pgffor loop with arithmetic expression

Question

I've grown fond of this xintexpr package for its nice syntax. I'd like to repeat a command M times and another one 5-M = N times. I'm getting an error due to the arithmetic expression of the latter being in the foreach loop. I thought using the \let cs was supposed to compute the expression? I also thought maybe the issue was related to expansion/

MWE below:

\documentclass[11pt,a4paper]{article}

\usepackage{pgffor}
\usepackage{xint}
\usepackage{xintexpr}

\begin{document}

    \newcommand{\M}{3}  
     
    % my attempts below
    \newcommand{\N}{\printnumber{{\xintiexpr5-\M\relax}}}
%   \edef\N{\printnumber{\xintiexpr5-\M\relax}}
%   \newcommand{\Na}{\xintiexpr5-\M\relax}  % intermediate value
%   \let\N\Na % tr
    
    \foreach \m in {1,...,\M}{M}
    \foreach \n in {1,...,\N}{N} % but the math wont work here
\end{document}

Answer 1

You want to do full expansion of the expression:

\documentclass[11pt,a4paper]{article}

\usepackage{pgffor}
\usepackage{xint}
\usepackage{xintexpr}

\begin{document}

\newcommand{\M}{3}

\edef\N{\xintthe\xintiexpr5-\M\relax}

\foreach \m in {1,...,\M}{M}
\foreach \n in {1,...,\N}{N}

\end{document}

This will print

MMM NN

With a different approach:

\documentclass[11pt,a4paper]{article}

\usepackage{pgffor}
\usepackage{xfp}

\begin{document}

\newcommand{\M}{3}

\foreach \m in {1,...,\M}{M}
\foreach \n in {1,...,\inteval{5-\M}}{N}

\end{document}