Source file src/fmt/doc.go

Documentation: fmt

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  /*
     6  	Package fmt implements formatted I/O with functions analogous
     7  	to C's printf and scanf.  The format 'verbs' are derived from C's but
     8  	are simpler.
     9  
    10  
    11  	Printing
    12  
    13  	The verbs:
    14  
    15  	General:
    16  		%v	the value in a default format
    17  			when printing structs, the plus flag (%+v) adds field names
    18  		%#v	a Go-syntax representation of the value
    19  		%T	a Go-syntax representation of the type of the value
    20  		%%	a literal percent sign; consumes no value
    21  
    22  	Boolean:
    23  		%t	the word true or false
    24  	Integer:
    25  		%b	base 2
    26  		%c	the character represented by the corresponding Unicode code point
    27  		%d	base 10
    28  		%o	base 8
    29  		%O	base 8 with 0o prefix
    30  		%q	a single-quoted character literal safely escaped with Go syntax.
    31  		%x	base 16, with lower-case letters for a-f
    32  		%X	base 16, with upper-case letters for A-F
    33  		%U	Unicode format: U+1234; same as "U+%04X"
    34  	Floating-point and complex constituents:
    35  		%b	decimalless scientific notation with exponent a power of two,
    36  			in the manner of strconv.FormatFloat with the 'b' format,
    37  			e.g. -123456p-78
    38  		%e	scientific notation, e.g. -1.234456e+78
    39  		%E	scientific notation, e.g. -1.234456E+78
    40  		%f	decimal point but no exponent, e.g. 123.456
    41  		%F	synonym for %f
    42  		%g	%e for large exponents, %f otherwise. Precision is discussed below.
    43  		%G	%E for large exponents, %F otherwise
    44  		%x	hexadecimal notation (with decimal power of two exponent), e.g. -0x1.23abcp+20
    45  		%X	upper-case hexadecimal notation, e.g. -0X1.23ABCP+20
    46  	String and slice of bytes (treated equivalently with these verbs):
    47  		%s	the uninterpreted bytes of the string or slice
    48  		%q	a double-quoted string safely escaped with Go syntax
    49  		%x	base 16, lower-case, two characters per byte
    50  		%X	base 16, upper-case, two characters per byte
    51  	Slice:
    52  		%p	address of 0th element in base 16 notation, with leading 0x
    53  	Pointer:
    54  		%p	base 16 notation, with leading 0x
    55  		The %b, %d, %o, %x and %X verbs also work with pointers,
    56  		formatting the value exactly as if it were an integer.
    57  
    58  	The default format for %v is:
    59  		bool:                    %t
    60  		int, int8 etc.:          %d
    61  		uint, uint8 etc.:        %d, %#x if printed with %#v
    62  		float32, complex64, etc: %g
    63  		string:                  %s
    64  		chan:                    %p
    65  		pointer:                 %p
    66  	For compound objects, the elements are printed using these rules, recursively,
    67  	laid out like this:
    68  		struct:             {field0 field1 ...}
    69  		array, slice:       [elem0 elem1 ...]
    70  		maps:               map[key1:value1 key2:value2 ...]
    71  		pointer to above:   &{}, &[], &map[]
    72  
    73  	Width is specified by an optional decimal number immediately preceding the verb.
    74  	If absent, the width is whatever is necessary to represent the value.
    75  	Precision is specified after the (optional) width by a period followed by a
    76  	decimal number. If no period is present, a default precision is used.
    77  	A period with no following number specifies a precision of zero.
    78  	Examples:
    79  		%f     default width, default precision
    80  		%9f    width 9, default precision
    81  		%.2f   default width, precision 2
    82  		%9.2f  width 9, precision 2
    83  		%9.f   width 9, precision 0
    84  
    85  	Width and precision are measured in units of Unicode code points,
    86  	that is, runes. (This differs from C's printf where the
    87  	units are always measured in bytes.) Either or both of the flags
    88  	may be replaced with the character '*', causing their values to be
    89  	obtained from the next operand (preceding the one to format),
    90  	which must be of type int.
