How to check for more elements a foreach loop?

Question

I want to check if there are more elements in a foreach loop. How can this be done?

This is an example for loop:

\foreach \p/\q in {a/{1,2,3}, b/{4,5,6}}
    \foreach \r in \q { (\r) }

EDIT#1

An example of code in a programming language for what I want to achieve would be:

a = [3,4,5,6]
for i, val in enumerate(a):
    print(val, sep='', end='')
    if i < len(a)-1:
        print(',', sep='', end='')

Hope that helps to clarify.

Answer 1

You can use the count feature of \foreach:

\documentclass{article}

\usepackage{pgffor}

\begin{document}

\foreach \i [count=\xi] in {a,b,c}
 {
  \ifnum\xi>1 and \fi \i
 }

\foreach \i [count=\xi] in {a}
 {
  \ifnum\xi>1 and \fi \i
 }

\end{document}

Answer 2

You could use a cunning (La)TeX trick:

\documentclass{article}

\usepackage{etoolbox}

\newcommand{\printlist}[2][,]{%
  \def\itemdelim{\def\itemdelim{#1}}% Item delimiter delayed by one cycle
  \renewcommand*{\do}[1]{\itemdelim##1}% How each item is processed
  \docsvlist{#2}}% Process CSV list

\begin{document}

\printlist{a,b,c}

\printlist[ and ]{a,b,c}

\printlist{}

\printlist{a}

\end{document}

The approach is to always print a separator, but the first is only a redefinition of the separator, actually printing nothing.