xintlagrange

\documentclass{article}
\usepackage{xintexpr,pgffor,listofitems}

\NewDocumentCommand\CreerPolynomeLagrange{ m }{%
    \setsepchar{§/,}%
    \readlist*\lagrangepoints{#1}%
    0%
    \foreach \i in {1,...,\lagrangepointslen}{%
        +\lagrangepoints[\i,2]%
        \foreach \j in {1,...,\lagrangepointslen}{%
            \xintifboolexpr{\i == \j}%
            {}%
            {*(x-(\lagrangepoints[\j,1]))/((\lagrangepoints[\i,1])-(\lagrangepoints[\j,1]))}%
        }%
    }%
}

\begin{document}

\CreerPolynomeLagrange{1,2§3,4§10,1} %l'affichage semble ok

\CreerPolynomeLagrange{1,2§3,4§6,-3§10,2} %idem

%\xintdeffloatfunc test(x) := \CreerPolynomeLagrange{1,2§3,4§10,1};  %ne veut rien savoir, j'ai essayé de stocker le résultat de \CreerPolynomeLagrange dans une macro et de faire \xintdeffloatfunc test(x) := \mymacro; mais idem...

\end{document}