Nested foreach loop in TikZ

Question

I would like to draw some vector fields on a triangle, and I am using nested \foreach loop to do that, however in the inner loop, you can't use the outer loop variable as the numeric range. For e.g. The following code does not work:

\begin{tikzpicture}
{\draw (0,0) -- ++(1,0) -- ++(-1,1) -- cycle;}
\foreach \x in {0,0.1,...,1}
  \foreach \y in {0,0.1,...,1-\x}
  {
  \draw [->] (\x,\y)--(-1 + \x + 2*\y,-\y);
  }
\end{tikzpicture}

Is there a way to do nested loop like this? Or I have to manually go through the loop for each \y which is \x dependent?

Answer 1

I assume that vector should come out from inside of triangle (in your post it is not true). I think you stuck because of imprecise float arithmetic, so you'd better switch to integers:

\documentclass{minimal}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
    \draw (0,0) -- ++(1,0) -- ++(-1,1) -- cycle;

    \foreach \x in {0, ..., 10}
        \foreach \y in {0, ..., \x} {
            \draw[->] (1 - 0.1*\x,0.1*\y) -- (-0.1*\x + 0.2*\y,-0.1*\y);
        }
\end{tikzpicture}
\end{document}

Answer 2

I find that \foreach can be a little fussy over what it accepts as endpoints of a range. Whilst I don't know the details of what does and doesn't work, I do know how to get your example (with the 1-\x) to work: do the computation before the \foreach and save the result in a new macro. Thus:

\documentclass{article}
% http://tex.stackexchange.com/q/16411/86
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\draw (0,0) -- ++(1,0) -- ++(-1,1) -- cycle;
\foreach \x in {0,.1,...,1}
{
  \pgfmathsetmacro{\z}{1-\x}
  \foreach \y in {0,.1,...,\z}
  {
  \draw [->] (\x,\y)--(-1 + \x + 2*\y,-\y);
  }
}
\end{tikzpicture}

\end{document}

(Note that I haven't corrected the other issues: even when \x is 1 then the loop is executed for \y being 0.1. Using integers gets round this as then the range can be specified as 0,...,\z since the default step-size is 1.)