINDEX)
	Printf("%.[2]d", 7):       %!d(BADINDEX)

All errors begin with the string "%!" followed sometimes by a single character (the verb) and end with a parenthesized description.

If an Error or String method triggers a panic when called by a print routine, the fmt package reformats the error message from the panic, decorating it with an indication that it came through the fmt package. For example, if a String method calls panic("bad"), the resulting formatted message will look like

%!s(PANIC=bad)

The %!s just shows the print verb in use when the failure occurred. If the panic is caused by a nil receiver to an Error or String method, however, the output is the undecorated string, "<nil>".

Scanning

An analogous set of functions scans formatted text to yield values. Scan, Scanf and Scanln read from os.Stdin; Fscan, Fscanf and Fscanln read from a specified io.Reader; Sscan, Sscanf and Sscanln read from an argument string.

Scan, Fscan, Sscan treat newlines in the input as spaces.

Scanln, Fscanln and Sscanln stop scanning at a newline and require that the items be followed by a newline or EOF.

Scanf, Fscanf, and Sscanf parse the arguments according to a format string, analogous to that of Printf. In the text that follows, 'space' means any Unicode whitespace character except newline.

In the format string, a verb introduced by the % character consumes and parses input; these verbs are described in more detail below. A character other than %, space, or newline in the format consumes exactly that input character, which must be present. A newline with zero or more spaces before it in the format string consumes zero or more spaces in the input followed by a single newline or the end of the input. A space following a newline in the format string consumes zero or more spaces in the input. Otherwise, any run of one or more spaces in the format string consumes as many spaces as possible in the input. Unless the run of spaces in the format string appears adjacent to a newline, the run must consume at least one space from the input or find the end of the input.

The handling of spaces and newlines differs from that of C's scanf family: in C, newlines are treated as any other space, and it is never an error when a run of spaces in the format string finds no spaces to consume in the input.

The verbs behave analogously to those of Printf. For example, %x will scan an integer as a hexadecimal number, and %v will scan the default representation format for the value. The Printf verbs %p and %T and the flags # and + are not implemented. The verbs %e %E %f %F %g and %G are all equivalent and scan any floating-point or complex value. For float and complex literals in scientific notation, both the decimal (e) and binary (p) exponent formats are supported (for example: "2.3e+7" and "4.5p-8").

Input processed by verbs is implicitly space-delimited: the implementation of every verb except %c starts by discarding leading spaces from the remaining input, and the %s verb (and %v reading into a string) stops consuming input at the first space or newline character.

The familiar base-setting prefixes 0 (octal) and 0x (hexadecimal) are accepted when scanning integers without a format or with the %v verb.

Width is interpreted in the input text but there is no syntax for scanning with a precision (no %5.2f, just %5f). If width is provided, it applies after leading spaces are trimmed and specifies the maximum number of runes to read to satisfy the verb. For example,

Sscanf(" 1234567 ", "%5s%d", &s, &i)

will set s to "12345" and i to 67 while

Sscanf(" 12 34 567 ", "%5s%d", &s, &i)

will set s to "12" and i to 34.

In all the scanning functions, a carriage return followed immediately by a newline is treated as a plain newline (\r\n means the same as \n).

In all the scanning functions, if an operand implements method Scan (that is, it implements the Scanner interface) that method will be used to scan the text for that operand. Also, if the number of arguments scanned is less than the number of arguments provided, an error is returned.

All arguments to be scanned must be either pointers to basic types or implementations of the Scanner interface.

Like Scanf and Fscanf, Sscanf need not consume its entire input. There is no way to recover how much of the input string Sscanf used.

Note: Fscan etc. can read one character (rune) past the input they return, which means that a loop calling a scan routine may skip some of the input. This is usually a problem only when there is no space between input values. If the reader provided to Fscan implements ReadRune, that method will be used to read characters. If the reader also implements UnreadRune, that method will be used to save the character and successive calls will not lose data. To attach ReadRune and UnreadRune methods to a reader without that capability, use bufio.NewReader.

Example (Formats)

These examples demonstrate the basics of printing using a format string. Printf, Sprintf, and Fprintf all take a format string that specifies how to format the subsequent arguments. For example, %d (we call that a 'verb') says to print the corresponding argument, which must be an integer (or something containing an integer, such as a slice of ints) in decimal. The verb %v ('v' for 'value') always formats the argument in its default form, just how Print or Println would show it. The special verb %T ('T' for 'Type') prints the type of the argument rather than its value. The examples are not exhaustive; see the package comment for all the details.

