[pstricks] Making Direction Fields; also coordinates as functions inside loops

[Denis Girou]

>>>>> "Michael.Sharpe" == Michael J Sharpe <msharpe at euclid.ucsd.edu> writes:

    Michael.Sharpe> Here's another way making use of the ability of \SpecialCoor pointed 
    Michael.Sharpe> out by Denis to do the picture using \pstcustom.

  This is clearly the best solution that we have (it allow also to use the
straightforward way to set the arrows). I can only suggest to parametrize
the direction of the vector field for greater flexibility.

\documentclass[a4paper]{article}

\usepackage{pst-plot}

% From Michael Sharpe

\SpecialCoor

\def\drawseg#1#2{%
\pscustom{%
  \translate(#1,#2)
  \rotate{(\dirseg{#1}{#2})}    % Vector in direction of field at x=#1,y=#2
\psline(-.25,0)(.25,0)}}

\def\dirseg#1#2{-#2,#1}         % Default direction field ($y'=-x/y$)

\pagestyle{empty}

\begin{document}

\begin{pspicture}(-6,-6)(6,6)
  \psaxes{<->}(0,0)(-6,-6)(6,6)
  \multido{\n=-5+1}{11}{%
    \multido{\N=-5+1}{11}{%
      \ifnum\N=0
        \ifnum\n=0
        \else
          \drawseg{\n}{\N}
        \fi
      \else
        \drawseg{\n}{\N}
      \fi}}
\end{pspicture}

\def\dirseg#1#2{#2,#1}          % Direction field example ($y'=x/y$)

\begin{pspicture}(-6,-6)(6,6)
  \psaxes{<->}(0,0)(-6,-6)(6,6)
  \psset{arrows=->}
  \multido{\n=-5+1}{11}{%
    \multido{\N=-5+1}{11}{%
      \ifnum\N=0
        \ifnum\n=0
        \else
          \drawseg{\n}{\N}
        \fi
      \else
        \drawseg{\n}{\N}
      \fi}}
\end{pspicture}

\end{document}

  Thanks Michael!

D.G.