# Using foreach to generate computed indices in a plot

I have a table with 8 columns, and I want to generate 4 plots (1 vs 0, 3 vs 2, 5 vs 4, and 7 vs 6). I was going to use foreach to generate the plots using

\begin{tikzpicture}
\begin{axis}
\foreach \x in {0,...,3} {
\addplot table[x index = \x * 2, y index = \x * 2 + 1]\table
}
\end{axis}
\end{tikzpicture}


where \table is a stored table.

But I don't seem to permitted to do arithmetic on the indices. Is there a way to do this ?

Update: not-quite-m WE:

\documentclass{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.7}
\begin{tikzpicture}
-1.5875 -1.5324 -1.4773 -1.4222 -1.3671 -1.312  -1.2569 -1.2018
-1.5875 -1.5324 -1.4773 -1.4222 -1.3671 -1.312  -1.2569 -1.2018
0.013166    0.013156    0.013147    0.013141    0.013137    0.013135    0.013135    0.013139
0.013156    0.013146    0.013138    0.013132    0.013128    0.013127    0.013128    0.013131
\begin{axis}
\foreach \x in {0,...,1} {
\pgfmathtruncatemacro{\Xcol}{\x * 2}
\pgfmathtruncatemacro{\Ycol}{1 + \x * 2}
\addplot[mark=,color=blue] table[x index =\Xcol,y index =\Ycol]\transtable;
}
\end{axis}
\end{tikzpicture}
\end{document}

-
I can't test it since you did not provide a MWE, but you could try \pgfmathtruncatemacro{\XColumn}{\x*2}, and then use x index = XColumn. Similarly for the y index. While code snippets are useful in explanations, it is always best to compose a fully compilable MWE that illustrates the problem including the \documentclass and the appropriate packages so that those trying to help don't have to recreate it. –  Peter Grill Dec 5 '12 at 2:58
I completely understand. it's just that my data source is a transposed table (hence the not-quite-minimal working example) and it took me a while to extract a meaningful snippet for testing. The above should be testable. –  Suresh Dec 5 '12 at 3:13

For this case I would just use simple number expressions otherwise you have to use \pgfmathparse{...}\pgfmathresult pairs and it would be tedious. Instead do the arithmetic using evaluate key and use the resulting macro. There are a few examples in the PGF/TikZ manual :

\documentclass{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.7}
0 1 2 3 4 5 6 7
5 8 6 4 0 6 7 8
}\mytable
\begin{document}
\begin{tikzpicture}
\begin{axis}
\foreach \x in {0,...,3} {