Source file src/pkg/fmt/doc.go
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 %q a single-quoted character literal safely escaped with Go syntax. 30 %x base 16, with lower-case letters for a-f 31 %X base 16, with upper-case letters for A-F 32 %U Unicode format: U+1234; same as "U+%04X" 33 Floating-point and complex constituents: 34 %b decimalless scientific notation with exponent a power of two, 35 in the manner of strconv.FormatFloat with the 'b' format, 36 e.g. -123456p-78 37 %e scientific notation, e.g. -1234.456e+78 38 %E scientific notation, e.g. -1234.456E+78 39 %f decimal point but no exponent, e.g. 123.456 40 %g whichever of %e or %f produces more compact output 41 %G whichever of %E or %f produces more compact output 42 String and slice of bytes: 43 %s the uninterpreted bytes of the string or slice 44 %q a double-quoted string safely escaped with Go syntax 45 %x base 16, lower-case, two characters per byte 46 %X base 16, upper-case, two characters per byte 47 Pointer: 48 %p base 16 notation, with leading 0x 49 50 There is no 'u' flag. Integers are printed unsigned if they have unsigned type. 51 Similarly, there is no need to specify the size of the operand (int8, int64). 52 53 The width and precision control formatting and are in units of Unicode 54 code points. (This differs from C's printf where the units are numbers 55 of bytes.) Either or both of the flags may be replaced with the 56 character '*', causing their values to be obtained from the next 57 operand, which must be of type int. 58 59 For numeric values, width sets the width of the field and precision 60 sets the number of places after the decimal, if appropriate. For 61 example, the format %6.2f prints 123.45. 62 63 For strings, width is the minimum number of characters to output, 64 padding with spaces if necessary, and precision is the maximum 65 number of characters to output, truncating if necessary. 66 67 Other flags: 68 + always print a sign for numeric values; 69 guarantee ASCII-only output for %q (%+q) 70 - pad with spaces on the right rather than the left (left-justify the field) 71 # alternate format: add leading 0 for octal (%#o), 0x for hex (%#x); 72 0X for hex (%#X); suppress 0x for %p (%#p); 73 print a raw (backquoted) string if possible for %q (%#q); 74 write e.g. U+0078 'x' if the character is printable for %U (%#U). 75 ' ' (space) leave a space for elided sign in numbers (% d); 76 put spaces between bytes printing strings or slices in hex (% x, % X) 77 0 pad with leading zeros rather than spaces 78 79 For each Printf-like function, there is also a Print function 80 that takes no format and is equivalent to saying %v for every 81 operand. Another variant Println inserts blanks between 82 operands and appends a newline. 83 84 Regardless of the verb, if an operand is an interface value, 85 the internal concrete value is used, not the interface itself. 86 Thus: 87 var i interface{} = 23 88 fmt.Printf("%v\n", i) 89 will print 23. 90 91 If an operand implements interface Formatter, that interface 92 can be used for fine control of formatting. 93 94 If the format (which is implicitly %v for Println etc.) is valid 95 for a string (%s %q %v %x %X), the following two rules also apply: 96 97 1. If an operand implements the error interface, the Error method 98 will be used to convert the object to a string, which will then 99 be formatted as required by the verb (if any). 100 101 2. If an operand implements method String() string, that method 102 will be used to convert the object to a string, which will then 103 be formatted as required by the verb (if any). 104 105 To avoid recursion in cases such as 106 type X string 107 func (x X) String() string { return Sprintf("<%s>", x) } 108 convert the value before recurring: 109 func (x X) String() string { return Sprintf("<%s>", string(x)) } 110 111 Format errors: 112 113 If an invalid argument is given for a verb, such as providing 114 a string to %d, the generated string will contain a 115 description of the problem, as in these examples: 116 117 Wrong type or unknown verb: %!verb(type=value) 118 Printf("%d", hi): %!d(string=hi) 119 Too many arguments: %!(EXTRA type=value) 120 Printf("hi", "guys"): hi%!(EXTRA string=guys) 121 Too few arguments: %!verb(MISSING) 122 Printf("hi%d"): hi %!d(MISSING) 123 Non-int for width or precision: %!(BADWIDTH) or %!(BADPREC) 124 Printf("%*s", 4.5, "hi"): %!(BADWIDTH)hi 125 Printf("%.*s", 4.5, "hi"): %!(BADPREC)hi 126 127 All errors begin with the string "%!" followed sometimes 128 by a single character (the verb) and end with a parenthesized 129 description. 130 131 132 Scanning 133 134 An analogous set of functions scans formatted text to yield 135 values. Scan, Scanf and Scanln read from os.Stdin; Fscan, 136 Fscanf and Fscanln read from a specified io.Reader; Sscan, 137 Sscanf and Sscanln read from an argument string. Scanln, 138 Fscanln and Sscanln stop scanning at a newline and require that 139 the items be followed by one; Sscanf, Fscanf and Sscanf require 140 newlines in the input to match newlines in the format; the other 141 routines treat newlines as spaces. 142 143 Scanf, Fscanf, and Sscanf parse the arguments according to a 144 format string, analogous to that of Printf. For example, %x 145 will scan an integer as a hexadecimal number, and %v will scan 146 the default representation format for the value. 147 148 The formats behave analogously to those of Printf with the 149 following exceptions: 150 151 %p is not implemented 152 %T is not implemented 153 %e %E %f %F %g %G are all equivalent and scan any floating point or complex value 154 %s and %v on strings scan a space-delimited token 155 156 The familiar base-setting prefixes 0 (octal) and 0x 157 (hexadecimal) are accepted when scanning integers without a 158 format or with the %v verb. 159 160 Width is interpreted in the input text (%5s means at most 161 five runes of input will be read to scan a string) but there 162 is no syntax for scanning with a precision (no %5.2f, just 163 %5f). 164 165 When scanning with a format, all non-empty runs of space 166 characters (except newline) are equivalent to a single 167 space in both the format and the input. With that proviso, 168 text in the format string must match the input text; scanning 169 stops if it does not, with the return value of the function 170 indicating the number of arguments scanned. 171 172 In all the scanning functions, if an operand implements method 173 Scan (that is, it implements the Scanner interface) that 174 method will be used to scan the text for that operand. Also, 175 if the number of arguments scanned is less than the number of 176 arguments provided, an error is returned. 177 178 All arguments to be scanned must be either pointers to basic 179 types or implementations of the Scanner interface. 180 181 Note: Fscan etc. can read one character (rune) past the input 182 they return, which means that a loop calling a scan routine 183 may skip some of the input. This is usually a problem only 184 when there is no space between input values. If the reader 185 provided to Fscan implements ReadRune, that method will be used 186 to read characters. If the reader also implements UnreadRune, 187 that method will be used to save the character and successive 188 calls will not lose data. To attach ReadRune and UnreadRune 189 methods to a reader without that capability, use 190 bufio.NewReader. 191 */ 192 package fmt