» 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.

format(spec, values...)

» Examples

> format("Hello, %s!", "Ander")
Hello, Ander!
> format("There are %d lights", 4)
There are 4 lights

Simple format verbs like %s and %d behave similarly to template interpolation syntax, which is often more readable:

> format("Hello, %s!", var.name)
Hello, Valentina!
> "Hello, ${var.name}!"
Hello, Valentina!

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 additional 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.

  • formatdate is a specialized formatting function for human-readable timestamps.
  • formatlist uses the same specification syntax to produce a list of strings.