    91  
    92  	For most values, width is the minimum number of runes to output,
    93  	padding the formatted form with spaces if necessary.
    94  
    95  	For strings, byte slices and byte arrays, however, precision
    96  	limits the length of the input to be formatted (not the size of
    97  	the output), truncating if necessary. Normally it is measured in
    98  	runes, but for these types when formatted with the %x or %X format
    99  	it is measured in bytes.
   100  
   101  	For floating-point values, width sets the minimum width of the field and
   102  	precision sets the number of places after the decimal, if appropriate,
   103  	except that for %g/%G precision sets the maximum number of significant
   104  	digits (trailing zeros are removed). For example, given 12.345 the format
   105  	%6.3f prints 12.345 while %.3g prints 12.3. The default precision for %e, %f
   106  	and %#g is 6; for %g it is the smallest number of digits necessary to identify
   107  	the value uniquely.
   108  
   109  	For complex numbers, the width and precision apply to the two
   110  	components independently and the result is parenthesized, so %f applied
   111  	to 1.2+3.4i produces (1.200000+3.400000i).
   112  
   113  	Other flags:
   114  		+	always print a sign for numeric values;
   115  			guarantee ASCII-only output for %q (%+q)
   116  		-	pad with spaces on the right rather than the left (left-justify the field)
   117  		#	alternate format: add leading 0b for binary (%#b), 0 for octal (%#o),
   118  			0x or 0X for hex (%#x or %#X); suppress 0x for %p (%#p);
   119  			for %q, print a raw (backquoted) string if strconv.CanBackquote
   120  			returns true;
   121  			always print a decimal point for %e, %E, %f, %F, %g and %G;
   122  			do not remove trailing zeros for %g and %G;
   123  			write e.g. U+0078 'x' if the character is printable for %U (%#U).
   124  		' '	(space) leave a space for elided sign in numbers (% d);
   125  			put spaces between bytes printing strings or slices in hex (% x, % X)
   126  		0	pad with leading zeros rather than spaces;
   127  			for numbers, this moves the padding after the sign
   128  
   129  	Flags are ignored by verbs that do not expect them.
   130  	For example there is no alternate decimal format, so %#d and %d
   131  	behave identically.
   132  
   133  	For each Printf-like function, there is also a Print function
   134  	that takes no format and is equivalent to saying %v for every
   135  	operand.  Another variant Println inserts blanks between
   136  	operands and appends a newline.
   137  
   138  	Regardless of the verb, if an operand is an interface value,
   139  	the internal concrete value is used, not the interface itself.
   140  	Thus:
   141  		var i interface{} = 23
   142  		fmt.Printf("%v\n", i)
   143  	will print 23.
   144  
   145  	Except when printed using the verbs %T and %p, special
   146  	formatting considerations apply for operands that implement
   147  	certain interfaces. In order of application:
   148  
   149  	1. If the operand is a reflect.Value, the operand is replaced by the
   150  	concrete value that it holds, and printing continues with the next rule.
   151  
   152  	2. If an operand implements the Formatter interface, it will
   153  	be invoked. Formatter provides fine control of formatting.
   154  
   155  	3. If the %v verb is used with the # flag (%#v) and the operand
   156  	implements the GoStringer interface, that will be invoked.
   157  
   158  	If the format (which is implicitly %v for Println etc.) is valid
   159  	for a string (%s %q %v %x %X), the following two rules apply:
   160  
   161  	4. If an operand implements the error interface, the Error method
   162  	will be invoked to convert the object to a string, which will then
   163  	be formatted as required by the verb (if any).
   164  
   165  	5. If an operand implements method String() string, that method
   166  	will be invoked to convert the object to a string, which will then
   167  	be formatted as required by the verb (if any).
   168  
   169  	For compound operands such as slices and structs, the format
   170  	applies to the elements of each operand, recursively, not to the
   171  	operand as a whole. Thus %q will quote each element of a slice
   172  	of strings, and %6.2f will control formatting for each element
   173  	of a floating-point array.
   174  
   175  	However, when printing a byte slice with a string-like verb
   176  	(%s %q %x %X), it is treated identically to a string, as a single item.
