Apply command on nth character of a word
Assuming "character" means "a single character token in input" (so just ASCII with pdftex but Unicode input with xetex or luatex) then
\documentclass{article}
\usepackage{xcolor}
\begin{document}
\makeatletter
\def\ColorNthChar#1#2{\xColorNthChar{#1}#2\@empty}
\def\xColorNthChar#1#2{\ifnum\ifx\@empty#21\else#1\fi=1 \textcolor{red}{#2}\expandafter\@gobbletwo
\else#2\fi\xColorNthChar{\numexpr#1-1\relax}}
\makeatother
% example of use
\ColorNthChar{3}{classification} \ColorNthChar{23}{examination} \ColorNthChar{8}{examination}
\end{document}
updated version that checks if the word is too short.
A simple implementation in expl3
:
\documentclass{article}
\usepackage{xparse,xcolor}
\ExplSyntaxOn
\NewDocumentCommand{\colornth}{O{red}mm}
{% #1 = color to use, #2 = position, #3 = word
% deliver all characters before the chosen position
\tl_range:nnn { #3 } { 1 } { #2 - 1 }
% deliver the character in the chosen position with the desired color
\textcolor{#1}{ \tl_item:nn { #3 } { #2 } }
% deliver all characters after the chosen position
\tl_range:nnn { #3 } { #2 + 1 } { -1 }
}
\ExplSyntaxOff
\begin{document}
\colornth{3}{examination}
\colornth[blue]{-3}{examination}
\end{document}
With negative numbers you start counting from the end.
Just for fun, how to color differently several characters.
\documentclass{article}
\usepackage{xparse,xcolor}
\ExplSyntaxOn
\NewDocumentCommand{\colornth}{O{red}mm}
{% #1 = color to use, #2 = number, #3 = word
\tl_range:nnn { #3 } { 1 } { #2 - 1 }
\textcolor{#1}{ \tl_item:nn { #3 } { #2 } }
\tl_range:nnn { #3 } { #2 + 1 } { -1 }
}
\NewDocumentCommand{\colorsome}{mm}
{% #1 = list of chars to color, #2 = word
% split the given token list
\seq_set_split:Nnn \l_tmpa_seq { } { #2 }
% with \seq_indexed_map_inline:Nn we have ##1=item number, ##2=item
\seq_indexed_map_inline:Nn \l_tmpa_seq
{
\textcolor{ \int_case:nnF { ##1 } { #1 } { . } }{ ##2 }
}
}
\ExplSyntaxOff
\begin{document}
\colornth{3}{examination}
\colornth[blue]{-3}{examination}
\colorsome{{3}{red}{6}{blue!75!red}}{examination}
\end{document}
The first argument to \colorsome
should be a list of pairs {number}{color}
; since \int_case:nnF
is fully expandable, at the end we will get either the color for a chosen position, or .
which denotes the current color.
\documentclass{article}
\usepackage{xcolor,xparse}
\begin{document}
\ExplSyntaxOn
\NewDocumentCommand\ColorNthChar {mm}
{ \int_zero:N\l_tmpa_int
\tl_map_inline:nn
{
#2
}
{
\int_incr:N\l_tmpa_int
\int_compare:nNnTF {\l_tmpa_int}={#1}
{
\textcolor{red}{##1}
}
{
##1
}
}
}
\ExplSyntaxOff
% example of use
% example of use
\ColorNthChar{3}{examination}
\ColorNthChar{23}{examination}
\ColorNthChar{8}{examination}
\end{document}