23
23
23
int *int
true true
42 42 2a 52 101010
3.141592653589793 3.141592653589793 3.14 (  3.14) 3.141593e+00
(110.7+22.5i) (110.7+22.5i) (110.70+22.50i) (1.11e+02+2.25e+01i)
128512 128512 😀 '😀' U+1F600 U+1F600 '😀'
foo "bar" foo "bar" "foo \"bar\"" `foo "bar"`
map[dachshund:false peanut:true] map[string]bool{"dachshund":false, "peanut":true}
{Kim 22} {Name:Kim Age:22} struct { Name string; Age int }{Name:"Kim", Age:22}
&{Kim 22} 0x0
[Katano Kobayashi Kurosawa Miyazaki Ozu] ["Katano" "Kobayashi" "Kurosawa" "Miyazaki" "Ozu"]
[Katano Kobayashi Kurosawa] ["Katano" "Kobayashi" "Kurosawa"] []string{"Katano", "Kobayashi", "Kurosawa"}
[97 226 140 152] [97 226 140 152] a⌘ "a⌘" 61e28c98 61 e2 8c 98
1973-11-29 21:33:09 +0000 UTC "1973-11-29 21:33:09 +0000 UTC"

Example (Printers)

Print, Println, and Printf lay out their arguments differently. In this example we can compare their behaviors. Println always adds blanks between the items it prints, while Print adds blanks only between non-string arguments and Printf does exactly what it is told. Sprint, Sprintln, Sprintf, Fprint, Fprintln, and Fprintf behave the same as their corresponding Print, Println, and Printf functions shown here.

The vector (3 4) has length 5.
The vector ( 3 4 ) has length 5 .
The vector (3 4) has length 5.

Index ▾

Constants
Variables
func Errorf(format string, a ...interface{}) error
func Fprint(w io.Writer, a ...interface{}) (n int, err error)
func Fprintf(w io.Writer, format string, a ...interface{}) (n int, err error)
func Fprintln(w io.Writer, a ...interface{}) (n int, err error)
func Fscan(r io.Reader, a ...interface{}) (n int, err error)
func Fscanf(r io.Reader, format string, a ...interface{}) (n int, err error)
func Fscanln(r io.Reader, a ...interface{}) (n int, err error)
func Print(a ...interface{}) (n int, err error)
func Printf(format string, a ...interface{}) (n int, err error)
func Println(a ...interface{}) (n int, err error)
func Scan(a ...interface{}) (n int, err error)
func Scanf(format string, a ...interface{}) (n int, err error)
func Scanln(a ...interface{}) (n int, err error)
func Sprint(a ...interface{}) string
func Sprintf(format string, a ...interface{}) string
func Sprintln(a ...interface{}) string
func Sscan(str string, a ...interface{}) (n int, err error)
func Sscanf(str string, format string, a ...interface{}) (n int, err error)
func Sscanln(str string, a ...interface{}) (n int, err error)
func errorHandler(errp *error)
func getField(v reflect.Value, i int) reflect.Value
func hexDigit(d rune) (int, bool)
func indexRune(s string, r rune) int
func intFromArg(a []interface{}, argNum int) (num int, isInt bool, newArgNum int)
func isSpace(r rune) bool
func newScanState(r io.Reader, nlIsSpace, nlIsEnd bool) (s *ss, old ssave)
func notSpace(r rune) bool
func parseArgNumber(format string) (index int, wid int, ok bool)
func parsenum(s string, start, end int) (num int, isnum bool, newi int)
func tooLarge(x int) bool
type Formatter
type GoStringer
type ScanState
type Scanner
type State
type Stringer
type buffer
    func (b *buffer) Write(p []byte)
    func (b *buffer) WriteByte(c byte)
    func (bp *buffer) WriteRune(r rune)
    func (b *buffer) WriteString(s string)
type fmt
    func (f *fmt) clearflags()
    func (f *fmt) fmtBoolean(v bool)
    func (f *fmt) fmtBs(b []byte)
    func (f *fmt) fmtBx(b []byte, digits string)
    func (f *fmt) fmtC(c uint64)
    func (f *fmt) fmtFloat(v float64, size int, verb rune, prec int)
    func (f *fmt) fmtInteger(u uint64, base int, isSigned bool, digits string)
    func (f *fmt) fmtQ(s string)
    func (f *fmt) fmtQc(c uint64)
    func (f *fmt) fmtS(s string)
    func (f *fmt) fmtSbx(s string, b []byte, digits string)
    func (f *fmt) fmtSx(s, digits string)
    func (f *fmt) fmtUnicode(u uint64)
    func (f *fmt) init(buf *buffer)
    func (f *fmt) pad(b []byte)
    func (f *fmt) padString(s string)
    func (f *fmt) truncate(b []byte) []byte
    func (f *fmt) truncateString(s string) string
    func (f *fmt) writePadding(n int)
type fmtFlags
type pp
    func newPrinter() *pp
    func (p *pp) Flag(b int) bool
    func (p *pp) Precision() (prec int, ok bool)
    func (p *pp) Width() (wid int, ok bool)
    func (p *pp) Write(b []byte) (ret int, err error)
    func (p *pp) WriteString(s string) (ret int, err error)
    func (p *pp) argNumber(argNum int, format string, i int, numArgs int) (newArgNum, newi int, found bool)
    func (p *pp) badArgNum(verb rune)
    func (p *pp) badVerb(verb rune)
    func (p *pp) catchPanic(arg interface{}, verb rune, method string)
    func (p *pp) doPrint(a []interface{})
    func (p *pp) doPrintf(format string, a []interface{})
    func (p *pp) doPrintln(a []interface{})
    func (p *pp) fmt0x64(v uint64, leading0x bool)
    func (p *pp) fmtBool(v bool, verb rune)
    func (p *pp) fmtBytes(v []byte, verb rune, typeString string)
    func (p *pp) fmtComplex(v complex128, size int, verb rune)
    func (p *pp) fmtFloat(v float64, size int, verb rune)
    func (p *pp) fmtInteger(v uint64, isSigned bool, verb rune)
    func (p *pp) fmtPointer(value reflect.Value, verb rune)
    func (p *pp) fmtString(v string, verb rune)
    func (p *pp) free()
    func (p *pp) handleMethods(verb rune) (handled bool)
    func (p *pp) missingArg(verb rune)
    func (p *pp) printArg(arg interface{}, verb rune)
    func (p *pp) printValue(value reflect.Value, verb rune, depth int)
    func (p *pp) unknownType(v reflect.Value)
type readRune
    func (r *readRune) ReadRune() (rr rune, size int, err error)
    func (r *readRune) UnreadRune() error
    func (r *readRune) readByte() (b byte, err error)
type scanError
type ss
    func (s *ss) Read(buf []byte) (n int, err error)
    func (s *ss) ReadRune() (r rune, size int, err error)
    func (s *ss) SkipSpace()
    func (s *ss) Token(skipSpace bool, f func(rune) bool) (tok []byte, err error)
    func (s *ss) UnreadRune() error
    func (s *ss) Width() (wid int, ok bool)
    func (s *ss) accept(ok string) bool
    func (s *ss) advance(format string) (i int)
    func (s *ss) complexTokens() (real, imag string)
    func (s *ss) consume(ok string, accept bool) bool
    func (s *ss) convertFloat(str string, n int) float64
    func (s *ss) convertString(verb rune) (str string)
    func (s *ss) doScan(a []interface{}) (numProcessed int, err error)
    func (s *ss) doScanf(format string, a []interface{}) (numProcessed int, err error)
    func (s *ss) error(err error)
    func (s *ss) errorString(err string)
    func (s *ss) floatToken() string
    func (s *ss) free(old ssave)
    func (s *ss) getBase(verb rune) (base int, digits string)
    func (s *ss) getRune() (r rune)
    func (s *ss) hexByte() (b byte, ok bool)
    func (s *ss) hexString() string
    func (s *ss) mustReadRune() (r rune)
    func (s *ss) notEOF()
    func (s *ss) okVerb(verb rune, okVerbs, typ string) bool
    func (s *ss) peek(ok string) bool
    func (s *ss) quotedString() string
    func (s *ss) scanBasePrefix() (base int, digits string, found bool)
    func (s *ss) scanBool(verb rune) bool
    func (s *ss) scanComplex(verb rune, n int) complex128
    func (s *ss) scanInt(verb rune, bitSize int) int64
    func (s *ss) scanNumber(digits string, haveDigits bool) string
    func (s *ss) scanOne(verb rune, arg interface{})
    func (s *ss) scanRune(bitSize int) int64
    func (s *ss) scanUint(verb rune, bitSize int) uint64
    func (s *ss) token(skipSpace bool, f func(rune) bool) []byte
type ssave
type stringReader
    func (r *stringReader) Read(b []byte) (n int, err error)

