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