fmt

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 Go to latest Published: Aug 26, 2011 License: BSD-3-Clause Imports: 9 Imported by: 0

Documentation

Overview

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
%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

Pointer: 

%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. Thus: 

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 description.

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 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 %5f).

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 bufio.NewReader.

Index

Constants

This section is empty.

Variables

This section is empty.

Functions

func Errorf 

func Errorf(format string, a ...interface{}) os.Error

Errorf formats according to a format specifier and returns the string converted to an os.ErrorString, which satisfies the os.Error interface.

func Fprint 

func Fprint(w io.Writer, a ...interface{}) (n int, err os.Error)

Fprint formats using the default formats for its operands and writes to w. Spaces are added between operands when neither is a string. It returns the number of bytes written and any write error encountered.

func Fprintf 

func Fprintf(w io.Writer, format string, a ...interface{}) (n int, err os.Error)

Fprintf formats according to a format specifier and writes to w. It returns the number of bytes written and any write error encountered.

func Fprintln 

func Fprintln(w io.Writer, a ...interface{}) (n int, err os.Error)

Fprintln formats using the default formats for its operands and writes to w. Spaces are always added between operands and a newline is appended. It returns the number of bytes written and any write error encountered.

func Fscan 

func Fscan(r io.Reader, a ...interface{}) (n int, err os.Error)

Fscan scans text read from r, storing successive space-separated values into successive arguments. Newlines count as space. It returns the number of items successfully scanned. If that is less than the number of arguments, err will report why.

func Fscanf 

func Fscanf(r io.Reader, format string, a ...interface{}) (n int, err os.Error)

Fscanf scans text read from r, storing successive space-separated values into successive arguments as determined by the format. It returns the number of items successfully parsed.

func Fscanln 

func Fscanln(r io.Reader, a ...interface{}) (n int, err os.Error)

Fscanln is similar to Fscan, but stops scanning at a newline and after the final item there must be a newline or EOF.

func Print 

func Print(a ...interface{}) (n int, err os.Error)

Print formats using the default formats for its operands and writes to standard output. Spaces are added between operands when neither is a string. It returns the number of bytes written and any write error encountered.

func Printf 

func Printf(format string, a ...interface{}) (n int, err os.Error)

Printf formats according to a format specifier and writes to standard output. It returns the number of bytes written and any write error encountered.

func Println 

func Println(a ...interface{}) (n int, err os.Error)

Println formats using the default formats for its operands and writes to standard output. Spaces are always added between operands and a newline is appended. It returns the number of bytes written and any write error encountered.

func Scan 

func Scan(a ...interface{}) (n int, err os.Error)

Scan scans text read from standard input, storing successive space-separated values into successive arguments. Newlines count as space. It returns the number of items successfully scanned. If that is less than the number of arguments, err will report why.

func Scanf 

func Scanf(format string, a ...interface{}) (n int, err os.Error)

Scanf scans text read from standard input, storing successive space-separated values into successive arguments as determined by the format. It returns the number of items successfully scanned.

func Scanln 

func Scanln(a ...interface{}) (n int, err os.Error)

Scanln is similar to Scan, but stops scanning at a newline and after the final item there must be a newline or EOF.

func Sprint 

func Sprint(a ...interface{}) string

Sprint formats using the default formats for its operands and returns the resulting string. Spaces are added between operands when neither is a string.

func Sprintf 

func Sprintf(format string, a ...interface{}) string

Sprintf formats according to a format specifier and returns the resulting string.

func Sprintln 

func Sprintln(a ...interface{}) string

Sprintln formats using the default formats for its operands and returns the resulting string. Spaces are always added between operands and a newline is appended.

func Sscan 

func Sscan(str string, a ...interface{}) (n int, err os.Error)

Sscan scans the argument string, storing successive space-separated values into successive arguments. Newlines count as space. It returns the number of items successfully scanned. If that is less than the number of arguments, err will report why.

func Sscanf 

func Sscanf(str string, format string, a ...interface{}) (n int, err os.Error)

Sscanf scans the argument string, storing successive space-separated values into successive arguments as determined by the format. It returns the number of items successfully parsed.

func Sscanln 

func Sscanln(str string, a ...interface{}) (n int, err os.Error)

Sscanln is similar to Sscan, but stops scanning at a newline and after the final item there must be a newline or EOF.

Types

type Formatter 

type Formatter interface {
	Format(f State, c int)
}

Formatter is the interface implemented by values with a custom formatter. The implementation of Format may call Sprintf or Fprintf(f) etc. to generate its output.

type GoStringer 

type GoStringer interface {
	GoString() string
}

GoStringer is implemented by any value that has a GoString method, which defines the Go syntax for that value. The GoString method is used to print values passed as an operand to a %#v format.

type ScanState 

type ScanState interface {
	// ReadRune reads the next rune (Unicode code point) from the input.
	// If invoked during Scanln, Fscanln, or Sscanln, ReadRune() will
	// return EOF after returning the first '\n' or when reading beyond
	// the specified width.
	ReadRune() (rune int, size int, err os.Error)
	// UnreadRune causes the next call to ReadRune to return the same rune.
	UnreadRune() os.Error
	// SkipSpace skips space in the input. Newlines are treated as space
	// unless the scan operation is Scanln, Fscanln or Sscanln, in which case
	// a newline is treated as EOF.
	SkipSpace()
	// Token skips space in the input if skipSpace is true, then returns the
	// run of Unicode code points c satisfying f(c).  If f is nil,
	// !unicode.IsSpace(c) is used; that is, the token will hold non-space
	// characters.  Newlines are treated as space unless the scan operation
	// is Scanln, Fscanln or Sscanln, in which case a newline is treated as
	// EOF.  The returned slice points to shared data that may be overwritten
	// by the next call to Token, a call to a Scan function using the ScanState
	// as input, or when the calling Scan method returns.
	Token(skipSpace bool, f func(int) bool) (token []byte, err os.Error)
	// Width returns the value of the width option and whether it has been set.
	// The unit is Unicode code points.
	Width() (wid int, ok bool)
	// Because ReadRune is implemented by the interface, Read should never be
	// called by the scanning routines and a valid implementation of
	// ScanState may choose always to return an error from Read.
	Read(buf []byte) (n int, err os.Error)
}

ScanState represents the scanner state passed to custom scanners. Scanners may do rune-at-a-time scanning or ask the ScanState to discover the next space-delimited token.

type Scanner 

type Scanner interface {
	Scan(state ScanState, verb int) os.Error
}

Scanner is implemented by any value that has a Scan method, which scans the input for the representation of a value and stores the result in the receiver, which must be a pointer to be useful. The Scan method is called for any argument to Scan, Scanf, or Scanln that implements it.

type State 

type State interface {
	// Write is the function to call to emit formatted output to be printed.
	Write(b []byte) (ret int, err os.Error)
	// Width returns the value of the width option and whether it has been set.
	Width() (wid int, ok bool)
	// Precision returns the value of the precision option and whether it has been set.
	Precision() (prec int, ok bool)

	// Flag returns whether the flag c, a character, has been set.
	Flag(c int) bool
}

State represents the printer state passed to custom formatters. It provides access to the io.Writer interface plus information about the flags and options for the operand's format specifier.

type Stringer 

type Stringer interface {
	String() string
}

Stringer is implemented by any value that has a String method, which defines the “native” format for that value. The String method is used to print values passed as an operand to a %s or %v format or to an unformatted printer such as Print.

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