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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 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).
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[]
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
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.
87 For most values, width is the minimum number of runes to output,
88 padding the formatted form with spaces if necessary.
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.
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.
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).
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
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.
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.
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.
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:
141 1. If the operand is a reflect.Value, the operand is replaced by the
142 concrete value that it holds, and printing continues with the next rule.
144 2. If an operand implements the Formatter interface, it will
145 be invoked. Formatter provides fine control of formatting.
147 3. If the %v verb is used with the # flag (%#v) and the operand
148 implements the GoStringer interface, that will be invoked.
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:
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).
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).
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.
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.
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.
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 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.
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.
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.
267 The formats behave analogously to those of Printf with the
268 following exceptions:
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.
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.
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.
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).
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.
300 All arguments to be scanned must be either pointers to basic
301 types or implementations of the Scanner interface.
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