format Function
format produces a string by formatting a number of other values according
to a specification string. It is similar to the printf function in C, and
other similar functions in other programming languages.
Examples
Simple format verbs like %s and %d behave similarly to template
interpolation syntax, which is often more readable:
The format function is therefore more useful when you use more complex format
specifications, as described in the following section.
Specification Syntax
The specification is a string that includes formatting verbs that are introduced
with the % character. The function call must then have one additional argument
for each verb sequence in the specification. The verbs are matched with
consecutive arguments and formatted as directed, as long as each given argument
is convertible to the type required by the format verb.
The specification may contain the following verbs:
| Verb | Result |
|---|---|
%% | Literal percent sign, consuming no value. |
%v | Default formatting based on the value type, as described below. |
%#v | JSON serialization of the value, as with jsonencode. |
%t | Convert to boolean and produce true or false. |
%b | Convert to integer number and produce binary representation. |
%d | Convert to integer number and produce decimal representation. |
%o | Convert to integer number and produce octal representation. |
%x | Convert to integer number and produce hexadecimal representation with lowercase letters. |
%X | Like %x, but use uppercase letters. |
%e | Convert to number and produce scientific notation, like -1.234456e+78. |
%E | Like %e, but use an uppercase E to introduce the exponent. |
%f | Convert to number and produce decimal fraction notation with no exponent, like 123.456. |
%g | Like %e for large exponents or like %f otherwise. |
%G | Like %E for large exponents or like %f otherwise. |
%s | Convert to string and insert the string's characters. |
%q | Convert to string and produce a JSON quoted string representation. |
When %v is used, one of the following format verbs is chosen based on the value type:
| Type | Verb |
|---|---|
string | %s |
number | %g |
bool | %t |
| any other | %#v |
Null values produce the string null if formatted with %v or %#v, and
cause an error for other verbs.
A width modifier can be included with an optional decimal number immediately
preceding the verb letter, to specify how many characters will be used to
represent the value. Precision can be specified after the (optional) width
with a period (.) followed by a decimal number. If width or precision are
omitted then default values are selected based on the given value. For example:
| Sequence | Result |
|---|---|
%f | Default width and precision. |
%9f | Width 9, default precision. |
%.2f | Default width, precision 2. |
%9.2f | Width 9, precision 2. |
The following additional symbols can be used immediately after the % symbol
to set additoinal flags:
| Symbol | Result |
|---|---|
| space | Leave a space where the sign would be if a number is positive. |
+ | Show the sign of a number even if it is positive. |
- | Pad the width with spaces on the left rather than the right. |
0 | Pad the width with leading zeros rather than spaces. |
By default, % sequences consume successive arguments starting with the first.
Introducing a [n] sequence immediately before the verb letter, where n is a
decimal integer, explicitly chooses a particular value argument by its
one-based index. Subsequent calls without an explicit index will then proceed
with n+1, n+2, etc.
The function produces an error if the format string requests an impossible conversion or access more arguments than are given. An error is produced also for an unsupported format verb.
Related Functions
formatdateis a specialized formatting function for human-readable timestamps.formatlistuses the same specification syntax to produce a list of strings.