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I am looking for a conditional command with the next structure

ifnum x is even DO THIS \else DO THIS \fi

usually ifnum command have the options = < >, but I would like to have the option even and odd.

Andrew Swann
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Gitano
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1 Answers1

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Among its primitive conditionals, TeX has \ifodd:

\ifodd<number><true text>\else<false text>\fi

Be careful though that testing

\ifodd\value{page}

(in LaTeX, it would be \ifodd\pageno in Plain TeX) is not reliable due to the asynchronous page breaking mechanism. Refer to If Then Else for odd page/even page for this specific problem.

On the other hand if you want to test whether another LaTeX counter is even or odd, you can safely test

\ifodd\value{<counter>}

(in Plain TeX, \ifodd\counter; <counter> or \counter stand for the specific name).

Anything that satisfies the syntax for a <number> can follow \ifodd; the usual caveat about expansion in this context apply; a space token following an explicit constant is looked for (with expansion) and ignored.

Community
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egreg
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  • no wonder \ifeven wasn't working... – PatrickT Sep 17 '18 at 17:40
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    @PatrickT You could do \unless\ifodd; but you can't do \def\ifeven{\unless\ifodd} if you plan to nest it in other conditionals. You could do \def\even{TT\fi\unless\ifodd} and use \if\even, though. Using \ifodd seems simpler. ;-) – egreg Sep 17 '18 at 17:54
  • Oh... great! \unless\ifodd could come in handy! One for the toolbox. Thanks egreg. – PatrickT Sep 17 '18 at 18:03