Package fmt
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.FormatFloat with the 'b' format, 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 %F synonym for %f %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).
Width is specified by an optional decimal number immediately following 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. (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 most values, width is the minimum number of characters to output, padding the formatted form with spaces if necessary. For strings, precision is the maximum number of characters to output, truncating if necessary.
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 it sets the total number of digits. For example, given 123.45 the format %6.2f prints 123.45 while %.4g prints 123.5. The default precision for %e and %f 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 0 for octal (%#o), 0x for hex (%#x); 0X for hex (%#X); suppress 0x for %p (%#p); for %q, print a raw (backquoted) string if strconv.CanBackquote returns true; 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 an operand implements the Formatter interface, it will be invoked. Formatter provides fine control of formatting.
2. 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:
3. 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).
4. 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.
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)) }
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], arguments n+1, n+2, etc. will be processed 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.
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. Scanln, Fscanln and Sscanln stop scanning at a newline and require that the items be followed by one; Scanf, 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 Flags # and + are not implemented.
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, 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.
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.
