fmt_doc

package fmt // import "fmt"

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.


Printing

The verbs:

General:

    %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

Boolean:

    %t	the word true or false

Integer:

    %b	base 2
    %c	the character represented by the corresponding Unicode code point
    %d	base 10
    %o	base 8
    %O	base 8 with 0o prefix
    %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.FormatFloat with the 'b' format,
    	e.g. -123456p-78
    %e	scientific notation, e.g. -1.234456e+78
    %E	scientific notation, e.g. -1.234456E+78
    %f	decimal point but no exponent, e.g. 123.456
    %F	synonym for %f
    %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

Slice:

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

Pointer:

    %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:

4. 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).

5. 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".

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
    Invalid or invalid use of argument index: %!(BADINDEX)
    	Printf("%*[2]d", 7):       %!d(BADINDEX)
    	Printf("%.[2]d", 7):       %!d(BADINDEX)

All errors begin with the string "%!" followed sometimes by a single
character (the verb) and end with a parenthesized description.

If an Error or String method triggers a panic when called by a print
routine, the fmt package reformats the error message from the panic,
decorating it with an indication that it came through the fmt package. For
example, if a String method calls panic("bad"), the resulting formatted
message will look like

    %!s(PANIC=bad)

The %!s just shows the print verb in use when the failure occurred. If the
panic is caused by a nil receiver to an Error or String method, however, the
output is the undecorated string, "<nil>".


Scanning

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 io.Reader; Sscan, Sscanf and Sscanln read from an argument string.

Scan, Fscan, Sscan treat newlines in the input as spaces.

Scanln, Fscanln and Sscanln stop scanning at a newline and require that the
items be followed by a newline or EOF.

Scanf, Fscanf, and Sscanf parse the arguments according to a format string,
analogous to that of Printf. In the text that follows, 'space' means any
Unicode whitespace character except newline.

In the format string, a verb introduced by the % character consumes and
parses input; these verbs are described in more detail below. A character
other than %, space, or newline in the format consumes exactly that input
character, which must be present. A newline with zero or more spaces before
it in the format string consumes zero or more spaces in the input followed
by a single newline or the end of the input. A space following a newline in
the format string consumes zero or more spaces in the input. Otherwise, any
run of one or more spaces in the format string consumes as many spaces as
possible in the input. Unless the run of spaces in the format string appears
adjacent to a newline, the run must consume at least one space from the
input or find the end of the input.

The handling of spaces and newlines differs from that of C's scanf family:
in C, newlines are treated as any other space, and it is never an error when
a run of spaces in the format string finds no spaces to consume in the
input.

The verbs behave analogously to those 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 Printf verbs %p and %T and the
flags # and + are not implemented. For floating-point and complex values,
all valid formatting verbs (%b %e %E %f %F %g %G %x %X and %v) are
equivalent and accept both decimal and hexadecimal notation (for example:
"2.3e+7", "0x4.5p-8") and digit-separating underscores (for example:
"3.14159_26535_89793").

Input processed by verbs is implicitly space-delimited: the implementation
of every verb except %c starts by discarding leading spaces from the
remaining input, and the %s verb (and %v reading into a string) stops
consuming input at the first space or newline character.

The familiar base-setting prefixes 0b (binary), 0o and 0 (octal), and 0x
(hexadecimal) are accepted when scanning integers without a format or with
the %v verb, as are digit-separating underscores.

Width is interpreted in the input text but there is no syntax for scanning
with a precision (no %5.2f, just %5f). If width is provided, it applies
after leading spaces are trimmed and specifies the maximum number of runes
to read to satisfy the verb. For example,

    Sscanf(" 1234567 ", "%5s%d", &s, &i)

will set s to "12345" and i to 67 while

    Sscanf(" 12 34 567 ", "%5s%d", &s, &i)

will set s to "12" and i to 34.

In all the scanning functions, a carriage return followed immediately by a
newline is treated as a plain newline (\r\n means the same as \n).

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.

Like Scanf and Fscanf, Sscanf need not consume its entire input. There is no
way to recover how much of the input string Sscanf used.

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 bufio.NewReader.

func Errorf(format string, a ...interface{}) error
func Fprint(w io.Writer, a ...interface{}) (n int, err error)
func Fprintf(w io.Writer, format string, a ...interface{}) (n int, err error)
func Fprintln(w io.Writer, a ...interface{}) (n int, err error)
func Fscan(r io.Reader, a ...interface{}) (n int, err error)
func Fscanf(r io.Reader, format string, a ...interface{}) (n int, err error)
func Fscanln(r io.Reader, a ...interface{}) (n int, err error)
func Print(a ...interface{}) (n int, err error)
func Printf(format string, a ...interface{}) (n int, err error)
func Println(a ...interface{}) (n int, err error)
func Scan(a ...interface{}) (n int, err error)
func Scanf(format string, a ...interface{}) (n int, err error)
func Scanln(a ...interface{}) (n int, err error)
func Sprint(a ...interface{}) string
func Sprintf(format string, a ...interface{}) string
func Sprintln(a ...interface{}) string
func Sscan(str string, a ...interface{}) (n int, err error)
func Sscanf(str string, format string, a ...interface{}) (n int, err error)
func Sscanln(str string, a ...interface{}) (n int, err error)
type Formatter interface{ ... }
type GoStringer interface{ ... }
type ScanState interface{ ... }
type Scanner interface{ ... }
type State interface{ ... }
type Stringer interface{ ... }