Package files

doc.go format.go print.go scan.go

Constants

const (
        ldigits = "0123456789abcdefx"
        udigits = "0123456789ABCDEFX"
)
const (
        signed   = true
        unsigned = false
)

Strings for use with buffer.WriteString. This is less overhead than using buffer.Write with byte arrays.

const (
        commaSpaceString  = ", "
        nilAngleString    = "<nil>"
        nilParenString    = "(nil)"
        nilString         = "nil"
        mapString         = "map["
        percentBangString = "%!"
        missingString     = "(MISSING)"
        badIndexString    = "(BADINDEX)"
        panicString       = "(PANIC="
        extraString       = "%!(EXTRA "
        badWidthString    = "%!(BADWIDTH)"
        badPrecString     = "%!(BADPREC)"
        noVerbString      = "%!(NOVERB)"
        invReflectString  = "<invalid reflect.Value>"
)

Numerical elements

const (
        binaryDigits      = "01"
        octalDigits       = "01234567"
        decimalDigits     = "0123456789"
        hexadecimalDigits = "0123456789aAbBcCdDeEfF"
        sign              = "+-"
        period            = "."
        exponent          = "eEp"
)
const (
        floatVerbs = "beEfFgGv"

        hugeWid = 1 << 30

        intBits     = 32 << (^uint(0) >> 63)
        uintptrBits = 32 << (^uintptr(0) >> 63)
)
const eof = -1

Variables

var boolError = errors.New("syntax error scanning boolean")
var complexError = errors.New("syntax error scanning complex number")
var ppFree = sync.Pool{
        New: func() interface{} { return new(pp) },
}

space is a copy of the unicode.White_Space ranges, to avoid depending on package unicode.

var space = [][2]uint16{
        {0x0009, 0x000d},
        {0x0020, 0x0020},
        {0x0085, 0x0085},
        {0x00a0, 0x00a0},
        {0x1680, 0x1680},
        {0x2000, 0x200a},
        {0x2028, 0x2029},
        {0x202f, 0x202f},
        {0x205f, 0x205f},
        {0x3000, 0x3000},
}
var ssFree = sync.Pool{
        New: func() interface{} { return new(ss) },
}

func Errorf

func Errorf(format string, a ...interface{}) error

Errorf formats according to a format specifier and returns the string as a value that satisfies error.