   177  
   178  	To avoid recursion in cases such as
   179  		type X string
   180  		func (x X) String() string { return Sprintf("<%s>", x) }
   181  	convert the value before recurring:
   182  		func (x X) String() string { return Sprintf("<%s>", string(x)) }
   183  	Infinite recursion can also be triggered by self-referential data
   184  	structures, such as a slice that contains itself as an element, if
   185  	that type has a String method. Such pathologies are rare, however,
   186  	and the package does not protect against them.
   187  
   188  	When printing a struct, fmt cannot and therefore does not invoke
   189  	formatting methods such as Error or String on unexported fields.
   190  
   191  	Explicit argument indexes:
   192  
   193  	In Printf, Sprintf, and Fprintf, the default behavior is for each
   194  	formatting verb to format successive arguments passed in the call.
   195  	However, the notation [n] immediately before the verb indicates that the
   196  	nth one-indexed argument is to be formatted instead. The same notation
   197  	before a '*' for a width or precision selects the argument index holding
   198  	the value. After processing a bracketed expression [n], subsequent verbs
   199  	will use arguments n+1, n+2, etc. unless otherwise directed.
   200  
   201  	For example,
   202  		fmt.Sprintf("%[2]d %[1]d\n", 11, 22)
   203  	will yield "22 11", while
   204  		fmt.Sprintf("%[3]*.[2]*[1]f", 12.0, 2, 6)
   205  	equivalent to
   206  		fmt.Sprintf("%6.2f", 12.0)
   207  	will yield " 12.00". Because an explicit index affects subsequent verbs,
   208  	this notation can be used to print the same values multiple times
   209  	by resetting the index for the first argument to be repeated:
   210  		fmt.Sprintf("%d %d %#[1]x %#x", 16, 17)
   211  	will yield "16 17 0x10 0x11".
   212  
   213  	Format errors:
   214  
   215  	If an invalid argument is given for a verb, such as providing
   216  	a string to %d, the generated string will contain a
   217  	description of the problem, as in these examples:
   218  
   219  		Wrong type or unknown verb: %!verb(type=value)
   220  			Printf("%d", "hi"):        %!d(string=hi)
   221  		Too many arguments: %!(EXTRA type=value)
   222  			Printf("hi", "guys"):      hi%!(EXTRA string=guys)
   223  		Too few arguments: %!verb(MISSING)
   224  			Printf("hi%d"):            hi%!d(MISSING)
   225  		Non-int for width or precision: %!(BADWIDTH) or %!(BADPREC)
   226  			Printf("%*s", 4.5, "hi"):  %!(BADWIDTH)hi
   227  			Printf("%.*s", 4.5, "hi"): %!(BADPREC)hi
   228  		Invalid or invalid use of argument index: %!(BADINDEX)
   229  			Printf("%*[2]d", 7):       %!d(BADINDEX)
   230  			Printf("%.[2]d", 7):       %!d(BADINDEX)
   231  
   232  	All errors begin with the string "%!" followed sometimes
   233  	by a single character (the verb) and end with a parenthesized
   234  	description.
   235  
   236  	If an Error or String method triggers a panic when called by a
   237  	print routine, the fmt package reformats the error message
   238  	from the panic, decorating it with an indication that it came
   239  	through the fmt package.  For example, if a String method
   240  	calls panic("bad"), the resulting formatted message will look
   241  	like
   242  		%!s(PANIC=bad)
   243  
   244  	The %!s just shows the print verb in use when the failure
   245  	occurred. If the panic is caused by a nil receiver to an Error
   246  	or String method, however, the output is the undecorated
   247  	string, "<nil>".
   248  
   249  	Scanning
   250  
   251  	An analogous set of functions scans formatted text to yield
   252  	values.  Scan, Scanf and Scanln read from os.Stdin; Fscan,
   253  	Fscanf and Fscanln read from a specified io.Reader; Sscan,
   254  	Sscanf and Sscanln read from an argument string.
   255  
   256  	Scan, Fscan, Sscan treat newlines in the input as spaces.
   257  
   258  	Scanln, Fscanln and Sscanln stop scanning at a newline and
   259  	require that the items be followed by a newline or EOF.
