How to calculate the variable in foreach with ifnum?

Question

I want to draw some points on a grid with the condition x-y>1, and I write the following code:

\documentclass[tikz]{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
  \foreach\x in{1,...,10}{
    \foreach\y in{1,...,10}{
    \ifnum \x-\y>1
        \shade[ball color=red] (\x,\y) circle (0.2cm);
    \else
        \relax
    \fi
    }
  }
\end{tikzpicture}
\end{docume

The code above can't be compiled successfully.

Answer 1

You need to evaluate the numeric calculation (\numexpr) before testing it as a condition (\ifnum).

\documentclass{article}

\usepackage{tikz}

\begin{document}

\begin{tikzpicture}
  \foreach\x in{1,...,10}{
    \foreach\y in{1,...,10}{
    \ifnum\numexpr\x-\y>1
      \shade[ball color=red] (\x,\y) circle (0.2cm);
    \fi
    }
  }
\end{tikzpicture}

\end{document}

The token > stops the evaluation of \numexpr\x-\y, leaving the evaluated result in the input stream for conditioning against 1.

---

For more complex evaluations, use \fpeval (from xfp/LaTeX3):

%...
\usepackage{xfp}
% ...
\ifnum\fpeval{\x^2+\y^2}<10
% or
\ifnum\fpeval{abs(\x-\y)}<3
%...