Example

The Errorf function lets us use formatting features to create descriptive error messages.

user "bueller" (id 17) not found

func Fprint

func Fprint(w io.Writer, a ...interface{}) (n int, err error)

Fprint formats using the default formats for its operands and writes to w. Spaces are added between operands when neither is a string. It returns the number of bytes written and any write error encountered.

Example

Kim is 22 years old.
21 bytes written.

func Fprintf

func Fprintf(w io.Writer, format string, a ...interface{}) (n int, err error)

Fprintf formats according to a format specifier and writes to w. It returns the number of bytes written and any write error encountered.

Example

Kim is 22 years old.
21 bytes written.

func Fprintln

func Fprintln(w io.Writer, a ...interface{}) (n int, err error)

Fprintln formats using the default formats for its operands and writes to w. Spaces are always added between operands and a newline is appended. It returns the number of bytes written and any write error encountered.

Example

Kim is 22 years old.
21 bytes written.

func Fscan

func Fscan(r io.Reader, a ...interface{}) (n int, err error)

Fscan scans text read from r, storing successive space-separated values into successive arguments. Newlines count as space. It returns the number of items successfully scanned. If that is less than the number of arguments, err will report why.

func Fscanf

func Fscanf(r io.Reader, format string, a ...interface{}) (n int, err error)

Fscanf scans text read from r, storing successive space-separated values into successive arguments as determined by the format. It returns the number of items successfully parsed. Newlines in the input must match newlines in the format.

Example

5 true gophers
3

func Fscanln

func Fscanln(r io.Reader, a ...interface{}) (n int, err error)

Fscanln is similar to Fscan, but stops scanning at a newline and after the final item there must be a newline or EOF.

Example

3: dmr, 1771, 1.618034
3: ken, 271828, 3.141590

func Print

func Print(a ...interface{}) (n int, err error)

Print formats using the default formats for its operands and writes to standard output. Spaces are added between operands when neither is a string. It returns the number of bytes written and any write error encountered.

Example

Kim is 22 years old.

func Printf

func Printf(format string, a ...interface{}) (n int, err error)

Printf formats according to a format specifier and writes to standard output. It returns the number of bytes written and any write error encountered.

Example

Kim is 22 years old.

func Println

func Println(a ...interface{}) (n int, err error)

Println formats using the default formats for its operands and writes to standard output. Spaces are always added between operands and a newline is appended. It returns the number of bytes written and any write error encountered.

Example

Kim is 22 years old.

func Scan

func Scan(a ...interface{}) (n int, err error)

Scan scans text read from standard input, storing successive space-separated values into successive arguments. Newlines count as space. It returns the number of items successfully scanned. If that is less than the number of arguments, err will report why.

func Scanf

func Scanf(format string, a ...interface{}) (n int, err error)

Scanf scans text read from standard input, storing successive space-separated values into successive arguments as determined by the format. It returns the number of items successfully scanned. If that is less than the number of arguments, err will report why. Newlines in the input must match newlines in the format. The one exception: the verb %c always scans the next rune in the input, even if it is a space (or tab etc.) or newline.

func Scanln

func Scanln(a ...interface{}) (n int, err error)

Scanln is similar to Scan, but stops scanning at a newline and after the final item there must be a newline or EOF.

func Sprint

func Sprint(a ...interface{}) string

Sprint formats using the default formats for its operands and returns the resulting string. Spaces are added between operands when neither is a string.

Example

Kim is 22 years old.

func Sprintf

func Sprintf(format string, a ...interface{}) string

Sprintf formats according to a format specifier and returns the resulting string.

Example

Kim is 22 years old.

func Sprintln

func Sprintln(a ...interface{}) string

Sprintln formats using the default formats for its operands and returns the resulting string. Spaces are always added between operands and a newline is appended.

Example

Kim is 22 years old.

func Sscan

func Sscan(str string, a ...interface{}) (n int, err error)

Sscan scans the argument string, storing successive space-separated values into successive arguments. Newlines count as space. It returns the number of items successfully scanned. If that is less than the number of arguments, err will report why.

func Sscanf

func Sscanf(str string, format string, a ...interface{}) (n int, err error)

Sscanf scans the argument string, storing successive space-separated values into successive arguments as determined by the format. It returns the number of items successfully parsed. Newlines in the input must match newlines in the format.

