[Tex/LaTex] Why is this let expression not working in TikZ (calculating a midpoint)

calculationstikz-pgf

I have a rectangle in TikZ and the top left and bottom right corners are called (topleft) and (bottomright) respectively. I wanted to have a label midway down the left hand side of the rectangle, so I tried the following, both of which failed with cryptic error messages:

\draw let \p1 = (topleft), \p2 = (bottomright) 
in (\x1,\pgfmathparse{0.5*(\y1 + \y2)}\pgfmathresult) node[right]{6 metres};

and

\draw let \p1 = (topleft), \p2 = (bottomright) 
in (\x1,{0.5*(\y1 + \y2)}) node[right]{6 metres};

I got the desired result in a roundabout way:

\draw let \p1 = (topleft), \p2 = (bottomright) 
in ($(\x1,\y1)!.5!(\x1,\y2)$) node[right]{6 metres};

But why did the first two attempts not work?

EDIT (in response to request for more detail):
If it helps, you can assume that the context of the command above is something like:

\usetikzlibrary{calc}
\begin{tikzpicture}
\draw (0,10) coordinate (topleft) rectangle (6,0) coordinate (bottomright);
%% relevant let statement would go just below here
\end{tikzpicture}

but I do not want a solution that uses the knowledge that the coordinates are (0,10) and (6,0).

Best Answer

Working with \pgfmathparse inside a path

If you need to do a calculation inside a path, I recommend to suspend this path, do the calculation, then resume the path. That "escaping" could be done by using the \pgfextra{code} macro executing code.

In your example, \pgfextra may contain the \pgfmathparse calculation, \pgfmathresult may be used later on in the path.

Here's the modification for your first example:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{calc}
\begin{document}
\begin{tikzpicture}
\draw (0,10) coordinate (topleft) rectangle (6,0) coordinate (bottomright);
\draw let \p1 = (topleft), \p2 = (bottomright) 
  in \pgfextra{\pgfmathparse{0.5*(\y1 + \y2)}}
    (\x1,\pgfmathresult pt) node[right]{6 metres};
\end{tikzpicture}
\end{document}

Output:

alt text

This advice is for the case that you need to do more complex calculation not easily done by simple expressions or perhaps intersections.

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