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 %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 There is no 'u' flag. Integers are printed unsigned if they have unsigned type. 52 Similarly, there is no need to specify the size of the operand (int8, int64). 53 54 The default format for %v is: 55 bool: %t 56 int, int8 etc.: %d 57 uint, uint8 etc.: %d, %x if printed with %#v 58 float32, complex64, etc: %g 59 string: %s 60 chan: %p 61 pointer: %p 62 For compound objects, the elements are printed using these rules, recursively, 63 laid out like this: 64 struct: {field0 field1 ...} 65 array, slice: [elem0 elem1 ...] 66 maps: map[key1:value1 key2:value2] 67 pointer to above: &{}, &[], &map[] 68 69 Width is specified by an optional decimal number immediately preceding the verb. 70 If absent, the width is whatever is necessary to represent the value. 71 Precision is specified after the (optional) width by a period followed by a 72 decimal number. If no period is present, a default precision is used. 73 A period with no following number specifies a precision of zero. 74 Examples: 75 %f default width, default precision 76 %9f width 9, default precision 77 %.2f default width, precision 2 78 %9.2f width 9, precision 2 79 %9.f width 9, precision 0 80 81 Width and precision are measured in units of Unicode code points, 82 that is, runes. (This differs from C's printf where the 83 units are always measured in bytes.) Either or both of the flags 84 may be replaced with the character '*', causing their values to be 85 obtained from the next operand, which must be of type int. 86 87 For most values, width is the minimum number of runes to output, 88 padding the formatted form with spaces if necessary. 89 90 For strings, byte slices and byte arrays, however, precision 91 limits the length of the input to be formatted (not the size of 92 the output), truncating if necessary. Normally it is measured in 93 runes, but for these types when formatted with the %x or %X format 94 it is measured in bytes. 95 96 For floating-point values, width sets the minimum width of the field and 97 precision sets the number of places after the decimal, if appropriate, 98 except that for %g/%G it sets the total number of digits. For example, 99 given 123.45 the format %6.2f prints 123.45 while %.4g prints 123.5. 100 The default precision for %e and %f is 6; for %g it is the smallest 101 number of digits necessary to identify the value uniquely. 102 103 For complex numbers, the width and precision apply to the two 104 components independently and the result is parenthesized, so %f applied 105 to 1.2+3.4i produces (1.200000+3.400000i). 106 107 Other flags: 108 + always print a sign for numeric values; 109 guarantee ASCII-only output for %q (%+q) 110 - pad with spaces on the right rather than the left (left-justify the field) 111 # alternate format: add leading 0 for octal (%#o), 0x for hex (%#x); 112 0X for hex (%#X); suppress 0x for %p (%#p); 113 for %q, print a raw (backquoted) string if strconv.CanBackquote 114 returns true; 115 write e.g. U+0078 'x' if the character is printable for %U (%#U). 116 ' ' (space) leave a space for elided sign in numbers (% d); 117 put spaces between bytes printing strings or slices in hex (% x, % X) 118 0 pad with leading zeros rather than spaces; 119 for numbers, this moves the padding after the sign 120 121 Flags are ignored by verbs that do not expect them. 122 For example there is no alternate decimal format, so %#d and %d 123 behave identically. 124 125 For each Printf-like function, there is also a Print function 126 that takes no format and is equivalent to saying %v for every 127 operand. Another variant Println inserts blanks between 128 operands and appends a newline. 129 130 Regardless of the verb, if an operand is an interface value, 131 the internal concrete value is used, not the interface itself. 132 Thus: 133 var i interface{} = 23 134 fmt.Printf("%v\n", i) 135 will print 23. 136 137 Except when printed using the verbs %T and %p, special 138 formatting considerations apply for operands that implement 139 certain interfaces. In order of application: 140 141 1. If the operand is a reflect.Value, the concrete value it 142 holds is printed as if it was the operand. 143 144 2. If an operand implements the Formatter interface, it will 145 be invoked. Formatter provides fine control of formatting. 146 147 3. If the %v verb is used with the # flag (%#v) and the operand 148 implements the GoStringer interface, that will be invoked. 