Example

2: Kim, 22

func Sscanln

func Sscanln(str string, a ...interface{}) (n int, err error)

Sscanln is similar to Sscan, but stops scanning at a newline and after the final item there must be a newline or EOF.

func errorHandler

func errorHandler(errp *error)

errorHandler turns local panics into error returns.

func getField

func getField(v reflect.Value, i int) reflect.Value

getField gets the i'th field of the struct value. If the field is itself is an interface, return a value for the thing inside the interface, not the interface itself.

func hexDigit

func hexDigit(d rune) (int, bool)

hexDigit returns the value of the hexadecimal digit.

func indexRune

func indexRune(s string, r rune) int

func intFromArg

func intFromArg(a []interface{}, argNum int) (num int, isInt bool, newArgNum int)

intFromArg gets the argNumth element of a. On return, isInt reports whether the argument has integer type.

func isSpace

func isSpace(r rune) bool

func newScanState

func newScanState(r io.Reader, nlIsSpace, nlIsEnd bool) (s *ss, old ssave)

newScanState allocates a new ss struct or grab a cached one.

func notSpace

func notSpace(r rune) bool

notSpace is the default scanning function used in Token.

func parseArgNumber

func parseArgNumber(format string) (index int, wid int, ok bool)

parseArgNumber returns the value of the bracketed number, minus 1 (explicit argument numbers are one-indexed but we want zero-indexed). The opening bracket is known to be present at format[0]. The returned values are the index, the number of bytes to consume up to the closing paren, if present, and whether the number parsed ok. The bytes to consume will be 1 if no closing paren is present.

func parsenum

func parsenum(s string, start, end int) (num int, isnum bool, newi int)

parsenum converts ASCII to integer. num is 0 (and isnum is false) if no number present.

func tooLarge

func tooLarge(x int) bool

tooLarge reports whether the magnitude of the integer is too large to be used as a formatting width or precision.

type Formatter

Formatter is the interface implemented by values with a custom formatter. The implementation of Format may call Sprint(f) or Fprint(f) etc. to generate its output.

type Formatter interface {
        Format(f State, c rune)
}

type GoStringer

GoStringer is implemented by any value that has a GoString method, which defines the Go syntax for that value. The GoString method is used to print values passed as an operand to a %#v format.

type GoStringer interface {
        GoString() string
}

Example

Person{Name: "Warren", Age: 31, Addr: &Address{City: "Denver", State: "CO", Country: "U.S.A."}}
Person{Name: "Theia", Age: 4}

type ScanState

ScanState represents the scanner state passed to custom scanners. Scanners may do rune-at-a-time scanning or ask the ScanState to discover the next space-delimited token.

type ScanState interface {
        // ReadRune reads the next rune (Unicode code point) from the input.
        // If invoked during Scanln, Fscanln, or Sscanln, ReadRune() will
        // return EOF after returning the first '\n' or when reading beyond
        // the specified width.
        ReadRune() (r rune, size int, err error)
        // UnreadRune causes the next call to ReadRune to return the same rune.
        UnreadRune() error
        // SkipSpace skips space in the input. Newlines are treated appropriately
        // for the operation being performed; see the package documentation
        // for more information.
        SkipSpace()
        // Token skips space in the input if skipSpace is true, then returns the
        // run of Unicode code points c satisfying f(c).  If f is nil,
        // !unicode.IsSpace(c) is used; that is, the token will hold non-space
        // characters. Newlines are treated appropriately for the operation being
        // performed; see the package documentation for more information.
        // The returned slice points to shared data that may be overwritten
        // by the next call to Token, a call to a Scan function using the ScanState
        // as input, or when the calling Scan method returns.
        Token(skipSpace bool, f func(rune) bool) (token []byte, err error)
        // Width returns the value of the width option and whether it has been set.
        // The unit is Unicode code points.
        Width() (wid int, ok bool)
        // Because ReadRune is implemented by the interface, Read should never be
        // called by the scanning routines and a valid implementation of
        // ScanState may choose always to return an error from Read.
        Read(buf []byte) (n int, err error)
}

type Scanner

Scanner is implemented by any value that has a Scan method, which scans the input for the representation of a value and stores the result in the receiver, which must be a pointer to be useful. The Scan method is called for any argument to Scan, Scanf, or Scanln that implements it.

type Scanner interface {
        Scan(state ScanState, verb rune) error
}

type State

State represents the printer state passed to custom formatters. It provides access to the io.Writer interface plus information about the flags and options for the operand's format specifier.

type State interface {
        // Write is the function to call to emit formatted output to be printed.
        Write(b []byte) (n int, err error)
        // Width returns the value of the width option and whether it has been set.
        Width() (wid int, ok bool)
        // Precision returns the value of the precision option and whether it has been set.
        Precision() (prec int, ok bool)

        // Flag reports whether the flag c, a character, has been set.
        Flag(c int) bool
}

type Stringer

Stringer is implemented by any value that has a String method, which defines the “native” format for that value. The String method is used to print values passed as an operand to any format that accepts a string or to an unformatted printer such as Print.

type Stringer interface {
        String() string
}

Example

Gopher (2)

type buffer

Use simple []byte instead of bytes.Buffer to avoid large dependency.

type buffer []byte

func (*buffer) Write

func (b *buffer) Write(p []byte)

func (*buffer) WriteByte

func (b *buffer) WriteByte(c byte)

func (*buffer) WriteRune

func (bp *buffer) WriteRune(r rune)

func (*buffer) WriteString

func (b *buffer) WriteString(s string)

type fmt

A fmt is the raw formatter used by Printf etc. It prints into a buffer that must be set up separately.

