A puzzle with TeX \ifnum

[Isaiah Shavitt]

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Thank you for your response.  Actually, I did try 

 	\ifdim\ylen<0pt\pssetlength{\ylen}{-\ylen}\fi

and also 

 	\ifdim\ylen<\z@\pssetlength{\ylen}{-\ylen}\fi

(with the appropriate declarations preceding, and with the correct 
setting for the @ catcode) and neither worked.  When I put in 
\typeout{\the\ylen} after the test, nothing was output and the 
\pssetlength was never executed.  But when I added another
\typeout{\the\ylen} *before* the test, the macro did work.  The 
complete code section that worked is:

\pssetlength{\ylen}{#3}\psaddtolength{\ylen}{-#2}
\typeout{\the\ylen}
\ifdim\ylen<\z@\typeout{\the\ylen}\pssetlength{\ylen}{-\ylen}\fi

(a \newlength{\ylen} declaration had appeared earlier}.  The values 
of \ylen were listed correctly, and the resetting of \ylen occurred.
But if I left out the second line in the above 3-line segment, the 
\pssetlength statement in the third line was never executed.

At 10:34 AM 10/5/99 +0200, Denis Girou wrote:
>>>>>> "Isaiah.Shavitt" == Isaiah Shavitt <shavitt at chemistry.ohio-state.edu> 
>writes:
>
>> Can somebody tell me why
>>
>> 	\ifnum\ylen>0\else\pssetlength{\ylen}{-\ylen}\fi
>>
>> works (forcing \ylen to be positive), while
>>
>> 	\ifnum\ylen<0 \pssetlength{\ylen}{-\ylen}\fi
>>
>> does not?  It does not work even if I put \relax after the <0, 
>> or put the \pssetlength command in braces, or try a few other 
>> variations.  The \pssetlength command is just not executed.
>> Or is there a better way to get the absolute value of \ylen 
>> without using PS code?
>
>  The \ifnum macro allow the comparison of two integers, but here you want to
>compare lengths (or dimensions). You obviously must have declare \ylen with
>\newlength if you use LaTeX or with \newdimen if you use plain.
>
>  So you must use the \ifdim macro and you must compare your \ylen own value
>with a length (dimension), not an integer.
>
>  P.S. If you want to build your own macros inside a personal package,
>you can better use the internal \z@ macro (which require as usual to have
>changed the catcode of @). You can take a look at some parts of the PSTricks
>code itself:
>...
>\newdimen\pslinearc
>...
>\def\psline at iii{%
>\ifdim\pslinearc>\z@
>...
>
>------------------------------------------------------------------------------
>
>\documentclass{article}
>\usepackage{pstricks}
>\newlength{\ylen}
>\pagestyle{empty}
>
>\begin{document}
>
>\typeout{Default value:\the\ylen}
>
>\setlength{\ylen}{-1pt}
>\ifdim\ylen>0pt\else\typeout{Point 1}\pssetlength{\ylen}{-\ylen}\fi
>
>\setlength{\ylen}{-1pt}
>\ifdim\ylen<0pt\typeout{Point 2}\pssetlength{\ylen}{-\ylen}\fi
>
>\end{document}
>
>D.G. 

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Isaiah Shavitt
Professor Emeritus
Department of Chemistry           Telephone: 614-292-1668
The Ohio State University         Fax:       614-292-1685
Columbus, OH 43210-1185           E-mail: shavitt at chemistry.ohio-state.edu
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