# Referring to tikz matrix coordinates using pgf mathematics operations

I would like to use pgf's math operations to refer to matrix coodinates in tikz. This works for the second coordinate, but apparently miraculously:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{matrix,arrows}
\begin{document}
$$\begin{tikzpicture} [description/.style={fill=white,inner sep=2pt},baseline=(current bounding box.center)] \matrix (m) [matrix of math nodes, row sep=1em, column sep=1.5em, text height=1.5ex, text depth=0.25ex] { 0 & \Omega^k_X & Z k k & Z k {k-1} & \cdots & Z k 1 & Z k 0 & 0\phantom .\\ 0 & \Lambda^k \mathcal E^\vee & Z k k & Z k {k-1} & \cdots & Z k 1 & Z k 0 & 0.\\ }; \foreach \a in {2,3,...,8} { \pgfmathsetmacro{\myresult}{\a-1} \draw[->] (m-1-\myresult) -- (m-1-\a); \draw[->] (m-2-\myresult) -- (m-2-\a); }; \end{tikzpicture}$$
\end{document}


The following, however, fails spectacularly.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{matrix,arrows}
\begin{document}
$$\begin{tikzpicture} [description/.style={fill=white,inner sep=2pt},baseline=(current bounding box.center)] \matrix (m) [matrix of math nodes, row sep=1em, column sep=1.5em, text height=1.5ex, text depth=0.25ex] { 0 \\ \Omega^k_X \\ Z k k \\ Z k {k-1} \\ cdots \\ Z k 1 \\ Z k 0 \\ }; \foreach \a in {2,3,...,7} { \pgfmathsetmacro{\myresult}{\a-1} \draw[->] (m-\myresult-1) -- (m-\a-1); }; \end{tikzpicture}$$
\end{document}


Apparently, this is because \myresult is 1.0, 2.0, etc, and tikz does not like. An otherwise fruitless google search turned up \pgfmathfloatint, but tikz barfs on \usetikzlibrary{fpu}. Is there any way to turn 1.0 into 1?

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You should post always complete, compilable examples so that people can simple cut&paste the code and run it for themselves. Otherwise people have to guess which packages you are using. This is especially true for TikZ which has many libraries. – Martin Scharrer Mar 30 '11 at 20:03
Will do, thanks for the advice. – Josh Guffin Mar 30 '11 at 20:17

There is \pgfmathtruncatemacro which works like \pgfmathsetmacro but truncates the result to an integer.
BTW: The reason why m-1-\myresult works but m-\myresult-1 isn't is because the first one will expand to m-1-2.0 which is iterpreted as node m-1-2 with anchor 0 (the . is a node.anchor separator). The 0 and any other number is taken as an angle, i.e. 0 degree (see also my answer to How to draw lines between nodes that point at the node center, but stop at the nodes edge). So it kind of works.
The second form m-\myresult-1 will expand to m-2.0-1 which is taken as node m-2 and anchor 0-1, where both most likely don't exit. The node might be, but there is no anchor with that name.
That's a useful command to know about. I've been using \pgfmathparse{int(... expression ...)}\edef\macro{\pgfmathresult} to accomplish the same end, but from now on I'll use this. – Loop Space Mar 30 '11 at 20:51
@Andrew: :-) I used to have some #1.#2{#1} macro to truncate it. I tried int before and that wasn't good enough as well. Assigning it to a counter didn't removed the fraction... – Martin Scharrer Mar 30 '11 at 21:16