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The \numexpr...\relax construction in eTeX allows to evaluate numerical expressions, and it expands tokens fully as it goes.

The \pdfstrcmp{...}{...} construction in pdfTeX lets us compare two lists of tokens after full expansion and conversion to a string (with \detokenize).

Are there specific token lists (parameter-less macros) \foo such that \the\numexpr\foo\relax correctly produces an integer, but \pdfstrcmp{\foo}{} causes a TeX error? It seems that the expansion behaviour is the same in both cases, but one converts its argument to an integer, and the other one to a string.

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\numexpr doesn't fully expand tokens in the same sense of \edef; it stops expanding as soon as it finds something that can't be interpreted as a numexpr; if this token happens to be \relax, it is swallowed. I'd say it's a particular case of what \pdfstrcmp does. – egreg Dec 20 '11 at 8:00
@egreg: true. Note that I put the requirement that \the\numexpr\foo\relax correctly produces an integer. So \def\foo{1+2X\junk} is not eligible for instance. – Bruno Le Floch Dec 20 '11 at 12:38
up vote 9 down vote accepted

I see two cases where \the\numexpr...\relax works, but \pdfstrcmp{}{...} will blow up, excluding the obvious case where ... is replaced by 0\relax\undefined, terminating the \numexpr prematurely.

  1. TeX interprets `\a as a number, without expanding \a. Hence, \the\numexpr`\a\relax expands to 97 (the character code of a), whereas \pdfstrcmp{}{`\a} blows up if \a is not defined.

  2. Using \protected control sequences can also cause trouble, because those are forcefully expanded "from the left" in a \numexpr, but will not be expanded by \pdfstrcmp. Take for instance

    \the\numexpr 0\gob\undefined  \relax

In the case of \numexpr, \gob is expanded and removes the \undefined control sequence. In the second case, however, the \edef-like expansion leaves the \protected control sequence \gob untouched, and goes on to expand \undefined, which is, well, undefined.

The original goal I had was to define a macro which takes in an argument which can be either empty or an integer expression, and evaluates the integer expression or puts a default value in the case of an empty argument. It seemed illogical to perform expansion in the \numexpr case but not for the emptyness test, and I was thinking of testing with \pdfstrcmp{}{...}. That can't work. An uglier but more correct choice is the following:



If the argument to \evaluate is empty or expands to an empty argument, the \numexpr expansion will go through all of it and reach the first \z@, evaluating that to 0 (default value), then stop because \z@ does not make sense in an integer expression there. The auxiliary cleans up.

On the other hand, if the argument to \evaluate is a correct integer expression, it is evaluated, and \numexpr stops expanding when encountering the first \z@, and the cleaning up macro removes both \z@.

I just thought of a better way: "f-expand" (expand fully from the left, stopping at the first non-expandable token, removing it in case it is a space) the argument before testing for emptyness:

\def\evaluate@#1{\the\numexpr\ifcat X\detokenize{#1}X\z@\fi#1\relax}

If the argument is empty or will expand to become empty, \romannumeral-`0#1 expands to nothing, and the test in \evaluate@ is true, which means we insert \z@ (default value). Otherwise #1 is evaluated.

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Good catch about \a. – egreg Dec 22 '11 at 21:12
@egreg Also, I realized now that when the default is 0, I can simply do \the\numexpr#1+\z@\relax. But I don't think that can be adapted to any other default. – Bruno Le Floch Dec 22 '11 at 21:16
If #1 is a correct integer expression, isn't also #1+3 again a correct integer expression? – egreg Dec 22 '11 at 21:25
@egreg yes, but I need {}=> default and {123}=>123, not 123+default. – Bruno Le Floch Dec 22 '11 at 21:57
Bruno: What of \the\numexpr 0\gobb\undefined\relax? – Ahmed Musa Dec 23 '11 at 5:02

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