# I am using algebraic option but why must I specify some constants that are only needed for expressions written in RPN?

Please consider \pstVerb{/#2 \csname#2\endcsname\space def} in the following code snippet.

\newcommand\const[3][3]{%
\edef\next{round(#3}%
\expandafter\FPeval\csname#2\expandafter\endcsname
\expandafter{\next:#1)}%
\pstVerb{/#2 \csname#2\endcsname\space def}%
}

Its purpose is to define a constant for expressions written in RPN notation.

My complete code below

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

\newcommand\const[3][3]{%
\edef\next{round(#3}%
\expandafter\FPeval\csname#2\expandafter\endcsname
\expandafter{\next:#1)}%
\pstVerb{/#2 \csname#2\endcsname\space def}%
}

\const{Vox}{2}
\const{Xo}{1}

\const{Voy}{4}
\const{Yo}{10}

\const{G}{10}

\def\X(#1){Xo+Vox*#1}
\def\Y(#1){Yo+Voy*#1-0.5*G*#1^2}

\const{Tp}{Voy/G}
\const{Toffset}{0.5}

\const{Tf}{2*Tp+Toffset}

\const{Xp}{\X(Tp)}
\const{Yp}{\Y(Tp)}
\const{Xf}{\X(Tf)}
\const{Yf}{\Y(Tf)}

\usepackage{fourier}
\begin{document}
\begin{pspicture}(0,\Yp)(\Xf,\Yf)
\psframe*[linecolor=lightgray](0,\Yf)(\Xo,\Yo)
\psparametricplot[algebraic,linecolor=red,linestyle=dashed]{0}{\Tf}{\X(t)|\Y(t)}
\rput[B]{45}(\Xp,\Yp){\bomb}
\end{pspicture}
\end{document}

use \psparametricplot[algebraic,linecolor=red,linestyle=dashed]{0}{\Tf}{\X(t)|\Y(t)} with algebraic option. As you can see there is no expressions written in RPN notation.

However, if I disable \pstVerb{/#2 \csname#2\endcsname\space def} (by commenting it with %), the compilation successes in latex and dvips phases but fails in ps2pdf phase with

Error: /undefined in Xo
... <other errors go here but omitted for brevity> ....

I am using algebraic option but why must I specify some constants that are only needed for expressions written in RPN?

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