Fill a matrix with multido

I would like to fulfill a matrix with multido (because I will have tons of matrices to write). I would like to do something like this but that does not work

documentclass[11pt]{article}
usepackage{amsmath,multido,ifthen}
begin{document}
$$begin{pMatrix}multido{i=1+1}{13}{%
multido{I=1+1}{7}{%
1ifthenelse{I<7}{&}{\}}}
end{pmatrix}$$
end{document}

If someone has a good idea…