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// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
Package fmt implements formatted I/O with functions analogous
to C's printf and scanf. The format 'verbs' are derived from C's but
are simpler.
The verbs:
%v the value in a default format.
when printing structs, the plus flag (%+v) adds field names
%#v a Go-syntax representation of the value
%T a Go-syntax representation of the type of the value
%% a literal percent sign; consumes no value
%t the word true or false
%b base 2
%c the character represented by the corresponding Unicode code point
%d base 10
%o base 8
%q a single-quoted character literal safely escaped with Go syntax.
%x base 16, with lower-case letters for a-f
%X base 16, with upper-case letters for A-F
%U Unicode format: U+1234; same as "U+%04X"
Floating-point and complex constituents:
%b decimalless scientific notation with exponent a power
of two, in the manner of strconv.Ftoa32, e.g. -123456p-78
%e scientific notation, e.g. -1234.456e+78
%E scientific notation, e.g. -1234.456E+78
%f decimal point but no exponent, e.g. 123.456
%g whichever of %e or %f produces more compact output
%G whichever of %E or %f produces more compact output
String and slice of bytes:
%s the uninterpreted bytes of the string or slice
%q a double-quoted string safely escaped with Go syntax
%x base 16, lower-case, two characters per byte
%X base 16, upper-case, two characters per byte
%p base 16 notation, with leading 0x
There is no 'u' flag. Integers are printed unsigned if they have unsigned type.
Similarly, there is no need to specify the size of the operand (int8, int64).
The width and precision control formatting and are in units of Unicode
code points. (This differs from C's printf where the units are numbers
of bytes.) Either or both of the flags may be replaced with the
character '*', causing their values to be obtained from the next
operand, which must be of type int.
For numeric values, width sets the width of the field and precision
sets the number of places after the decimal, if appropriate. For
example, the format %6.2f prints 123.45.
For strings, width is the minimum number of characters to output,
padding with spaces if necessary, and precision is the maximum
number of characters to output, truncating if necessary.
Other flags:
+ always print a sign for numeric values;
guarantee ASCII-only output for %q (%+q)
- pad with spaces on the right rather than the left (left-justify the field)
# alternate format: add leading 0 for octal (%#o), 0x for hex (%#x);
0X for hex (%#X); suppress 0x for %p (%#p);
print a raw (backquoted) string if possible for %q (%#q);
write e.g. U+0078 'x' if the character is printable for %U (%#U).
' ' (space) leave a space for elided sign in numbers (% d);
put spaces between bytes printing strings or slices in hex (% x, % X)
0 pad with leading zeros rather than spaces
For each Printf-like function, there is also a Print function
that takes no format and is equivalent to saying %v for every
operand. Another variant Println inserts blanks between
operands and appends a newline.
Regardless of the verb, if an operand is an interface value,
the internal concrete value is used, not the interface itself.
var i interface{} = 23
fmt.Printf("%v\n", i)
will print 23.
If an operand implements interface Formatter, that interface
can be used for fine control of formatting.
If an operand implements method String() string that method
will be used to convert the object to a string, which will then
be formatted as required by the verb (if any). To avoid
recursion in cases such as
type X int
func (x X) String() string { return Sprintf("%d", x) }
cast the value before recurring:
func (x X) String() string { return Sprintf("%d", int(x)) }
Format errors:
If an invalid argument is given for a verb, such as providing
a string to %d, the generated string will contain a
description of the problem, as in these examples:
Wrong type or unknown verb: %!verb(type=value)
Printf("%d", hi): %!d(string=hi)
Too many arguments: %!(EXTRA type=value)
Printf("hi", "guys"): hi%!(EXTRA string=guys)
Too few arguments: %!verb(MISSING)
Printf("hi%d"): hi %!d(MISSING)
Non-int for width or precision: %!(BADWIDTH) or %!(BADPREC)
Printf("%*s", 4.5, "hi"): %!(BADWIDTH)hi
Printf("%.*s", 4.5, "hi"): %!(BADPREC)hi
All errors begin with the string "%!" followed sometimes
by a single character (the verb) and end with a parenthesized
An analogous set of functions scans formatted text to yield
values. Scan, Scanf and Scanln read from os.Stdin; Fscan,
Fscanf and Fscanln read from a specified os.Reader; Sscan,
Sscanf and Sscanln read from an argument string. Scanln,
Fscanln and Sscanln stop scanning at a newline and require that
the items be followed by one; Sscanf, Fscanf and Sscanf require
newlines in the input to match newlines in the format; the other
routines treat newlines as spaces.
Scanf, Fscanf, and Sscanf parse the arguments according to a
format string, analogous to that of Printf. For example, %x
will scan an integer as a hexadecimal number, and %v will scan
the default representation format for the value.
The formats behave analogously to those of Printf with the
following exceptions:
%p is not implemented
%T is not implemented
%e %E %f %F %g %G are all equivalent and scan any floating point or complex value
%s and %v on strings scan a space-delimited token
The familiar base-setting prefixes 0 (octal) and 0x
(hexadecimal) are accepted when scanning integers without a
format or with the %v verb.
Width is interpreted in the input text (%5s means at most
five runes of input will be read to scan a string) but there
is no syntax for scanning with a precision (no %5.2f, just
When scanning with a format, all non-empty runs of space
characters (except newline) are equivalent to a single
space in both the format and the input. With that proviso,
text in the format string must match the input text; scanning
stops if it does not, with the return value of the function
indicating the number of arguments scanned.
In all the scanning functions, if an operand implements method
Scan (that is, it implements the Scanner interface) that
method will be used to scan the text for that operand. Also,
if the number of arguments scanned is less than the number of
arguments provided, an error is returned.
All arguments to be scanned must be either pointers to basic
types or implementations of the Scanner interface.
Note: Fscan etc. can read one character (rune) past the input
they return, which means that a loop calling a scan routine
may skip some of the input. This is usually a problem only
when there is no space between input values. If the reader
provided to Fscan implements ReadRune, that method will be used
to read characters. If the reader also implements UnreadRune,
that method will be used to save the character and successive
calls will not lose data. To attach ReadRune and UnreadRune
methods to a reader without that capability, use
package fmt