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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 %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. Precision is discussed below.
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 Slice:
49 %p address of 0th element in base 16 notation, with leading 0x
50 Pointer:
51 %p base 16 notation, with leading 0x
52 The %b, %d, %o, %x and %X verbs also work with pointers,
53 formatting the value exactly as if it were an integer.
55 The default format for %v is:
56 bool: %t
57 int, int8 etc.: %d
58 uint, uint8 etc.: %d, %#x if printed with %#v
59 float32, complex64, etc: %g
60 string: %s
61 chan: %p
62 pointer: %p
63 For compound objects, the elements are printed using these rules, recursively,
64 laid out like this:
65 struct: {field0 field1 ...}
66 array, slice: [elem0 elem1 ...]
67 maps: map[key1:value1 key2:value2 ...]
68 pointer to above: &{}, &[], &map[]
70 Width is specified by an optional decimal number immediately preceding the verb.
71 If absent, the width is whatever is necessary to represent the value.
72 Precision is specified after the (optional) width by a period followed by a
73 decimal number. If no period is present, a default precision is used.
74 A period with no following number specifies a precision of zero.
75 Examples:
76 %f default width, default precision
77 %9f width 9, default precision
78 %.2f default width, precision 2
79 %9.2f width 9, precision 2
80 %9.f width 9, precision 0
82 Width and precision are measured in units of Unicode code points,
83 that is, runes. (This differs from C's printf where the
84 units are always measured in bytes.) Either or both of the flags
85 may be replaced with the character '*', causing their values to be
86 obtained from the next operand (preceding the one to format),
87 which must be of type int.
89 For most values, width is the minimum number of runes to output,
90 padding the formatted form with spaces if necessary.
92 For strings, byte slices and byte arrays, however, precision
93 limits the length of the input to be formatted (not the size of
94 the output), truncating if necessary. Normally it is measured in
95 runes, but for these types when formatted with the %x or %X format
96 it is measured in bytes.
98 For floating-point values, width sets the minimum width of the field and
99 precision sets the number of places after the decimal, if appropriate,
100 except that for %g/%G precision sets the maximum number of significant
101 digits (trailing zeros are removed). For example, given 12.345 the format
102 %6.3f prints 12.345 while %.3g prints 12.3. The default precision for %e, %f
103 and %#g is 6; for %g it is the smallest number of digits necessary to identify
104 the value uniquely.
106 For complex numbers, the width and precision apply to the two
107 components independently and the result is parenthesized, so %f applied
108 to 1.2+3.4i produces (1.200000+3.400000i).
110 Other flags:
111 + always print a sign for numeric values;
112 guarantee ASCII-only output for %q (%+q)
113 - pad with spaces on the right rather than the left (left-justify the field)
114 # alternate format: add leading 0 for octal (%#o), 0x for hex (%#x);
115 0X for hex (%#X); suppress 0x for %p (%#p);
116 for %q, print a raw (backquoted) string if strconv.CanBackquote
117 returns true;
118 always print a decimal point for %e, %E, %f, %F, %g and %G;
119 do not remove trailing zeros for %g and %G;
120 write e.g. U+0078 'x' if the character is printable for %U (%#U).
121 ' ' (space) leave a space for elided sign in numbers (% d);
122 put spaces between bytes printing strings or slices in hex (% x, % X)
123 0 pad with leading zeros rather than spaces;
124 for numbers, this moves the padding after the sign
126 Flags are ignored by verbs that do not expect them.
127 For example there is no alternate decimal format, so %#d and %d
128 behave identically.
130 For each Printf-like function, there is also a Print function
131 that takes no format and is equivalent to saying %v for every
132 operand. Another variant Println inserts blanks between
133 operands and appends a newline.
135 Regardless of the verb, if an operand is an interface value,
136 the internal concrete value is used, not the interface itself.
137 Thus:
138 var i interface{} = 23
139 fmt.Printf("%v\n", i)
140 will print 23.
142 Except when printed using the verbs %T and %p, special
143 formatting considerations apply for operands that implement
144 certain interfaces. In order of application:
146 1. If the operand is a reflect.Value, the operand is replaced by the
147 concrete value that it holds, and printing continues with the next rule.