type fmt struct {
        buf *buffer

        fmtFlags

        wid  int // width
        prec int // precision

        // intbuf is large enough to store %b of an int64 with a sign and
        // avoids padding at the end of the struct on 32 bit architectures.
        intbuf [68]byte
}

func (*fmt) clearflags

func (f *fmt) clearflags()

func (*fmt) fmtBoolean

func (f *fmt) fmtBoolean(v bool)

fmtBoolean formats a boolean.

func (*fmt) fmtBs

func (f *fmt) fmtBs(b []byte)

fmtBs formats the byte slice b as if it was formatted as string with fmtS.

func (*fmt) fmtBx

func (f *fmt) fmtBx(b []byte, digits string)

fmtBx formats a byte slice as a hexadecimal encoding of its bytes.

func (*fmt) fmtC

func (f *fmt) fmtC(c uint64)

fmtC formats an integer as a Unicode character. If the character is not valid Unicode, it will print '\ufffd'.

func (*fmt) fmtFloat

func (f *fmt) fmtFloat(v float64, size int, verb rune, prec int)

fmtFloat formats a float64. It assumes that verb is a valid format specifier for strconv.AppendFloat and therefore fits into a byte.

func (*fmt) fmtInteger

func (f *fmt) fmtInteger(u uint64, base int, isSigned bool, digits string)

fmtInteger formats signed and unsigned integers.

func (*fmt) fmtQ

func (f *fmt) fmtQ(s string)

fmtQ formats a string as a double-quoted, escaped Go string constant. If f.sharp is set a raw (backquoted) string may be returned instead if the string does not contain any control characters other than tab.

func (*fmt) fmtQc

func (f *fmt) fmtQc(c uint64)

fmtQc formats an integer as a single-quoted, escaped Go character constant. If the character is not valid Unicode, it will print '\ufffd'.

func (*fmt) fmtS

func (f *fmt) fmtS(s string)

fmtS formats a string.

func (*fmt) fmtSbx

func (f *fmt) fmtSbx(s string, b []byte, digits string)

fmtSbx formats a string or byte slice as a hexadecimal encoding of its bytes.

func (*fmt) fmtSx

func (f *fmt) fmtSx(s, digits string)

fmtSx formats a string as a hexadecimal encoding of its bytes.

func (*fmt) fmtUnicode

func (f *fmt) fmtUnicode(u uint64)

fmtUnicode formats a uint64 as "U+0078" or with f.sharp set as "U+0078 'x'".

func (*fmt) init

func (f *fmt) init(buf *buffer)

func (*fmt) pad

func (f *fmt) pad(b []byte)

pad appends b to f.buf, padded on left (!f.minus) or right (f.minus).

func (*fmt) padString

func (f *fmt) padString(s string)

padString appends s to f.buf, padded on left (!f.minus) or right (f.minus).

func (*fmt) truncate

func (f *fmt) truncate(b []byte) []byte

truncate truncates the byte slice b as a string of the specified precision, if present.

func (*fmt) truncateString

func (f *fmt) truncateString(s string) string

truncate truncates the string s to the specified precision, if present.

func (*fmt) writePadding

func (f *fmt) writePadding(n int)

writePadding generates n bytes of padding.

type fmtFlags

flags placed in a separate struct for easy clearing.

type fmtFlags struct {
        widPresent  bool
        precPresent bool
        minus       bool
        plus        bool
        sharp       bool
        space       bool
        zero        bool

        // For the formats %+v %#v, we set the plusV/sharpV flags
        // and clear the plus/sharp flags since %+v and %#v are in effect
        // different, flagless formats set at the top level.
        plusV  bool
        sharpV bool
}

type pp

pp is used to store a printer's state and is reused with sync.Pool to avoid allocations.

type pp struct {
        buf buffer

        // arg holds the current item, as an interface{}.
        arg interface{}

        // value is used instead of arg for reflect values.
        value reflect.Value

        // fmt is used to format basic items such as integers or strings.
        fmt fmt

        // reordered records whether the format string used argument reordering.
        reordered bool
        // goodArgNum records whether the most recent reordering directive was valid.
        goodArgNum bool
        // panicking is set by catchPanic to avoid infinite panic, recover, panic, ... recursion.
        panicking bool
        // erroring is set when printing an error string to guard against calling handleMethods.
        erroring bool
}

func newPrinter

func newPrinter() *pp

newPrinter allocates a new pp struct or grabs a cached one.

func (*pp) Flag

func (p *pp) Flag(b int) bool

func (*pp) Precision

func (p *pp) Precision() (prec int, ok bool)

func (*pp) Width

func (p *pp) Width() (wid int, ok bool)

func (*pp) Write

func (p *pp) Write(b []byte) (ret int, err error)

Implement Write so we can call Fprintf on a pp (through State), for recursive use in custom verbs.

func (*pp) WriteString

func (p *pp) WriteString(s string) (ret int, err error)

Implement WriteString so that we can call io.WriteString on a pp (through state), for efficiency.

func (*pp) argNumber

func (p *pp) argNumber(argNum int, format string, i int, numArgs int) (newArgNum, newi int, found bool)

argNumber returns the next argument to evaluate, which is either the value of the passed-in argNum or the value of the bracketed integer that begins format[i:]. It also returns the new value of i, that is, the index of the next byte of the format to process.

