# Replace letter of command (control sequence)

Question on commands and control sequences. The following command

\newcommand{\startwiths}[1]{\csname s\expandafter\@gobble\string#1\endcsname}


adds the letter s the beginning of the sequence in the argument. For example, \startwiths{\plot} returns \splot.

I would like to define a similar command that makes the first letter of the original command uppercase on top of adding the initial s. Let us call this new command \startwithsUpper. For example, \startwithsUpper{\plot} returns \sPlot. I am not an expert in these manipulations and any help is very welcome :)

EDIT: \startwiths{\plot} and \startwithsUpper{\plot} are now correct (in the first version I forgot the backslash).

• I'm curious: what's the broader purpose?
– jon
Nov 2, 2016 at 22:07
• the code you post would need input {\plot} not {plot} as you show. which do you need (it is always best to post complete test document to avoid such ambiguities) Nov 2, 2016 at 22:28
• @jon: the purpose is to implement it for some manipulation after this question that was solved by egreg Nov 2, 2016 at 23:48

It's possible in several ways. Here's a (non expandable) one:

\documentclass{article}

\makeatletter
\newcommand{\startwithsUpper}[1]{%
\startwithsUpper@aux#1\relax
}
\def\startwithsUpper@aux#1#2\relax{%
\uppercase{\csname\initial@s #1}#2\endcsname
}
\def\initial@s{s}
\makeatother

\newcommand{\sPlot}{Here it is}

\begin{document}

\startwithsUpper{plot}

\end{document}


Here's an expandable one, with expl3:

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn
\NewExpandableDocumentCommand{\startwithsUpper}{m}
{
% \use:c is essentially \csname...\endcsname
\use:c
{
s % add the initial s
\text_titlecase_first:n { #1 }
}
}
\ExplSyntaxOff

\newcommand{\sPlot}{Here it is}

\begin{document}

\startwithsUpper{plot}

\end{document}


The following (more complicated) version accepts either a string or a control sequence as argument.

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn
\NewExpandableDocumentCommand{\startwithsUpper}{m}
{
\montecarlo_start_with_s_upper:n { #1 }
}

\cs_new:Nn \montecarlo_start_with_s_upper:n
{
\tl_if_single:nTF { #1 }
{
\token_if_cs:NTF #1
{
\__montecarlo_start_with_s_upper:e { \cs_to_str:N #1 }
}
{
\__montecarlo_start_with_s_upper:n { #1 }
}
}
{
\__montecarlo_start_with_s_upper:n { #1 }
}
}
\cs_new:Nn \__montecarlo_start_with_s_upper:n
{
\use:c
{
s
\char_titlecase:N #1
}
}
\cs_generate_variant:Nn \__montecarlo_start_with_s_upper:n { e }
\ExplSyntaxOff

\newcommand{\sPlot}{Here it is}

\begin{document}

X\startwithsUpper{plot}X

X\startwithsUpper{\plot}X

\end{document}

• I've updated here to reflect expl3 changes, but I wonder if \text_titlecase_first:n might now be the best choice. Jan 15, 2020 at 16:35
• @JosephWright I’ll have a look to this one and another Jan 15, 2020 at 16:39

The question is inconsistent, \expandafter\@gobble\string#1 indicates that the argument is a command sequence and the code strips the starting backslash. Then the usage cases are:

\startwiths{\plot} -> \splot
\startwithsUpper{\plot} -> \sPlot


Macro \startwithsUpper can be implemented in an expandable manner by using \@Alph instead of \uppercase for converting a simple ASCII letter to its uppercase version:

\makeatletter
\newcommand*{\startwithsUpper}[1]{%
\csname
s%
\expandafter\expandafter\expandafter\FirstUpper
\expandafter\@gobble\string#1%
\endcsname
}
\def\FirstUpper#1{%
\ifnum#1>Z % case distinction between lowercase and uppercase letter
\@Alph{\numexpr#1-a+1\relax}% Convert lowercase letter
\else
#1% Keep uppercase letter
\fi
}

\makeatother

% Test
\edef\x{%
\unexpanded\expandafter\expandafter\expandafter{%
\startwithsUpper{\plot}%
}%
}
\makeatletter
\typeout{\string\startwithsUpper{\string\plot} -> \detokenize\expandafter{\x}}
\makeatother
\stop


Result:

\startwithsUpper{\plot} -> \sPlot


Here's a LuaLaTeX-based solution.

\documentclass{article}
\usepackage{luacode}
\begin{luacode}
function startwithsUpper ( s )
tex.sprint ( "\\s" .. string.gsub( s , "^%l", string.upper) )
end
\end{luacode}
\newcommand\startwithsUpper[1]{\directlua{startwithsUpper(\luastring{#1})}}

\newcommand\sPlot{Hello World}  % dummy definition of "\sPlot"

\begin{document}

\startwithsUpper{plot}

\end{document}