[Tex/LaTex] How to check if a value is not equal using TeX conditionals

conditionalscontextcontext-mkivluatex-core

I have a value defined in a macro, e.g.:

\mymacro{apple}

I need to check if a value does not equal a string, e.g.:

IF #1 NOT EQUAL TO "apple" THEN
    PRINT "It is not a apple, it is #1."
FI

I have tried using this, but it does not work:

\ifx#1="apple"
\else
    It is not a apple, it is #1.
\fi

I have also tried using this, but it also does not work:

\startlua
    if #1 ~= "apple" then
        context("It is not a apple, it is #1.")
    end
\stoplua

The macro might be set to any value.
If anything other than "apple" appears inside, including TeX commands which do not create output, it should still be considered a negative result.

How can I create a plain TeX or Lua conditional which checks if the value is not equal?

Best Answer

ConTeXt provides a \doif... series of macros to do string comparisons. See the ConTeXt wiki for details. For example, if you want check if #1 is the same as a previously defined macro \fakeapple, then you can use:

\def\checkapple#1%
    {\doifnot\fakeapple{#1}
       {It is not an apple, it is #1}}

(1) Tests for empty token list as input

You can try

\def\temp{#1}\ifx\temp\empty
  <EMPTY>%
\else
  <NON EMPTY>%
\fi

If you know that a token, say \hfuzz, will not appear in #1, then

\ifx\hfuzz#1\hfuzz
  <EMPTY>%
\else
  <NON EMPTY>%
\fi

This, differently from the previous test, is expandable.

The safest test uses e-TeX:

\if\relax\detokenize{#1}\relax
  <EMPTY>%
\else
  <NON EMPTY>%
\fi

Using this last one shouldn't be a problem, but be warned that it doesn't work with "Knuth TeX".

Comments

You should know also that \ifx compares the two tokens that follow it and has no = sign. Similarly, \if compares the two tokens that follow it, but after having done complete expansion.

So, how does the last test work? With \detokenize{#1} the argument is transformed into a sequence of characters of category code 12, none of which is \if-equivalent to \relax. Thus with empty #1 the test would compare \relax with \relax and so return True; with #1 non empty, say abc, the code would be

\if\relax abc\relax<EMPTY>\else<NOT EMPTY>\fi

and the comparison between \relax and a returns False, so only the <NOT EMPTY> code would remain.

(2) Test for no printed output

In order to test for no printed output you can't have an expandable test:

\setbox0=\hbox{#1\unskip}\ifdim\wd0=0pt
  <EMPTY>%
\else
  <NOT EMPTY>%
\fi

This assumes that #1 doesn't contain vertical material, such as \vfill. Remove the \unskip if also "space output" should be considered. However, this can go wrong when \mymacro{\hskip1pt\hskip1pt} is called; it mostly depends on what you expect to go in the argument how to cope with limit cases.

If e-TeX is used, one can define a \foreverunspace macro that will remove all trailing spaces (assuming no whatsit appears):

\def\foreverunspace{%
  \ifnum\lastnodetype=11
    \unskip\foreverunspace
  \else
    \ifnum\lastnodetype=12
      \unkern\foreverunspace
    \else
      \ifnum\lastnodetype=13
        \unpenalty\foreverunspace
      \fi
    \fi
  \fi
}

and so the test can be

\setbox0=\hbox{#1\foreverunspace}\ifdim\wd0=0pt
  <EMPTY>%
\else
  <NOT EMPTY>%
\fi

that would remove any trailing combination of glues, kerns and penalties.

[Tex/LaTex] How to create a conditional which checks if one or another condition is true in plain TeX

\if\ifnum#1>\variable T\else\ifnum#1=0 T\else F\fi\fi T%
  TRUE
\else
  FALSE
\fi

\if expands tokens until two non-expandable tokens remain. If one of the conditions is true, then the expansion is T, otherwise F. This is compared with the last T in the line.

Advantage: TRUE is not duplicated as in

\ifnum#1>\variable
  TRUE
\else
  \ifnum#1=0 %
    TRUE
  \else
    FALSE
  \fi
\fi

Advantage: All number parsings are stopped by well defined tokens. Example: The token T stops the scanning of the number \variable. If the TRUE part starts with a number 123abc and \variable is defined as 5
```
\def\variable{5}
\ifnum#1>\variable
  123abc
\fi
```
Then #1 must be greater than 5123!

Depending on the definition of \variable the evaluation of \variable as number might skip a first space of the TRUE part.
Advantage: The conditionals are well matched, thus this construct can also be used inside other conditionals. If TeX skips a conditional branch it only checks the command tokens for real \if... primitives, \else, \fi, and \or (for \ifcase).

For example with a macro \ifOR the following is not well matched:
```
\ifOR{\ifnum#1>\variable}{\ifnum#1=0}
  TRUE
\else
  FALSE
\fi
```
\ifOR does not count as macro, but there are two \ifnum, but only one \fi.
Disadvantage: The \if condition is not easy to read.

Best Answer

Related Solutions

[Tex/LaTex] How to check if a macro value is empty or will not create text with plain TeX conditionals

(1) Tests for empty token list as input

Comments

(2) Test for no printed output

[Tex/LaTex] How to create a conditional which checks if one or another condition is true in plain TeX

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