   260  
   261  	Scanf, Fscanf, and Sscanf parse the arguments according to a
   262  	format string, analogous to that of Printf. In the text that
   263  	follows, 'space' means any Unicode whitespace character
   264  	except newline.
   265  
   266  	In the format string, a verb introduced by the % character
   267  	consumes and parses input; these verbs are described in more
   268  	detail below. A character other than %, space, or newline in
   269  	the format consumes exactly that input character, which must
   270  	be present. A newline with zero or more spaces before it in
   271  	the format string consumes zero or more spaces in the input
   272  	followed by a single newline or the end of the input. A space
   273  	following a newline in the format string consumes zero or more
   274  	spaces in the input. Otherwise, any run of one or more spaces
   275  	in the format string consumes as many spaces as possible in
   276  	the input. Unless the run of spaces in the format string
   277  	appears adjacent to a newline, the run must consume at least
   278  	one space from the input or find the end of the input.
   279  
   280  	The handling of spaces and newlines differs from that of C's
   281  	scanf family: in C, newlines are treated as any other space,
   282  	and it is never an error when a run of spaces in the format
   283  	string finds no spaces to consume in the input.
   284  
   285  	The verbs behave analogously to those of Printf.
   286  	For example, %x will scan an integer as a hexadecimal number,
   287  	and %v will scan the default representation format for the value.
   288  	The Printf verbs %p and %T and the flags # and + are not implemented.
   289  	For floating-point and complex values, all valid formatting verbs
   290  	(%b %e %E %f %F %g %G %x %X and %v) are equivalent and accept
   291  	both decimal and hexadecimal notation (for example: "2.3e+7", "0x4.5p-8")
   292  	and digit-separating underscores (for example: "3.14159_26535_89793").
   293  
   294  	Input processed by verbs is implicitly space-delimited: the
   295  	implementation of every verb except %c starts by discarding
   296  	leading spaces from the remaining input, and the %s verb
   297  	(and %v reading into a string) stops consuming input at the first
   298  	space or newline character.
   299  
   300  	The familiar base-setting prefixes 0b (binary), 0o and 0 (octal),
   301  	and 0x (hexadecimal) are accepted when scanning integers
   302  	without a format or with the %v verb, as are digit-separating
   303  	underscores.
   304  
   305  	Width is interpreted in the input text but there is no
   306  	syntax for scanning with a precision (no %5.2f, just %5f).
   307  	If width is provided, it applies after leading spaces are
   308  	trimmed and specifies the maximum number of runes to read
   309  	to satisfy the verb. For example,
   310  	   Sscanf(" 1234567 ", "%5s%d", &s, &i)
   311  	will set s to "12345" and i to 67 while
   312  	   Sscanf(" 12 34 567 ", "%5s%d", &s, &i)
   313  	will set s to "12" and i to 34.
   314  
   315  	In all the scanning functions, a carriage return followed
   316  	immediately by a newline is treated as a plain newline
   317  	(\r\n means the same as \n).
   318  
   319  	In all the scanning functions, if an operand implements method
   320  	Scan (that is, it implements the Scanner interface) that
   321  	method will be used to scan the text for that operand.  Also,
   322  	if the number of arguments scanned is less than the number of
   323  	arguments provided, an error is returned.
   324  
   325  	All arguments to be scanned must be either pointers to basic
   326  	types or implementations of the Scanner interface.
   327  
   328  	Like Scanf and Fscanf, Sscanf need not consume its entire input.
   329  	There is no way to recover how much of the input string Sscanf used.
   330  
   331  	Note: Fscan etc. can read one character (rune) past the input
   332  	they return, which means that a loop calling a scan routine
   333  	may skip some of the input.  This is usually a problem only
   334  	when there is no space between input values.  If the reader
   335  	provided to Fscan implements ReadRune, that method will be used
   336  	to read characters.  If the reader also implements UnreadRune,
   337  	that method will be used to save the character and successive
   338  	calls will not lose data.  To attach ReadRune and UnreadRune
   339  	methods to a reader without that capability, use
   340  	bufio.NewReader.
   341  */
   342  package fmt
   343  

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