Finding Greatest Common Divisor using LuaLatex

Question

Here is my code to find Greatest Common Divisor of two positive integers.

\documentclass{article}
\usepackage{luacode}

\begin{luacode}
function gcd(a,b)
    if b ~= 0 then
        return gcd(b, a % b)
    else
        return math.abs(a)
    end
end
\end{luacode}
\newcommand\findgcd[2]{\directlua{tex.sprint(gcd(#1,#2))}}

\begin{document}
\findgcd{5,10}
\end{document}

It throws error. The expected output is simply gcd of 5 and 10. Is % sign in lua code causing the error?

Answer 1

You have defined \findgcd with two arguments, but you only supply one:

\findgcd{5}{10}

will work.

If you want the syntax \findgcd{5,10}, then declare it:

\documentclass{article}
\usepackage{luacode}

\begin{luacode}
function gcd(a,b)
    if b ~= 0 then
        return gcd(b, a % b)
    else
        return math.abs(a)
    end
end
\end{luacode}

\newcommand\findgcd[1]{\directlua{tex.sprint(gcd(#1))}}

\begin{document}
\findgcd{5,10}
\end{document}

Here's a simple package code, save as gcd.sty:

\ProvidesPackage{gcd}[2019/07/22]
\RequirePackage{luacode}

\begin{luacode}
function gcd(a,b)
    if b ~= 0 then
        return gcd(b, a % b)
    else
        return math.abs(a)
    end
end
\end{luacode}

\newcommand\findgcd[2]{\directlua{tex.sprint(gcd(#1,#2))}}

\endinput

Now your document, as soon as gcd.sty is in a directory read by the TeX engines, can be

\documentclass{article}
\usepackage{gcd}

\begin{document}
\findgcd{5}{10}
\end{document}