pgfmathparse in foreach, use result as node coordinates

Question

I'm trying to use a counter inside a TikZ picture, and use values resulting on calculations with this counter. Let's say for instance, I want i to be ranging from 0 to 9, j to be i*5 and k to be i*10. I've managed to get this working this way:

\documentclass[tikz]{standalone}
\begin{document}
\begin{tikzpicture}
\foreach \i in {0,...,9}
{
\def \j {\pgfmathparse{\i * 5}\pgfmathresult}
\def \k {\pgfmathparse{\i * 10}\pgfmathresult}
\node at (0, -\i) {\tiny \i,\j,\k};
}
\end{tikzpicture}
\end{document}

This works as I intend to, the output being:

0,0.0,0.0

1,5.0,10.0

2,10.0,20.0

3,15.0,30.0

4,20.0,40.0

5,25.0,50.0

6,30.0,60.0

7,35.0,70.0

8,40.0,80.0

9,45.0,90.0

Now, and this is where it gets tricky, I want to use the secondary counter, let's say j as a coordinate of the nodes:

\documentclass[tikz]{standalone}
\begin{document}
\begin{tikzpicture}
\foreach \i in {0,...,9}
{
\def \j {\pgfmathparse{\i * 5}\pgfmathresult}
\def \k {\pgfmathparse{\i * 10}\pgfmathresult}
\node at (0, \j) {\tiny \i,\j,\k};
}
\end{tikzpicture}
\end{document}

I end up with this error:

! Incomplete \iffalse; all text was ignored after line 9.
<inserted text> 
\fi 
<*> mwe.tex

Why doesn't it work? What would be the proper way to achieve what I want to do?

use \pgfmathsetmacro{\j}{\i*5} etc (untested) – daleif Mar 09 '17 at 15:15 — daleif, Mar 09 '17 at 15:15

score 10 · Accepted Answer · answered Mar 09 '17 at 20:37

Your attempt does not work because the \pgfmathparse{…}\pgfmathresult is not expanded until \j is used within the coordinate to the \node. TikZ, of course, does not expect this, so it fails to parse the coordinate. To fix this, you can expand the macros at definition time.

Here is, I believe, the most direct analogue of your code:

\documentclass[tikz]{standalone}

\begin{document}

\begin{tikzpicture}
  \foreach \i in {0,...,9} {
    \pgfmathparse{\i * 5}\edef\j{\pgfmathresult}
    \pgfmathparse{\i * 10}\edef\k{\pgfmathresult}
    \node at (0, \j) {\tiny \i,\j,\k};
  }
\end{tikzpicture}

\end{document}

This is a common pattern, so \pgfmathsetmacro provides a shortcut:

\documentclass[tikz]{standalone}

\begin{document}

\begin{tikzpicture}
  \foreach \i in {0,...,9} {
    \pgfmathsetmacro{\j}{\i * 5}
    \pgfmathsetmacro{\k}{\i * 10}
    \node at (0, \j) {\tiny \i,\j,\k};
  }
\end{tikzpicture}

\end{document}

Both yield the same result.

All the answers were great but your one does have the explanation I needed. Thanks. — nicoco, Mar 10 '17 at 08:21

score 8 · Answer 2 · answered Mar 09 '17 at 15:15

8

Not entirely sure why you get that, but use evaluate instead.

\documentclass[tikz]{standalone}
\begin{document}
\begin{tikzpicture}
\foreach [evaluate=\i as \j using \i*5,evaluate=\i as \k using \i*10] \i in {0,...,9}
{
\node at (0, \j) {\tiny \i,\j,\k};
}
\end{tikzpicture}
\end{document}

answered Mar 09 '17 at 15:15

Torbjørn T.

206,688

2

You could also have used \node at (0,\i*5) .... – Torbjørn T. Mar 09 '17 at 15:16

score 5 · Answer 3 · answered Mar 09 '17 at 15:36

A solution using \pgfmathparse implicitly.

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{calc}
\begin{document}
\begin{tikzpicture}[y=1mm]
\foreach \i in {0,...,9}
  \draw let \n1 = {5*\i}, \n2 = {10*\i} in
  (0, -\n1) node {\tiny \i,\n1,\n2};
\end{tikzpicture}
\end{document}

pgfmathparse in foreach, use result as node coordinates

3 Answers3

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