149 2. If an operand implements the Formatter interface, it will
150 be invoked. Formatter provides fine control of formatting.
152 3. If the %v verb is used with the # flag (%#v) and the operand
153 implements the GoStringer interface, that will be invoked.
155 If the format (which is implicitly %v for Println etc.) is valid
156 for a string (%s %q %v %x %X), the following two rules apply:
158 4. If an operand implements the error interface, the Error method
159 will be invoked to convert the object to a string, which will then
160 be formatted as required by the verb (if any).
162 5. If an operand implements method String() string, that 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 For compound operands such as slices and structs, the format
167 applies to the elements of each operand, recursively, not to the
168 operand as a whole. Thus %q will quote each element of a slice
169 of strings, and %6.2f will control formatting for each element
170 of a floating-point array.
172 However, when printing a byte slice with a string-like verb
173 (%s %q %x %X), it is treated identically to a string, as a single item.
175 To avoid recursion in cases such as
176 type X string
177 func (x X) String() string { return Sprintf("<%s>", x) }
178 convert the value before recurring:
179 func (x X) String() string { return Sprintf("<%s>", string(x)) }
180 Infinite recursion can also be triggered by self-referential data
181 structures, such as a slice that contains itself as an element, if
182 that type has a String method. Such pathologies are rare, however,
183 and the package does not protect against them.
185 When printing a struct, fmt cannot and therefore does not invoke
186 formatting methods such as Error or String on unexported fields.
188 Explicit argument indexes:
190 In Printf, Sprintf, and Fprintf, the default behavior is for each
191 formatting verb to format successive arguments passed in the call.
192 However, the notation [n] immediately before the verb indicates that the
193 nth one-indexed argument is to be formatted instead. The same notation
194 before a '*' for a width or precision selects the argument index holding
195 the value. After processing a bracketed expression [n], subsequent verbs
196 will use arguments n+1, n+2, etc. unless otherwise directed.
198 For example,
199 fmt.Sprintf("%[2]d %[1]d\n", 11, 22)
200 will yield "22 11", while
201 fmt.Sprintf("%[3]*.[2]*[1]f", 12.0, 2, 6)
202 equivalent to
203 fmt.Sprintf("%6.2f", 12.0)
204 will yield " 12.00". Because an explicit index affects subsequent verbs,
205 this notation can be used to print the same values multiple times
206 by resetting the index for the first argument to be repeated:
207 fmt.Sprintf("%d %d %#[1]x %#x", 16, 17)
208 will yield "16 17 0x10 0x11".
210 Format errors:
212 If an invalid argument is given for a verb, such as providing
213 a string to %d, the generated string will contain a
214 description of the problem, as in these examples:
216 Wrong type or unknown verb: %!verb(type=value)
217 Printf("%d", hi): %!d(string=hi)
218 Too many arguments: %!(EXTRA type=value)
219 Printf("hi", "guys"): hi%!(EXTRA string=guys)
220 Too few arguments: %!verb(MISSING)
221 Printf("hi%d"): hi%!d(MISSING)
222 Non-int for width or precision: %!(BADWIDTH) or %!(BADPREC)
223 Printf("%*s", 4.5, "hi"): %!(BADWIDTH)hi
224 Printf("%.*s", 4.5, "hi"): %!(BADPREC)hi
225 Invalid or invalid use of argument index: %!(BADINDEX)
226 Printf("%*[2]d", 7): %!d(BADINDEX)
227 Printf("%.[2]d", 7): %!d(BADINDEX)
229 All errors begin with the string "%!" followed sometimes
230 by a single character (the verb) and end with a parenthesized
231 description.
233 If an Error or String method triggers a panic when called by a
234 print routine, the fmt package reformats the error message
235 from the panic, decorating it with an indication that it came
236 through the fmt package. For example, if a String method
237 calls panic("bad"), the resulting formatted message will look
238 like
239 %!s(PANIC=bad)
241 The %!s just shows the print verb in use when the failure
242 occurred. If the panic is caused by a nil receiver to an Error
243 or String method, however, the output is the undecorated
244 string, "<nil>".
