Source file src/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 %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. In this case the interpretation of verbs and flags is 154 controlled by that implementation. 155 156 3. If the %v verb is used with the # flag (%#v) and the operand 157 implements the GoStringer interface, that will be invoked. 158 159 If the format (which is implicitly %v for Println etc.) is valid 160 for a string (%s %q %v %x %X), the following two rules apply: 161 162 4. If an operand implements the error interface, the Error method 163 will be invoked to convert the object to a string, which will then 164 be formatted as required by the verb (if any). 165 166 5. If an operand implements method String() string, that method 167 will be invoked to convert the object to a string, which will then 168 be formatted as required by the verb (if any). 169 170 For compound operands such as slices and structs, the format 171 applies to the elements of each operand, recursively, not to the 172 operand as a whole. Thus %q will quote each element of a slice 173 of strings, and %6.2f will control formatting for each element 174 of a floating-point array. 175 176 However, when printing a byte slice with a string-like verb 177 (%s %q %x %X), it is treated identically to a string, as a single item. 178 179 To avoid recursion in cases such as 180 type X string 181 func (x X) String() string { return Sprintf("<%s>", x) } 182 convert the value before recurring: 183 func (x X) String() string { return Sprintf("<%s>", string(x)) } 184 Infinite recursion can also be triggered by self-referential data 185 structures, such as a slice that contains itself as an element, if 186 that type has a String method. Such pathologies are rare, however, 187 and the package does not protect against them. 188 189 When printing a struct, fmt cannot and therefore does not invoke 190 formatting methods such as Error or String on unexported fields. 191 192 Explicit argument indexes 193 194 In Printf, Sprintf, and Fprintf, the default behavior is for each 195 formatting verb to format successive arguments passed in the call. 196 However, the notation [n] immediately before the verb indicates that the 197 nth one-indexed argument is to be formatted instead. The same notation 198 before a '*' for a width or precision selects the argument index holding 199 the value. After processing a bracketed expression [n], subsequent verbs 200 will use arguments n+1, n+2, etc. unless otherwise directed. 201 202 For example, 203 fmt.Sprintf("%[2]d %[1]d\n", 11, 22) 204 will yield "22 11", while 205 fmt.Sprintf("%[3]*.[2]*[1]f", 12.0, 2, 6) 206 equivalent to 207 fmt.Sprintf("%6.2f", 12.0) 208 will yield " 12.00". Because an explicit index affects subsequent verbs, 209 this notation can be used to print the same values multiple times 210 by resetting the index for the first argument to be repeated: 211 fmt.Sprintf("%d %d %#[1]x %#x", 16, 17) 212 will yield "16 17 0x10 0x11". 213 214 Format errors 215 216 If an invalid argument is given for a verb, such as providing 217 a string to %d, the generated string will contain a 218 description of the problem, as in these examples: 219 220 Wrong type or unknown verb: %!verb(type=value) 221 Printf("%d", "hi"): %!d(string=hi) 222 Too many arguments: %!(EXTRA type=value) 223 Printf("hi", "guys"): hi%!(EXTRA string=guys) 224 Too few arguments: %!verb(MISSING) 225 Printf("hi%d"): hi%!d(MISSING) 226 Non-int for width or precision: %!(BADWIDTH) or %!(BADPREC) 227 Printf("%*s", 4.5, "hi"): %!(BADWIDTH)hi 228 Printf("%.*s", 4.5, "hi"): %!(BADPREC)hi 229 Invalid or invalid use of argument index: %!(BADINDEX) 230 Printf("%*[2]d", 7): %!d(BADINDEX) 231 Printf("%.[2]d", 7): %!d(BADINDEX) 232 233 All errors begin with the string "%!" followed sometimes 234 by a single character (the verb) and end with a parenthesized 235 description. 236 237 If an Error or String method triggers a panic when called by a 238 print routine, the fmt package reformats the error message 239 from the panic, decorating it with an indication that it came 240 through the fmt package. For example, if a String method 241 calls panic("bad"), the resulting formatted message will look 242 like 243 %!s(PANIC=bad) 244 245 The %!s just shows the print verb in use when the failure 246 occurred. If the panic is caused by a nil receiver to an Error 247 or String method, however, the output is the undecorated 248 string, "<nil>". 