Count number of occurrences of a character in the content of a macro string output

Question

How to generalize this macro to make it print the number of times the string occurs and allow arbitrary separator including blank space? Thanks.

Outputs could be:

0 if FALSE; 1,2,3... number of times it occurs if TRUE.

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\NewDocumentCommand{\ifmember}{mmmm}
 {
  \clist_if_in:onTF { #2 } { #1 } { #3 } { #4 }
 }
\cs_generate_variant:Nn \clist_if_in:nnTF { o }
\ExplSyntaxOff
\begin{document}
\ifmember{3}{3,4}{True}{False} (expected: True)
\ifmember{3}{ 3 ,4}{True}{False} (expected: True)
\newcommand{\foo}{3,4}
\ifmember{3}{\foo}{True}{False} (expected: True)
\renewcommand{\foo}{3, 4}
\ifmember{4}{\foo}{True}{False} (expected: True)
\ifmember{foo}{\foo}{True}{False} (expected: False)
\end{document}

Answer 1

You can map the list and check every item against the given token list, incrementing the counter if they match. At the end, you print the counter's value.

The separator (default a comma) is specified in an optional argument.

\documentclass{article}
%\usepackage{xparse} % no longer needed
\ExplSyntaxOn
\NewDocumentCommand{\countmember}{O{,}mm}
 {
  \krantz_countmember:onn { #3 } { #2 } { #1 }
 }
\seq_new:N \l__krantz_countmember_items_seq
\int_new:N \l__krantz_countmember_count_int
\cs_new_protected:Nn \krantz_countmember:nnn
 {
  \int_zero:N \l__krantz_countmember_count_int
  \seq_set_split:Nnn \l__krantz_countmember_items_seq { #3 } { #1 }
  \seq_map_inline:Nn \l__krantz_countmember_items_seq
   {
    \tl_if_eq:nnT { ##1 } { #2 } { \int_incr:N \l__krantz_countmember_count_int }
   }
  \int_to_arabic:n { \l__krantz_countmember_count_int }
 }
\cs_generate_variant:Nn \krantz_countmember:nnn { o }
\ExplSyntaxOff
\begin{document}
\countmember{3}{3,4} (expected: 1)
\countmember{3}{ 3 ,4} (expected: 1)
\newcommand{\foo}{3,4}
\countmember{3}{\foo} (expected: 1)
\renewcommand{\foo}{3, 4, 4}
\countmember{4}{\foo} (expected: 2)
\countmember{foo}{\foo} (expected: 0)
\countmember[\]{1}{1 \ 2 \ 1 \ 0} (expected: 2)
\countmember[ ]{2}{1 2 1 0} (expected: 1)
\renewcommand{\foo}{1 2 1 0}
\countmember[ ]{2}{\foo} (expected: 1)
\end{document}

It is possible to add a trailing optional argument that should be a control sequence in which the result will be stored for later use.

\documentclass{article}
%\usepackage{xparse} % no longer needed
\ExplSyntaxOn
\NewDocumentCommand{\countmember}{O{,}mmo}
 {
  \krantz_countmember:onn { #3 } { #2 } { #1 }
  \IfNoValueTF { #4 }
   {% just print the count
    \int_to_arabic:n { \l__krantz_countmember_count_int }
   }
   {% store
    \tl_clear_new:N #4
    \tl_set:Nx #4 { \int_to_arabic:n { \l__krantz_countmember_count_int } }
   }
 }
\seq_new:N \l__krantz_countmember_items_seq
\int_new:N \l__krantz_countmember_count_int
\cs_new_protected:Nn \krantz_countmember:nnn
 {
  \int_zero:N \l__krantz_countmember_count_int
  \seq_set_split:Nnn \l__krantz_countmember_items_seq { #3 } { #1 }
  \seq_map_inline:Nn \l__krantz_countmember_items_seq
   {
    \tl_if_eq:nnT { ##1 } { #2 } { \int_incr:N \l__krantz_countmember_count_int }
   }
 }
\cs_generate_variant:Nn \krantz_countmember:nnn { o }
\ExplSyntaxOff
\begin{document}
\countmember{3}{3,4} (expected: 1)
\countmember{3}{ 3 ,4} (expected: 1)
\newcommand{\foo}{3,4}
\countmember{3}{\foo} (expected: 1)
\renewcommand{\foo}{3, 4, 4}
\countmember{4}{\foo} (expected: 2)
\countmember{foo}{\foo} (expected: 0)
\countmember[\]{1}{1 \ 2 \ 1 \ 0} (expected: 2)
\countmember[ ]{2}{1 2 1 0} (expected: 1)
\renewcommand{\foo}{1 2 1 0}
\countmember[ ]{2}{\foo} (expected: 1)
% store in \foo
\countmember{1}{1,2,2,1,1,2}[\foo]
\foo
\end{document}

The output is the same as before, but you will also see “3” at the end.