go 软件包fmt使用与C的printf和scanf类似的功能实现格式化的I / O。


%v	默认格式的值,打印结构时,加号(%+ v)添加字段名称
%#v	该值的Go语法表示形式
%T	值类型的Go语法表示形式
%%	文字百分号;没有任何价值


%t	单词true或false


%b	2进制
%c	相应的Unicode代码点表示的字符
%d	10进制
%o	8进制
%O	以0o为前缀8进制
%q	使用Go语法安全地转义的单引号字符文字。
%x	16进制的a-f小写字母
%X	16进制的A-F大写字母
%U	Unicode格式:U + 1234;与“ U +%04X”相同

浮点和复杂 constituents:

%b	指数为2的幂的十进制科学计数法,
以strconv.FormatFloat的方式使用“ b”格式,
%e	科学记数法-1.234456e + 78
%E	科学记数法-1.234456E + 78
%f	小数点,但没有指数,例如123.456
%F	%f的同义词
%g	%e for large exponents, %f otherwise. Precision is discussed below.
%G	%E for large exponents, %F otherwise
%x	hexadecimal notation (with decimal power of two exponent), e.g. -0x1.23abcp+20
%X	upper-case hexadecimal notation, e.g. -0X1.23ABCP+20
String and slice of bytes (treated equivalently with these verbs):

%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	address of 0th element in base 16 notation, with leading 0x


%p	base 16 notation, with leading 0x
The %b, %d, %o, %x and %X verbs also work with pointers,
formatting the value exactly as if it were an integer.

The default format for %v is:

bool: %t
int, int8 etc.: %d
uint, uint8 etc.: %d, %#x if printed with %#v
float32, complex64, etc: %g
string: %s
chan: %p
pointer: %p

For compound objects, the elements are printed using these rules, recursively, laid out like this:

struct: {field0 field1 ...}
array, slice: [elem0 elem1 ...]
maps: map[key1:value1 key2:value2 ...]
pointer to above: &{}, &[], &map[]

Width is specified by an optional decimal number immediately preceding the verb. If absent, the width is whatever is necessary to represent the value. Precision is specified after the (optional) width by a period followed by a decimal number. If no period is present, a default precision is used. A period with no following number specifies a precision of zero. Examples:

%f default width, default precision
%9f width 9, default precision
%.2f default width, precision 2
%9.2f width 9, precision 2
%9.f width 9, precision 0

Width and precision are measured in units of Unicode code points, that is, runes. (This differs from C's printf where the units are always measured in bytes.) Either or both of the flags may be replaced with the character '*', causing their values to be obtained from the next operand (preceding the one to format), which must be of type int.

For most values, width is the minimum number of runes to output, padding the formatted form with spaces if necessary.

For strings, byte slices and byte arrays, however, precision limits the length of the input to be formatted (not the size of the output), truncating if necessary. Normally it is measured in runes, but for these types when formatted with the %x or %X format it is measured in bytes.

For floating-point values, width sets the minimum width of the field and precision sets the number of places after the decimal, if appropriate, except that for %g/%G precision sets the maximum number of significant digits (trailing zeros are removed). For example, given 12.345 the format %6.3f prints 12.345 while %.3g prints 12.3. The default precision for %e, %f and %#g is 6; for %g it is the smallest number of digits necessary to identify the value uniquely.

For complex numbers, the width and precision apply to the two components independently and the result is parenthesized, so %f applied to 1.2+3.4i produces (1.200000+3.400000i).

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 0b for binary (%#b), 0 for octal (%#o),

0x or 0X for hex (%#x or %#X); suppress 0x for %p (%#p);
for %q, print a raw (backquoted) string if strconv.CanBackquote
returns true;
always print a decimal point for %e, %E, %f, %F, %g and %G;
do not remove trailing zeros for %g and %G;
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 numbers, this moves the padding after the sign

Flags are ignored by verbs that do not expect them. For example there is no alternate decimal format, so %#d and %d behave identically.

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. Thus:

var i interface{} = 23
fmt.Printf("%v\n", i)
will print 23.

Except when printed using the verbs %T and %p, special formatting considerations apply for operands that implement certain interfaces. In order of application:

  1. If the operand is a reflect.Value, the operand is replaced by the concrete value that it holds, and printing continues with the next rule.
  2. If an operand implements the Formatter interface, it will be invoked. Formatter provides fine control of formatting.
  3. If the %v verb is used with the # flag (%#v) and the operand implements the GoStringer interface, that will be invoked.

If the format (which is implicitly %v for Println etc.) is valid for a string (%s %q %v %x %X), the following two rules apply:

  1. If an operand implements the error interface, the Error method will be invoked to convert the object to a string, which will then be formatted as required by the verb (if any).
  2. If an operand implements method String() string, that method will be invoked to convert the object to a string, which will then be formatted as required by the verb (if any).

For compound operands such as slices and structs, the format applies to the elements of each operand, recursively, not to the operand as a whole. Thus %q will quote each element of a slice of strings, and %6.2f will control formatting for each element of a floating-point array.

However, when printing a byte slice with a string-like verb (%s %q %x %X), it is treated identically to a string, as a single item.

To avoid recursion in cases such as

type X string
func (x X) String() string { return Sprintf("<%s>", x) }
convert the value before recurring:

func (x X) String() string { return Sprintf("<%s>", string(x)) }
Infinite recursion can also be triggered by self-referential data structures, such as a slice that contains itself as an element, if that type has a String method. Such pathologies are rare, however, and the package does not protect against them.

When printing a struct, fmt cannot and therefore does not invoke formatting methods such as Error or String on unexported fields.

Explicit argument indexes:

In Printf, Sprintf, and Fprintf, the default behavior is for each formatting verb to format successive arguments passed in the call. However, the notation [n] immediately before the verb indicates that the nth one-indexed argument is to be formatted instead. The same notation before a '*' for a width or precision selects the argument index holding the value. After processing a bracketed expression [n], subsequent verbs will use arguments n+1, n+2, etc. unless otherwise directed.

For example,

fmt.Sprintf("%[2]d %[1]d\n", 11, 22)
will yield "22 11", while

fmt.Sprintf("%[3].[2][1]f", 12.0, 2, 6)
equivalent to

fmt.Sprintf("%6.2f", 12.0)
will yield " 12.00". Because an explicit index affects subsequent verbs, this notation can be used to print the same values multiple times by resetting the index for the first argument to be repeated:

fmt.Sprintf("%d %d %#[1]x %#x", 16, 17)
will yield "16 17 0x10 0x11".