Editing Printf (section)

===Parameter field===

The parameter field is optional. If included, then matching specifiers to values is {{em|not}} sequential. The numeric value {{samp|n}} selects the n-th value parameter. This is a [[POSIX]] extension; not [[C99]].{{needs citation|date=April 2025}}

{{Table alignment}}
{| class="wikitable col1center"
|-
! Text
! Description
|-
| {{tt|''n''$}}
| ''n'' is the index of the value parameter to [[serialization|serialize]] using this format specifier
|}

This field allows for using the same value multiple times in a format string instead of having to pass the value multiple times. If a specifier includes this field, then subsequent specifiers must also.

For example, 
<syntaxhighlight lang="c">
printf("%2$d %2$#x; %1$d %1$#x",16,17);
</syntaxhighlight>
outputs: {{samp|17 0x11; 16 0x10}}

This field is particularly useful for [[Localization (computing)|localizing]] messages to different [[natural language]]s that use different [[word order]]s.

In [[Microsoft Windows API|Windows API]], support for this feature is via a different function, {{code|printf_p}}.