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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
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
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[]
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
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.
84 For most values, width is the minimum number of runes to output,
85 padding the formatted form with spaces if necessary.
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.
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.
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).
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
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.
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.
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.
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:
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.
141 2. If an operand implements the Formatter interface, it will
142 be invoked. Formatter provides fine control of formatting.
144 3. If the %v verb is used with the # flag (%#v) and the operand
145 implements the GoStringer interface, that will be invoked.
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:
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).
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).
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.
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.
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.
177 When printing a struct, fmt cannot and therefore does not invoke
178 formatting methods such as Error or String on unexported fields.
180 Explicit argument indexes:
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.
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".
202 Format errors:
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:
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)
221 All errors begin with the string "%!" followed sometimes
222 by a single character (the verb) and end with a parenthesized
223 description.
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)
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>".
238 Scanning
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.
245 Scan, Fscan, Sscan treat newlines in the input as spaces.
247 Scanln, Fscanln and Sscanln stop scanning at a newline and
248 require that the items be followed by a newline or EOF.
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.
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.
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.
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.
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.
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.
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.
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).
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.
311 All arguments to be scanned must be either pointers to basic
312 types or implementations of the Scanner interface.
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.
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