Deleting an element from a comma separated list

Question

In this answer we learnt how to remove the n-th element from a comma-separated list.

How can I remove an element from a list by its value, rather than its position? For example: if my global variable \citylist currently has the value derby,leicester,nottingham, then after executing \removecity{leicester} I would like it to have the value derby,nottingham.

I already have an \addcity macro (which I made by adapting a macro by David Carlisle):

\newcommand\addcity[1]{%
  \ifx\citylist\@empty\else\g@addto@macro\citylist{,}\fi
  \g@addto@macro\citylist{#1}
}

Answer 1

In simple cases, you can use LaTeX's \@removeelement:

\documentclass{article}

\newcommand*{\citylist}{derby,leicester,nottingham}

\makeatletter
\@expandtwoargs\@removeelement{leicester}\citylist\citylist
\makeatother

\typeout{\citylist}

\begin{document}
\end{document}

Result:

derby,nottingham

Simple means, the list is normalized without optional spaces around the comma. The list elements does not contain commas and macros.

A variant with package kvsetkeys:

\documentclass{article}

\usepackage{kvsetkeys}

\makeatletter
\newcommand*{\addcity}[1]{%
  \ifx\citylist\@empty\else\g@addto@macro\citylist{,}\fi
  \g@addto@macro\citylist{#1}
}

\newcommand*{\removecity}[1]{%
  \def\unwanted@city{#1}%
  \edef\citylist{\expandafter}%
  \expandafter\comma@parse\expandafter{\citylist}\@removecity
}
\newcommand*{\@removecity}[1]{%
  \def\param@city{#1}%
  \ifx\param@city\unwanted@city
  \else
    \addcity{#1}%
  \fi
}
\makeatother

\newcommand*{\citylist}{derby,leicester,nottingham}
\removecity{leicester}
\typeout{\citylist}

\begin{document}
\end{document}

Then spaces do not disturb, e.g.:

\newcommand*{\citylist}{derby, leicester, nottingham}

The city names may contain commands and fragile macros, they are not expanded.

With the following definition for \addcity, the city names might even contain commas:

\newcommand*{\addcity}[1]{%
  \ifx\citylist\@empty\else\g@addto@macro\citylist{,}\fi
  \g@addto@macro\citylist{{#1}}
}

Example for a list:

{derby},{leicester},{berlin, west},{berlin, east}

Answer 2

The expl3 suite can manage comma separated lists in many ways.

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\NewDocumentCommand{\createlist}{mm}
 {
  \clist_new:c { g_wickerson_data_#1_clist }
  \clist_gset:cn { g_wickerson_data_#1_clist } { #2 }
 }
\NewDocumentCommand{\addtolist}{mm}
 {
  \clist_gput_right:cn { g_wickerson_data_#1_clist } { #2 }
 }
\NewDocumentCommand{\removefromlist}{mm}
 {
  \clist_gremove_all:cn { g_wickerson_data_#1_clist } { #2 }
 }
\NewDocumentCommand{\purgelist}{m}
 {
  \clist_gremove_duplicates:c { g_wickerson_data_#1_clist }
 }
\NewDocumentCommand{\printlist}{m}
 {
  \clist_use:cnnn { g_wickerson_data_#1_clist } { ,~ } { ,~ } { ,~ }
 }
\cs_generate_variant:Nn \clist_use:Nnnn { c }
\ExplSyntaxOff

%%% working with a specific list without needing two args
\newcommand{\addcity}[1]{\addtolist{city}{#1}}
\newcommand{\removecity}[1]{\removefromlist{city}{#1}}
\newcommand{\printcity}{\printlist{city}}

\createlist{city}{derby,leicester} % initialization

\begin{document}

\addcity{nottingham}

Cities: \printcity

\bigskip

\removecity{leicester}

Cities: \printcity

\end{document}

However, comma lists are not the most efficient data type: sequences are better. And the internal implementation shouldn't be a concern.

The same is achieved in a faster way with

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\NewDocumentCommand{\createlist}{mm}
 {
  \seq_new:c { g_wickerson_data_#1_seq }
  \seq_gset_from_clist:cn { g_wickerson_data_#1_seq } { #2 }
 }
\NewDocumentCommand{\addtolist}{mm}
 {
  \seq_gput_right:cn { g_wickerson_data_#1_seq } { #2 }
 }
\NewDocumentCommand{\removefromlist}{mm}
 {
  \seq_gremove_all:cn { g_wickerson_data_#1_seq } { #2 }
 }
\NewDocumentCommand{\purgelist}{m}
 {
  \seq_gremove_duplicates:c { g_wickerson_data_#1_seq }
 }
\NewDocumentCommand{\printlist}{m}
 {
  \seq_use:cnnn { g_wickerson_data_#1_seq } { ,~ } { ,~ } { ,~ }
 }
\cs_generate_variant:Nn \seq_use:Nnnn { c }
\ExplSyntaxOff

%%% working with a specific list without needing two args
\newcommand{\addcity}[1]{\addtolist{city}{#1}}
\newcommand{\removecity}[1]{\removefromlist{city}{#1}}
\newcommand{\printcity}{\printlist{city}}

\createlist{city}{derby,leicester} % initialize

\begin{document}

\addcity{nottingham}

Cities: \printcity

\bigskip

\removecity{leicester}

Cities: \printcity

\end{document}