149 150 If the format (which is implicitly %v for Println etc.) is valid 151 for a string (%s %q %v %x %X), the following two rules apply: 152 153 4. If an operand implements the error interface, the Error method 154 will be invoked to convert the object to a string, which will then 155 be formatted as required by the verb (if any). 156 157 5. If an operand implements method String() string, that method 158 will be invoked to convert the object to a string, which will then 159 be formatted as required by the verb (if any). 160 161 For compound operands such as slices and structs, the format 162 applies to the elements of each operand, recursively, not to the 163 operand as a whole. Thus %q will quote each element of a slice 164 of strings, and %6.2f will control formatting for each element 165 of a floating-point array. 166 167 However, when printing a byte slice with a string-like verb 168 (%s %q %x %X), it is treated identically to a string, as a single item. 169 170 To avoid recursion in cases such as 171 type X string 172 func (x X) String() string { return Sprintf("<%s>", x) } 173 convert the value before recurring: 174 func (x X) String() string { return Sprintf("<%s>", string(x)) } 175 Infinite recursion can also be triggered by self-referential data 176 structures, such as a slice that contains itself as an element, if 177 that type has a String method. Such pathologies are rare, however, 178 and the package does not protect against them. 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 require that (after skipping spaces) 251 newlines in the format are matched by newlines in the input 252 and vice versa. This behavior differs from the corresponding 253 routines in C, which uniformly treat newlines as spaces. 254 255 When scanning with Scanf, Fscanf, and Sscanf, all non-empty 256 runs of space characters (except newline) are equivalent 257 to a single space in both the format and the input. With 258 that proviso, text in the format string must match the input 259 text; scanning stops if it does not, with the return value 260 of the function indicating the number of arguments scanned. 261 262 Scanf, Fscanf, and Sscanf parse the arguments according to a 263 format string, analogous to that of Printf. For example, %x 264 will scan an integer as a hexadecimal number, and %v will scan 265 the default representation format for the value. 266 267 The formats behave analogously to those of Printf with the 268 following exceptions: 269 270 %p is not implemented 271 %T is not implemented 272 %e %E %f %F %g %G are all equivalent and scan any floating point or complex value 273 %s and %v on strings scan a space-delimited token 274 Flags # and + are not implemented. 275 276 The familiar base-setting prefixes 0 (octal) and 0x 277 (hexadecimal) are accepted when scanning integers without 278 a format or with the %v verb. 279 280 Width is interpreted in the input text but there is no 281 syntax for scanning with a precision (no %5.2f, just %5f). 282 If width is provided, it applies after leading spaces are 283 trimmed and specifies the maximum number of runes to read 284 to satisfy the verb. For example, 285 Sscanf(" 1234567 ", "%5s%d", &s, &i) 286 will set s to "12345" and i to 67 while 287 Sscanf(" 12 34 567 ", "%5s%d", &s, &i) 288 will set s to "12" and i to 34. 289 290 In all the scanning functions, a carriage return followed 291 immediately by a newline is treated as a plain newline 292 (\r\n means the same as \n). 293 294 In all the scanning functions, if an operand implements method 295 Scan (that is, it implements the Scanner interface) that 296 method will be used to scan the text for that operand. Also, 297 if the number of arguments scanned is less than the number of 298 arguments provided, an error is returned. 299 300 All arguments to be scanned must be either pointers to basic 301 types or implementations of the Scanner interface. 302 303 Note: Fscan etc. can read one character (rune) past the input 304 they return, which means that a loop calling a scan routine 305 may skip some of the input. This is usually a problem only 306 when there is no space between input values. If the reader 307 provided to Fscan implements ReadRune, that method will be used 308 to read characters. If the reader also implements UnreadRune, 309 that method will be used to save the character and successive 310 calls will not lose data. To attach ReadRune and UnreadRune 311 methods to a reader without that capability, use 312 bufio.NewReader. 313 */ 314 package fmt 315