func (*pp) badArgNum

func (p *pp) badArgNum(verb rune)

func (*pp) badVerb

func (p *pp) badVerb(verb rune)

func (*pp) catchPanic

func (p *pp) catchPanic(arg interface{}, verb rune, method string)

func (*pp) doPrint

func (p *pp) doPrint(a []interface{})

func (*pp) doPrintf

func (p *pp) doPrintf(format string, a []interface{})

func (*pp) doPrintln

func (p *pp) doPrintln(a []interface{})

doPrintln is like doPrint but always adds a space between arguments and a newline after the last argument.

func (*pp) fmt0x64

func (p *pp) fmt0x64(v uint64, leading0x bool)

fmt0x64 formats a uint64 in hexadecimal and prefixes it with 0x or not, as requested, by temporarily setting the sharp flag.

func (*pp) fmtBool

func (p *pp) fmtBool(v bool, verb rune)

func (*pp) fmtBytes

func (p *pp) fmtBytes(v []byte, verb rune, typeString string)

func (*pp) fmtComplex

func (p *pp) fmtComplex(v complex128, size int, verb rune)

fmtComplex formats a complex number v with r = real(v) and j = imag(v) as (r+ji) using fmtFloat for r and j formatting.

func (*pp) fmtFloat

func (p *pp) fmtFloat(v float64, size int, verb rune)

fmtFloat formats a float. The default precision for each verb is specified as last argument in the call to fmt_float.

func (*pp) fmtInteger

func (p *pp) fmtInteger(v uint64, isSigned bool, verb rune)

fmtInteger formats a signed or unsigned integer.

func (*pp) fmtPointer

func (p *pp) fmtPointer(value reflect.Value, verb rune)

func (*pp) fmtString

func (p *pp) fmtString(v string, verb rune)

func (*pp) free

func (p *pp) free()

free saves used pp structs in ppFree; avoids an allocation per invocation.

func (*pp) handleMethods

func (p *pp) handleMethods(verb rune) (handled bool)

func (*pp) missingArg

func (p *pp) missingArg(verb rune)

func (*pp) printArg

func (p *pp) printArg(arg interface{}, verb rune)

func (*pp) printValue

func (p *pp) printValue(value reflect.Value, verb rune, depth int)

printValue is similar to printArg but starts with a reflect value, not an interface{} value. It does not handle 'p' and 'T' verbs because these should have been already handled by printArg.

func (*pp) unknownType

func (p *pp) unknownType(v reflect.Value)

type readRune

readRune is a structure to enable reading UTF-8 encoded code points from an io.Reader. It is used if the Reader given to the scanner does not already implement io.RuneScanner.

type readRune struct {
        reader   io.Reader
        buf      [utf8.UTFMax]byte // used only inside ReadRune
        pending  int               // number of bytes in pendBuf; only >0 for bad UTF-8
        pendBuf  [utf8.UTFMax]byte // bytes left over
        peekRune rune              // if >=0 next rune; when <0 is ^(previous Rune)
}

func (*readRune) ReadRune

func (r *readRune) ReadRune() (rr rune, size int, err error)

ReadRune returns the next UTF-8 encoded code point from the io.Reader inside r.

func (*readRune) UnreadRune

func (r *readRune) UnreadRune() error

func (*readRune) readByte

func (r *readRune) readByte() (b byte, err error)

readByte returns the next byte from the input, which may be left over from a previous read if the UTF-8 was ill-formed.

type scanError

scanError represents an error generated by the scanning software. It's used as a unique signature to identify such errors when recovering.

type scanError struct {
        err error
}

type ss

ss is the internal implementation of ScanState.

type ss struct {
        rs    io.RuneScanner // where to read input
        buf   buffer         // token accumulator
        count int            // runes consumed so far.
        atEOF bool           // already read EOF
        ssave
}

func (*ss) Read

func (s *ss) Read(buf []byte) (n int, err error)

The Read method is only in ScanState so that ScanState satisfies io.Reader. It will never be called when used as intended, so there is no need to make it actually work.

func (*ss) ReadRune

func (s *ss) ReadRune() (r rune, size int, err error)

func (*ss) SkipSpace

func (s *ss) SkipSpace()

SkipSpace provides Scan methods the ability to skip space and newline characters in keeping with the current scanning mode set by format strings and Scan/Scanln.

func (*ss) Token

func (s *ss) Token(skipSpace bool, f func(rune) bool) (tok []byte, err error)

func (*ss) UnreadRune

func (s *ss) UnreadRune() error

func (*ss) Width

func (s *ss) Width() (wid int, ok bool)

func (*ss) accept

func (s *ss) accept(ok string) bool

accept checks the next rune in the input. If it's a byte (sic) in the string, it puts it in the buffer and returns true. Otherwise it return false.

func (*ss) advance

func (s *ss) advance(format string) (i int)

advance determines whether the next characters in the input match those of the format. It returns the number of bytes (sic) consumed in the format. All runs of space characters in either input or format behave as a single space. Newlines are special, though: newlines in the format must match those in the input and vice versa. This routine also handles the %% case. If the return value is zero, either format starts with a % (with no following %) or the input is empty. If it is negative, the input did not match the string.