246 Scanning
248 An analogous set of functions scans formatted text to yield
249 values. Scan, Scanf and Scanln read from os.Stdin; Fscan,
250 Fscanf and Fscanln read from a specified io.Reader; Sscan,
251 Sscanf and Sscanln read from an argument string.
253 Scan, Fscan, Sscan treat newlines in the input as spaces.
255 Scanln, Fscanln and Sscanln stop scanning at a newline and
256 require that the items be followed by a newline or EOF.
258 Scanf, Fscanf, and Sscanf parse the arguments according to a
259 format string, analogous to that of Printf. In the text that
260 follows, 'space' means any Unicode whitespace character
261 except newline.
263 In the format string, a verb introduced by the % character
264 consumes and parses input; these verbs are described in more
265 detail below. A character other than %, space, or newline in
266 the format consumes exactly that input character, which must
267 be present. A newline with zero or more spaces before it in
268 the format string consumes zero or more spaces in the input
269 followed by a single newline or the end of the input. A space
270 following a newline in the format string consumes zero or more
271 spaces in the input. Otherwise, any run of one or more spaces
272 in the format string consumes as many spaces as possible in
273 the input. Unless the run of spaces in the format string
274 appears adjacent to a newline, the run must consume at least
275 one space from the input or find the end of the input.
277 The handling of spaces and newlines differs from that of C's
278 scanf family: in C, newlines are treated as any other space,
279 and it is never an error when a run of spaces in the format
280 string finds no spaces to consume in the input.
282 The verbs behave analogously to those of Printf.
283 For example, %x will scan an integer as a hexadecimal number,
284 and %v will scan the default representation format for the value.
285 The Printf verbs %p and %T and the flags # and + are not implemented.
286 The verbs %e %E %f %F %g and %G are all equivalent and scan any
287 floating-point or complex value. For float and complex literals in
288 scientific notation, both the decimal (e) and binary (p) exponent
289 formats are supported (for example: "2.3e+7" and "4.5p-8").
291 Input processed by verbs is implicitly space-delimited: the
292 implementation of every verb except %c starts by discarding
293 leading spaces from the remaining input, and the %s verb
294 (and %v reading into a string) stops consuming input at the first
295 space or newline character.
297 The familiar base-setting prefixes 0 (octal) and 0x
298 (hexadecimal) are accepted when scanning integers without
299 a format or with the %v verb.
301 Width is interpreted in the input text but there is no
302 syntax for scanning with a precision (no %5.2f, just %5f).
303 If width is provided, it applies after leading spaces are
304 trimmed and specifies the maximum number of runes to read
305 to satisfy the verb. For example,
306 Sscanf(" 1234567 ", "%5s%d", &s, &i)
307 will set s to "12345" and i to 67 while
308 Sscanf(" 12 34 567 ", "%5s%d", &s, &i)
309 will set s to "12" and i to 34.
311 In all the scanning functions, a carriage return followed
312 immediately by a newline is treated as a plain newline
313 (\r\n means the same as \n).
315 In all the scanning functions, if an operand implements method
316 Scan (that is, it implements the Scanner interface) that
317 method will be used to scan the text for that operand. Also,
318 if the number of arguments scanned is less than the number of
319 arguments provided, an error is returned.
321 All arguments to be scanned must be either pointers to basic
322 types or implementations of the Scanner interface.
324 Like Scanf and Fscanf, Sscanf need not consume its entire input.
325 There is no way to recover how much of the input string Sscanf used.
327 Note: Fscan etc. can read one character (rune) past the input
328 they return, which means that a loop calling a scan routine
329 may skip some of the input. This is usually a problem only
330 when there is no space between input values. If the reader
331 provided to Fscan implements ReadRune, that method will be used
332 to read characters. If the reader also implements UnreadRune,
333 that method will be used to save the character and successive
334 calls will not lose data. To attach ReadRune and UnreadRune
335 methods to a reader without that capability, use
336 bufio.NewReader.
337 */
338 package fmt