249 250 Scanning 251 252 An analogous set of functions scans formatted text to yield 253 values. Scan, Scanf and Scanln read from os.Stdin; Fscan, 254 Fscanf and Fscanln read from a specified io.Reader; Sscan, 255 Sscanf and Sscanln read from an argument string. 256 257 Scan, Fscan, Sscan treat newlines in the input as spaces. 258 259 Scanln, Fscanln and Sscanln stop scanning at a newline and 260 require that the items be followed by a newline or EOF. 261 262 Scanf, Fscanf, and Sscanf parse the arguments according to a 263 format string, analogous to that of Printf. In the text that 264 follows, 'space' means any Unicode whitespace character 265 except newline. 266 267 In the format string, a verb introduced by the % character 268 consumes and parses input; these verbs are described in more 269 detail below. A character other than %, space, or newline in 270 the format consumes exactly that input character, which must 271 be present. A newline with zero or more spaces before it in 272 the format string consumes zero or more spaces in the input 273 followed by a single newline or the end of the input. A space 274 following a newline in the format string consumes zero or more 275 spaces in the input. Otherwise, any run of one or more spaces 276 in the format string consumes as many spaces as possible in 277 the input. Unless the run of spaces in the format string 278 appears adjacent to a newline, the run must consume at least 279 one space from the input or find the end of the input. 280 281 The handling of spaces and newlines differs from that of C's 282 scanf family: in C, newlines are treated as any other space, 283 and it is never an error when a run of spaces in the format 284 string finds no spaces to consume in the input. 285 286 The verbs behave analogously to those of Printf. 287 For example, %x will scan an integer as a hexadecimal number, 288 and %v will scan the default representation format for the value. 289 The Printf verbs %p and %T and the flags # and + are not implemented. 290 For floating-point and complex values, all valid formatting verbs 291 (%b %e %E %f %F %g %G %x %X and %v) are equivalent and accept 292 both decimal and hexadecimal notation (for example: "2.3e+7", "0x4.5p-8") 293 and digit-separating underscores (for example: "3.14159_26535_89793"). 294 295 Input processed by verbs is implicitly space-delimited: the 296 implementation of every verb except %c starts by discarding 297 leading spaces from the remaining input, and the %s verb 298 (and %v reading into a string) stops consuming input at the first 299 space or newline character. 300 301 The familiar base-setting prefixes 0b (binary), 0o and 0 (octal), 302 and 0x (hexadecimal) are accepted when scanning integers 303 without a format or with the %v verb, as are digit-separating 304 underscores. 305 306 Width is interpreted in the input text but there is no 307 syntax for scanning with a precision (no %5.2f, just %5f). 308 If width is provided, it applies after leading spaces are 309 trimmed and specifies the maximum number of runes to read 310 to satisfy the verb. For example, 311 Sscanf(" 1234567 ", "%5s%d", &s, &i) 312 will set s to "12345" and i to 67 while 313 Sscanf(" 12 34 567 ", "%5s%d", &s, &i) 314 will set s to "12" and i to 34. 315 316 In all the scanning functions, a carriage return followed 317 immediately by a newline is treated as a plain newline 318 (\r\n means the same as \n). 319 320 In all the scanning functions, if an operand implements method 321 Scan (that is, it implements the Scanner interface) that 322 method will be used to scan the text for that operand. Also, 323 if the number of arguments scanned is less than the number of 324 arguments provided, an error is returned. 325 326 All arguments to be scanned must be either pointers to basic 327 types or implementations of the Scanner interface. 328 329 Like Scanf and Fscanf, Sscanf need not consume its entire input. 330 There is no way to recover how much of the input string Sscanf used. 331 332 Note: Fscan etc. can read one character (rune) past the input 333 they return, which means that a loop calling a scan routine 334 may skip some of the input. This is usually a problem only 335 when there is no space between input values. If the reader 336 provided to Fscan implements ReadRune, that method will be used 337 to read characters. If the reader also implements UnreadRune, 338 that method will be used to save the character and successive 339 calls will not lose data. To attach ReadRune and UnreadRune 340 methods to a reader without that capability, use 341 bufio.NewReader. 342 */ 343 package fmt 344