func (*ss) complexTokens

func (s *ss) complexTokens() (real, imag string)

complexTokens returns the real and imaginary parts of the complex number starting here. The number might be parenthesized and has the format (N+Ni) where N is a floating-point number and there are no spaces within.

func (*ss) consume

func (s *ss) consume(ok string, accept bool) bool

consume reads the next rune in the input and reports whether it is in the ok string. If accept is true, it puts the character into the input token.

func (*ss) convertFloat

func (s *ss) convertFloat(str string, n int) float64

convertFloat converts the string to a float64value.

func (*ss) convertString

func (s *ss) convertString(verb rune) (str string)

convertString returns the string represented by the next input characters. The format of the input is determined by the verb.

func (*ss) doScan

func (s *ss) doScan(a []interface{}) (numProcessed int, err error)

doScan does the real work for scanning without a format string.

func (*ss) doScanf

func (s *ss) doScanf(format string, a []interface{}) (numProcessed int, err error)

doScanf does the real work when scanning with a format string. At the moment, it handles only pointers to basic types.

func (*ss) error

func (s *ss) error(err error)

func (*ss) errorString

func (s *ss) errorString(err string)

func (*ss) floatToken

func (s *ss) floatToken() string

floatToken returns the floating-point number starting here, no longer than swid if the width is specified. It's not rigorous about syntax because it doesn't check that we have at least some digits, but Atof will do that.

func (*ss) free

func (s *ss) free(old ssave)

free saves used ss structs in ssFree; avoid an allocation per invocation.

func (*ss) getBase

func (s *ss) getBase(verb rune) (base int, digits string)

getBase returns the numeric base represented by the verb and its digit string.

func (*ss) getRune

func (s *ss) getRune() (r rune)

The public method returns an error; this private one panics. If getRune reaches EOF, the return value is EOF (-1).

func (*ss) hexByte

func (s *ss) hexByte() (b byte, ok bool)

hexByte returns the next hex-encoded (two-character) byte from the input. It returns ok==false if the next bytes in the input do not encode a hex byte. If the first byte is hex and the second is not, processing stops.

func (*ss) hexString

func (s *ss) hexString() string

hexString returns the space-delimited hexpair-encoded string.

func (*ss) mustReadRune

func (s *ss) mustReadRune() (r rune)

mustReadRune turns io.EOF into a panic(io.ErrUnexpectedEOF). It is called in cases such as string scanning where an EOF is a syntax error.

func (*ss) notEOF

func (s *ss) notEOF()

func (*ss) okVerb

func (s *ss) okVerb(verb rune, okVerbs, typ string) bool

okVerb verifies that the verb is present in the list, setting s.err appropriately if not.

func (*ss) peek

func (s *ss) peek(ok string) bool

peek reports whether the next character is in the ok string, without consuming it.

func (*ss) quotedString

func (s *ss) quotedString() string

quotedString returns the double- or back-quoted string represented by the next input characters.

func (*ss) scanBasePrefix

func (s *ss) scanBasePrefix() (base int, digits string, found bool)

scanBasePrefix reports whether the integer begins with a 0 or 0x, and returns the base, digit string, and whether a zero was found. It is called only if the verb is %v.

func (*ss) scanBool

func (s *ss) scanBool(verb rune) bool

scanBool returns the value of the boolean represented by the next token.

func (*ss) scanComplex

func (s *ss) scanComplex(verb rune, n int) complex128

convertComplex converts the next token to a complex128 value. The atof argument is a type-specific reader for the underlying type. If we're reading complex64, atof will parse float32s and convert them to float64's to avoid reproducing this code for each complex type.

func (*ss) scanInt

func (s *ss) scanInt(verb rune, bitSize int) int64

scanInt returns the value of the integer represented by the next token, checking for overflow. Any error is stored in s.err.

func (*ss) scanNumber

func (s *ss) scanNumber(digits string, haveDigits bool) string

scanNumber returns the numerical string with specified digits starting here.

func (*ss) scanOne

func (s *ss) scanOne(verb rune, arg interface{})

scanOne scans a single value, deriving the scanner from the type of the argument.

func (*ss) scanRune

func (s *ss) scanRune(bitSize int) int64

scanRune returns the next rune value in the input.

func (*ss) scanUint

func (s *ss) scanUint(verb rune, bitSize int) uint64

scanUint returns the value of the unsigned integer represented by the next token, checking for overflow. Any error is stored in s.err.

func (*ss) token

func (s *ss) token(skipSpace bool, f func(rune) bool) []byte

token returns the next space-delimited string from the input. It skips white space. For Scanln, it stops at newlines. For Scan, newlines are treated as spaces.

type ssave

ssave holds the parts of ss that need to be saved and restored on recursive scans.

type ssave struct {
        validSave bool // is or was a part of an actual ss.
        nlIsEnd   bool // whether newline terminates scan
        nlIsSpace bool // whether newline counts as white space
        argLimit  int  // max value of ss.count for this arg; argLimit <= limit
        limit     int  // max value of ss.count.
        maxWid    int  // width of this arg.
}

type stringReader

type stringReader string

func (*stringReader) Read

func (r *stringReader) Read(b []byte) (n int, err error)