[Tex/LaTex] Trouble using pgfmathsetmacro in foreach loop more than once

foreachpgfmathtikz-pgf

I'm trying to draw a series of lines where the x-coordinate is given by a geometric sequence, as in the following example:

\begin{tikzpicture}
    \foreach \i in {0,...,3}
        \pgfmathsetmacro{\result}{7-pow(2,-\i)}
        \draw [red, ultra thick] (\result,3) -- (\result,5);
        \draw [blue, ultra thick] (\result,2.5) -- (\result,3);
        \draw [green, ultra thick] (\result,5) -- (\result,5.5);
\end{tikzpicture}

However, I can't seem to use \result more than once. If I comment out 2 of the \draw lines it works, yet if 2 or more \draw lines are uncommented I receive an error:

! Undefined control sequence.
<argument> \result

Best Answer

There are curly braces around the block inside the foreach missing:

\begin{tikzpicture}
    \foreach \i in {0,...,3} {
        \pgfmathsetmacro{\result}{7-pow(2,-\i)}
        \draw [red, ultra thick] (\result,3) -- (\result,5);
        \draw [blue, ultra thick] (\result,2.5) -- (\result,3);
        \draw [green, ultra thick] (\result,5) -- (\result,5.5);
    }
\end{tikzpicture}

works for me.

Related Solutions

[Tex/LaTex] Increment loop variable in inner foreach loop

You're right, it's a bit of a shame that the expressions aren't evaluated before the loop is started.

Three approaches:

Use the count=\xi expression to make the outer loop counter accessible. In this case, the outer loop only has to run from 2 to 4, while the counter will run from 1 to 3, which happens to be the starting point of the inner loop. This solution only works in this special case, though, as soon as you need the inner loop to be larger by a different value, you'll have to resort to something else.

Alternatively, you can parse the expression at the start of the outer loop using \pgfmathtruncatemacro, which will save the result as an integer, or using the evaluate=\x as \result using <expression> syntax.

Note that the outer loop actually only needs to run from 1 to 3.

\documentclass{article}
\usepackage{tikz}

\begin{document}
\begin{tikzpicture}
\foreach \x [count=\xi] in {2,...,4} {
  \foreach \y  in {\x,...,4} {
    \node at (\xi,\y) {(\xi,\y)};
  }
}
\end{tikzpicture}

\begin{tikzpicture}
\foreach \x in {1,...,3} {
    \pgfmathtruncatemacro\ystart{\x+1}
    \foreach \y in {\ystart,...,4} {
      \node at (\x,\y) {(\x,\y)};
    }
}
\end{tikzpicture}

\begin{tikzpicture}
\foreach \x [evaluate=\x as \ystart using int(\x+1)] in {1,...,3} {
    \foreach \y in {\ystart,...,4} {
      \node at (\x,\y) {(\x,\y)};
    }
}
\end{tikzpicture}
\end{document}

[Tex/LaTex] Using \Alph on \foreach loop argument

\Alph expects as its argument a counter name; in your case LaTeX looks for the inexistent counters named 1, 2 and so on.

A way out is to use the internal command that transforms numbers to letters, that is \@Alph:

\makeatletter
\newcommand{\myAlph}[1]{\expandafter\@Alph#1}
\makeatother

\begin{document}
\begin{tikzpicture}
  \foreach \x in {1,2,3,4,5}
  {
    \draw (\x,1) node{\myAlph{\x}};
  }
\end{tikzpicture}
\end{document}

Alternatively, the more esoteric

\newcommand{\myAlph}[1]{\char\numexpr`A-1+#1\relax}

will do the same, but is quite different from the other one in that the former leaves the letters in the input stream. while the latter leaves the instructions to print the letters.

Best Answer

Related Solutions

[Tex/LaTex] Increment loop variable in inner foreach loop

[Tex/LaTex] Using \Alph on \foreach loop argument

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