pgfmath does not work

Question

I want to use the pgfmath engine, but the result is not what I expect. In this example, I want both lines to have the same length.

\documentclass{standalone}
\usepackage{tikz}
\usepackage{pgfmath}
\begin{document}
    \begin{tikzpicture}
        \draw[color=blue, very thick] (0,0) -- (2,0);
        \pgfmathparse{ceil(1.7)} % Why is the red line shorter? It should be 2 units in length
        \draw[color=red, very thick] (0,0.5) -- (\pgfmathresult,0.5);
    \end{tikzpicture}
\end{document}

Sorry if this question seems too stupid, but I don't know what's wrong. Thanks for your help.

Answer 1

PGF is used everywhere in TikZ. At the point where you invoke \pgfmathresult, the result of your calculation has already been overwritten by something from the last \draw statement before the point where you used \pgfmathresult.

In order to avoid this problem, you can store the the result right after computing it:

\documentclass{standalone}
\usepackage{tikz}

\begin{document}
\begin{tikzpicture}
  \draw[color=blue, very thick] (0,0) -- (2,0);
  \pgfmathparse{ceil(1.7)}
  \let\mylen\pgfmathresult
  % This would also work, but \let means less work to do
  % for TeX (thanks to egreg for the suggestion!)
  % \edef\mylen{\pgfmathresult}
  \draw[color=red, very thick] (0,0.5) -- (\mylen,0.5);
\end{tikzpicture}
\end{document}

but there is a shortcut for this, which is to use \pgfmathsetmacro:

\documentclass{standalone}
\usepackage{tikz}

\begin{document}
\begin{tikzpicture}
  \draw[color=blue, very thick] (0,0) -- (2,0);
  \pgfmathsetmacro{\mylen}{ceil(1.7)}
  \draw[color=red, very thick] (0,0.5) -- (\mylen,0.5);
\end{tikzpicture}
\end{document}