Using \def in \psplot

Question

For reasons I cannot explain \def's don't seem to always work in/with \psplot as shown below

\documentclass[english, landscape, a4paper]{slides}

\usepackage{graphics}
\usepackage{graphicx}
\usepackage{babel}
\usepackage{fp}
\usepackage{pst-plot}
\usepackage{pstricks}

\usepackage[active,tightpage]{preview}
\PreviewBorder=4pt
\PreviewEnvironment{pspicture}

\begin{document}
    \begin{center}
        \begin{pspicture*}(-1.5,-1)(1.5,1.5)
            \def\tI{0.0}
            \psplot{0.0}{1.0}{x \tI sub}   % does not work
            % \psplot{0.0}{1.0}{x \tI sub} % works
        \end{pspicture*}
    \end{center}
\end{document}

I compile the above file with xelatex file and get the error

Error: /undefined in 0.0sub
Operand stack:
   --nostringval--   0.0   0.0
Execution stack:
   %interp_exit   .runexec2   --nostringval--   --nostringval--   --nostringval--   2   %s
  --nostringval--   --nostringval--   false   1   %stopped_push   1910   1   3   %oparray_
pop   1893   1   3   %oparray_pop   1787   1   3   %oparray_pop   --nostringval--   %error
ingval--   --nostringval--   --nostringval--   2   %stopped_push   --nostringval--   --nos
...
** WARNING ** Filtering file via command -->mgs.exe -q -dNOPAUSE -dBATCH -sPAPERSIZE=a0 -s
yLevel=1.3 -dAutoFilterGrayImages=false -dGrayImageFilter=/FlateEncode -dAutoFilterColorIm
/FlateEncode -dUseFlateCompression=t

Can someone please suggest a workaround for this problem?

Answer 1

Comments for your MWE:

1. When you want to get a tight, separate PDF output for each pspicture, you can use standalone class instead of preview package to save some keystrokes.
2. Only use graphicx (the modern one) and it loads graphics internally.
3. Remove unnecessary packages.
4. No need to use center environment if you need to generate a tight, separate PDF output for each pspicture.
5. Only use pspicture* to trim the objects outside the canvas. If no objects outside the canvas exist, just use pspicture without stars.

\documentclass[pstricks,border=12pt]{standalone}
\usepackage{pst-plot}

\begin{document}
\begin{pspicture}[showgrid](-1.5,-1)(1.5,1.5)
\def\tI{0.0}
\psplot{0.0}{1.0}{x \tI\space sub}
\end{pspicture}
\end{document}

Now let's discuss your problem. Control words will eat spaces following them. So x \tI sub becomes x 0.0sub that is no meaning. To preserve the spaces, you need to use \space as shown in the code above.

Another solution:

Make sure you put at least one space before the closing } in your macro definition. See the code below.

\documentclass[pstricks,border=12pt]{standalone}
\usepackage{pst-plot}

\begin{document}
\begin{pspicture}[showgrid](-1.5,-1)(1.5,1.5)
\def\tI{0.0 }% can you see there is one space before the closing }?
\psplot{0.0}{1.0}{x \tI sub}
\end{pspicture}
\end{document}

Answer 2

use always a definition with a space at the end:

\def\tI{0.0 }


then you do not need the \space macro for the function.