build and use a capitalized CS from a lowercase string

Question

I'm looking for a way to get the following done

I want to have a command

\setcitation[2]

with the following definition

\def\setcitation#1#2{\lowercase{\expandafter\gdef\csname mycommoncitation#1\endcsname}{#2}}

Essentially, what it does is each time it is called with arguments xXx and yyy, it creates a new command

\mycommoncitationxxx

defined as

\yyy

Now, given an input, xxX, I would like to get yyy out. So what has to happen: convert xxX to lowercase xxx, call \mycommoncitationxxx

I tried doing this as follows:

\newcommand\getcitation[1]{%
\lowercase{\csuse{mycommoncitation#1}}
}

And as follows:

\newcommand\getcitation[1]{\lowercase{\expandafter\gdef\csname mycommoncitation#1\endcsname}}

However niether work.

For the first part (the definition of my setcitation command) I based my solution on build a capitalized CS from a lowercase string and to give it a definition

For the second part, I don't find any help yet.

Complete example, that also shows how I want to use these commands:

\documentclass{article} 
\usepackage{etoolbox} 
\newcommand{\setcitation}[2]{\lowercase{\csdef{mycommoncitation#1}}{#2}} 
\newcommand{\getcitation}[1]{\lowercase{\csuse{mycommoncitation#1}}} 

\newcommand{\getcitationifexistsotherwiseinput}[1]{\lowercase{\ifcsdef{mycommoncitation#1}{\getcitation{#1}}{#1}}}


\begin{document} 
\setcitation{Foo}{Bar} 
\getcitation{Foo} 
\mycommoncitationfoo
\getcitationifexistsotherwiseinput{Foo} %My expectation: Bar
\getcitationifexistsotherwiseinput{foo} %my expectation: Bar
\getcitationifexistsotherwiseinput{boo} %my expectation: boo
%Now I want a command that returns 
% \getcitation{FOO} 

\cite{\getcitation{foo}} %my expectaiton: "undefined citation Bar"
\end{document}

here, in the line \cite{...} things start to go wrong.

Answer 1

You need an expandable version of \lowercase:

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\setcitation}{mm}
 {
  \prop_gput:Nxn \g_bartbog_citations_prop { \str_lowercase:n { #1 } } { #2 }
 }
\DeclareExpandableDocumentCommand{\getcitation}{m}
 {
  \prop_item:Nf \g_bartbog_citations_prop { \str_lowercase:n { #1 } }
 }

\cs_generate_variant:Nn \prop_gput:Nnn { Nx }
\cs_generate_variant:Nn \prop_item:Nn { Nf }
\prop_new:N \g_bartbog_citations_prop
\ExplSyntaxOff

\setcitation{Foo}{Bar} 

\begin{document}
\getcitation{Foo} 
\getcitation{foo} 
\getcitation{FOO} 

\cite{\getcitation{Foo}},
\cite{\getcitation{foo}},
\cite{\getcitation{FOO}}

\begin{thebibliography}{1}

\bibitem{Bar} Whatever

\end{thebibliography}

\end{document}

I used a property list for compactness.

Answer 2

I just posted another answer to Expandably change letter case and use inside \csname, without a package.

That answer can be found at https://tex.stackexchange.com/a/349895/118714 .

That answer contains a routine \UD@ExpandableLowercase which delivers lowercasing of those 26 catcode-11(letter)-character-tokens that correspond to the 26 letters of the latin alphabet within two expansion steps. That routine does not require eTeX or the like extensions.

(!!!Be aware that having TeX read and tokenize input consisting of characters of the latin alphabet usually yields catcode-11(letter)-character tokens while expanding \string, \meaning, \jobmane and the like primitives delivers catcode-12(other)-character-tokens!!!)

As a (probably disliked) side effect that routine does replace pairs of matching catcode-1-character-tokens and catcode-2-character-tokens by pairs of matching curly catcode-1-brace-tokens and curly catcode-2-brace-tokens, i.e. { and }.

I think this should not be a problem with plain TeX and LaTeX2e as with these formats usually the only character with category code 1 is the opening curly brace { and the only character with category code 2 is the closing curly brace }.