Calculating marking points exam class

Question

I want to calculate marking points for an exam by multiplying a percentage with the total number of points (e.g., 0.5 * \numpoints{}) to make a grading table. Problem is that before I run the compilation I cannot access \numpoints as a number and get an error. I have been trying to use some sort of if-statement to solve it but that has rendered nothing useful. A working example is below, my current solution is to comment these lines, compile the file and then uncomment and compile again. Is there an easy way to use an if-statement to solve this issue?

\documentclass[addpoints, 12pt]{exam}
\printanswers
\usepackage[utf8]{inputenc}
\usepackage{fp}

\begin{document}
\Large{Exam}\\

% Marking points COMMENT line 1-3 below to run first time
\FPeval{\gradethree}{clip(\numpoints{} * 0.5)}
\FPeval{\gradefour}{clip(\numpoints{} * 0.67)}
\FPeval{\gradefive}{clip(\numpoints{} * 0.83)}

% Marking table COMMENT lines 2-4 below to run first time
\noindent{}Total points: \numpoints{} \\
Grade 3:  \gradethree \space points \\
Grade 4:  \gradefour  \space points \\
Grade 5:  \gradefive  \space points \\

\begin{questions}

\question[2]
Who is Mark Twain?
\begin{solution}
Not your daddy.
\end{solution}

\question[2]
Who is Mark Twain?
\begin{solution}
Not your daddy.
\end{solution}

\question[2]
Who is Mark Twain?
\begin{solution}
Not your daddy.
\end{solution}

\question[2]
Who is Mark Twain?
\begin{solution}
Not your daddy.
\end{solution}

\end{questions}


\end{document}

The output after my comment workaround looks like this (which is exactly what I want to achieve):

Answer 1

The package ifthenx which offers the \ispositiveinteger function does the trick:

\documentclass[addpoints, 12pt]{exam}
\printanswers
\usepackage[utf8]{inputenc}
\usepackage{fp}
\usepackage{ifthenx}

\begin{document}
\noindent\Large{Exam}\\

% calculates marking points for exam grades
\ifthenelse{\ispositiveinteger{\numpoints}}
    {   \FPeval{\gradethree}{clip(\numpoints{} * 0.5)}
        \FPeval{\gradefour}{clip(\numpoints{} * 0.67)}
        \FPeval{\gradefive}{clip(\numpoints{} * 0.83)}  }

% grading table:
\noindent{}Total points: \numpoints{} \\
\ifthenelse{\ispositiveinteger{\numpoints}}
    {   Grade 3:  \gradethree \space points \\
        Grade 4:  \gradefour  \space points \\
        Grade 5:  \gradefive  \space points \\  }
    % Printed if \numpoints is not available.
    {Sorry no grading table \textbf{yet}.}
    
\normalsize
\begin{questions}

\question[2]
Who is Mark Twain?
\begin{solution}
Not your daddy.
\end{solution}

\question[2]
Who is Mark Twain?
\begin{solution}
Not your daddy.
\end{solution}

\question[2]
Who is Mark Twain?
\begin{solution}
Not your daddy.
\end{solution}

\question[2]
Who is Mark Twain?
\begin{solution}
Not your daddy.
\end{solution}

\end{questions}


\end{document}

Gives of course the same output as above when numpoints exists, otherwise prints that the marking table is not available.