rjohnson@vela.acs.oakland.edu (R o d Johnson) (03/14/91)
This is undoubtedly a question with a simple answer that I just can't
seem to get. I want to have a macro (for use in LaTeX documents) that
can have three arguments, two of which can be null. It's invoked like
this:
\prt{arg1}{arg2}{arg3}
but could also look like any of the following:
\prt{}{arg2}{arg3}
\prt{arg1}{arg2}{}
\prt{}{arg2}{}
(arg2 will always be present). I want to define an new \if (I guess)
that would allow me to define a macro like the following:
\def\part#1#2#3{% % or should there be braces around
[some code] % the arguments in the definition?
%
\ifnull{#1} \relax \else % I guess it would be simpler to have
[some more code] % an \ifnotnull to do this
\fi
%
\ifnull{#3} \relax \else
[some more code]
\fi
}
I just can't figure out how to come up with an \ifnull or \ifnotnull.
Any suggestions? Thanks in advance.
--
Rod Johnson * rjohnson@vela.acs.oakland.edu * (313) 650 2315
"Life has a throat" --Peter Blegvadgvr@cs.brown.edu (George V. Reilly) (03/14/91)
In article <5396@vela.acs.oakland.edu> rjohnson@vela.acs.oakland.edu (R o d Johnson) writes: | | This is undoubtedly a question with a simple answer that I just can't | seem to get. I want to have a macro (for use in LaTeX documents) that | can have three arguments, two of which can be null. It's invoked like | this: | | \prt{arg1}{arg2}{arg3} | | but could also look like any of the following: | | \prt{}{arg2}{arg3} | \prt{arg1}{arg2}{} | \prt{}{arg2}{} | | (arg2 will always be present). I want to define an new \if (I guess) | that would allow me to define a macro like the following: | | \def\part#1#2#3{% % or should there be braces around | [some code] % the arguments in the definition? | % | \ifnull{#1} \relax \else % I guess it would be simpler to have | [some more code] % an \ifnotnull to do this | \fi | % | \ifnull{#3} \relax \else | [some more code] | \fi | } | | I just can't figure out how to come up with an \ifnull or \ifnotnull. | Any suggestions? Thanks in advance. Try this: \def\part#1#2#3{% \def\first{#1} % Or something with more mnemonic value \def\third{#1} [some code] % \ifx\first\empty % do nothing \else [some more code] \fi [etc.] } You might find it more convenient to define \part using strings to delimit the three arguments, e.g.: \def\part #1:#2:#3!{%... so you can say \part :arg2:! or \part :arg2:arg3! or \part arg1:arg2:! or \part arg1:arg2:arg3! or you can use multicharacter strings instead of the `:'s and the `!', of course. ________________ George V. Reilly `LiveLong&Fester' gvr@cs.brown.edu +1 (401) 863-7684 uunet!brunix!gvr gvr@browncs.bitnet Box 1910, Brown U, Prov, RI 02912
em@dce.ie (Eamonn McManus) (03/15/91)
gvr@cs.brown.edu (George V. Reilly) writes: >\def\part#1#2#3{% > \def\first{#1} % Or something with more mnemonic value > \def\third{#1} > [some code] > % > \ifx\first\empty > % do nothing > \else > [some more code] > \fi Another way of testing for null arguments is exemplified by this macro: \def\showempty#1{\message{\ifx\relax#1\relax empty\else not empty\fi}} This has the advantage that it can be done entirely in TeX's mouth, whereas the \def's in Geo's solution cannot. It does however fail badly if #1 begins with \relax (e.g., \showempty{\relax...}). I have the feeling that there is a better solution. , Eamonn
rjohnson@vela.acs.oakland.edu (R o d Johnson) (03/15/91)
Thanks to everyone that sent help on this, especially Raymond Chen,
Richard Hughey (who contributed an idea unlike any other) and Bill
Mitchell, whose version I actually used.
Incidentally, Raymond's solution was
\def\ifnull#1{\def\@tempa{#1}\ifx\@tempa\empty}
which doesn't work. I'm still not sure why. If instead we use
\def\ifnull#1{\def\@tempa{#1}\def\@tempb{}\ifx\@tempa\@tempb}
it works fine. Since \@tempb is defined the same way as \empty I'm
puzzled by this. Can anyone confirm this (mis)behavior or explain it?
--
Rod Johnson * rjohnson@vela.acs.oakland.edu * (313) 650 2315
"House, bridge, well, gate, jug, olive tree, window" --Rilkejeffrey@cs.chalmers.se (Alan Jeffrey) (03/15/91)
In article <5396@vela.acs.oakland.edu> rjohnson@vela.acs.oakland.edu (R o d Johnson) writes: [...] >I just can't figure out how to come up with an \ifnull or \ifnotnull. >Any suggestions? Thanks in advance. One simple solution is to test \def\unlikely{This is rather unlikely} \def\ifnull#1{\ifx\unlikely#1\unlikely} Then \ifnull{} A \else B \fi = \ifx\unlikely\unlikely A \else B \fi = A \ifnull{xyz} A \else B \fi = \ifx\unlikely xyz\unlikely A \else B \fi = B Actually, this is true if #1 is empty, or #1 begins with \unlikely. So as long as \unlikely is an unlikely control sequence to appear, you're OK. What a hack... Alan. Alan Jeffrey Tel: +46 31 72 10 98 jeffrey@cs.chalmers.se Department of Computer Sciences, Chalmers University, Gothenburg, Sweden