Package fmt
import "fmt"
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
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 %x base 16, with lower-case letters for a-f %X base 16, with upper-case letters for A-F
Floating-point and complex constituents:
%e scientific notation, e.g. -1234.456e+78 %E scientific notation, e.g. -1234.456E+78 %f decimal point but no exponent, e.g. 123.456 %g whichever of %e or %f produces more compact output %G whichever of %E or %f produces more compact output
String and slice of bytes:
%s the uninterpreted bytes of the string or slice %q a double-quoted string safely escaped with Go syntax %x base 16 notation with two characters per byte
Pointer:
%p base 16 notation, with leading 0x
There is no 'u' flag. Integers are printed unsigned if they have unsigned type. Similarly, there is no need to specify the size of the operand (int8, int64).
For numeric values, the width and precision flags control formatting; width sets the width of the field, precision the number of places after the decimal, if appropriate. The format %6.2f prints 123.45. The width of a field is the number of Unicode code points in the string. This differs from C's printf where the field width is the number of bytes.
Other flags:
+ always print a sign for numeric values - 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); print a raw (backquoted) string if possible for %q (%#q) ' ' (space) leave a space for elided sign in numbers (% d); put spaces between bytes printing strings or slices in hex (% x) 0 pad with leading zeros rather than spaces
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.
If an operand implements interface Formatter, that interface can be used for fine control of formatting.
If an operand implements method String() string that method will be used to conver the object to a string, which will then be formatted as required by the verb (if any). To avoid recursion in cases such as
type X int
func (x X) String() string { return Sprintf("%d", x) }
cast the value before recurring:
func (x X) String() string { return Sprintf("%d", int(x)) }
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 os.Reader; Sscan, Sscanf and Sscanln read from an argument string. Sscanln, Fscanln and Sscanln stop scanning at a newline and require that the items be followed by one; Sscanf, Fscanf and Sscanf require newlines in the input to match newlines in the format; the other routines treat newlines as spaces.
Scanf, Fscanf, and Sscanf parse the arguments according to a format string, analogous to that 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 formats behave analogously to those of Printf with the following exceptions:
%p is not implemented %T is not implemented %e %E %f %F %g %g are all equivalent and scan any floating
point or complex value
%s and %v on strings scan a space-delimited token
Width is interpreted in the input text (%5s means at most five runes of input will be read to scan a string) but there is no syntax for scanning with a precision (no %5.2f, just %5f).
When scanning with a format, all non-empty runs of space characters (except newline) are equivalent to a single space in both the format and the input. With that proviso, text in the format string must match the input text; scanning stops if it does not, with the return value of the function indicating the number of arguments scanned.
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.
Package files
doc.go format.go print.go scan.goConstants
const EOF = -1
func Fprint
func Fprint(w io.Writer, a ...interface{}) (n int, error os.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.
func Fprintf
func Fprintf(w io.Writer, format string, a ...interface{}) (n int, error os.Error)
Fprintf formats according to a format specifier and writes to w. It returns the number of bytes written and any write error encountered.
func Fprintln
func Fprintln(w io.Writer, a ...interface{}) (n int, error os.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.
func Fscan
func Fscan(r io.Reader, a ...interface{}) (n int, err os.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 os.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.
func Fscanln
func Fscanln(r io.Reader, a ...interface{}) (n int, err os.Error)
Fscanln is similar to Fscan, but stops scanning at a newline and after the final item there must be a newline or EOF.
func Print
func Print(a ...interface{}) (n int, errno os.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.
func Printf
func Printf(format string, a ...interface{}) (n int, errno os.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.
func Println
func Println(a ...interface{}) (n int, errno os.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.
func Scan
func Scan(a ...interface{}) (n int, err os.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 os.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.
func Scanln
func Scanln(a ...interface{}) (n int, err os.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. It returns the number of bytes written.
func Sprintf
func Sprintf(format string, a ...interface{}) string
Sprintf formats according to a format specifier and returns the resulting string. It returns the number of bytes written.
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. It returns the number of bytes written.
func Sscan
func Sscan(str string, a ...interface{}) (n int, err os.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 os.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.
func Sscanln
func Sscanln(str string, a ...interface{}) (n int, err os.Error)
Sscanln is similar to Sscan, but stops scanning at a newline and after the final item there must be a newline or EOF.
type Formatter
Formatter is the interface implemented by values with a custom formatter. The implementation of Format may call Sprintf or Fprintf(f) etc. to generate its output.
type Formatter interface {
Format(f State, c int)
}
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
}
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 {
// GetRune reads the next rune (Unicode code point) from the input.
GetRune() (rune int, err os.Error)
// UngetRune causes the next call to GetRune to return the rune.
UngetRune(rune int)
// 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)
// Token returns the next space-delimited token from the input. If
// a width has been specified, the returned token will be no longer
// than the width.
Token() (token string, err os.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 or Scanln that implements it.
type Scanner interface {
Scan(state ScanState, verb int) os.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) (ret int, err os.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 returns whether the flag c, a character, has been set.
Flag(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 a %s or %v format or to an unformatted printer such as Print.
type Stringer interface {
String() string
}