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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.
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.
11 Printing
13 The verbs:
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
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.
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[]
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
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.
92 For most values, width is the minimum number of runes to output,
93 padding the formatted form with spaces if necessary.
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.
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.
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).
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
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.
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.
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.
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:
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.
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.
156 3. If the %v verb is used with the # flag (%#v) and the operand
157 implements the GoStringer interface, that will be invoked.
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:
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).
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).
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.
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.
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.
189 When printing a struct, fmt cannot and therefore does not invoke
190 formatting methods such as Error or String on unexported fields.
192 Explicit argument indexes
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.
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".
214 Format errors
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:
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)
233 All errors begin with the string "%!" followed sometimes
234 by a single character (the verb) and end with a parenthesized
235 description.
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)
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>".
250 Scanning
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.
257 Scan, Fscan, Sscan treat newlines in the input as spaces.
259 Scanln, Fscanln and Sscanln stop scanning at a newline and
260 require that the items be followed by a newline or EOF.
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.
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.
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.
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").
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.
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.
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.
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).
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.
326 All arguments to be scanned must be either pointers to basic
327 types or implementations of the Scanner interface.
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.
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