(m4.info.gz) Ifelse

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 Comparing strings
 =================
 
    The other conditional, `ifelse', is much more powerful.  It can be
 used as a way to introduce a long comment, as an if-else construct, or
 as a multibranch, depending on the number of arguments supplied:
 
      ifelse(COMMENT)
      ifelse(STRING-1, STRING-2, EQUAL, opt NOT-EQUAL)
      ifelse(STRING-1, STRING-2, EQUAL, ...)
 
 Used with only one argument, the `ifelse' simply discards it and
 produces no output.  This is a common `m4' idiom for introducing a
 block comment, as an alternative to repeatedly using `dnl'.  This
 special usage is recognized by GNU `m4', so that in this case, the
 warning about missing arguments is never triggered.
 
    If called with three or four arguments, `ifelse' expands into EQUAL,
 if STRING-1 and STRING-2 are equal (character for character), otherwise
 it expands to NOT-EQUAL.
 
      ifelse(foo, bar, `true')
      =>
      ifelse(foo, foo, `true')
      =>true
      ifelse(foo, bar, `true', `false')
      =>false
      ifelse(foo, foo, `true', `false')
      =>true
 
    However, `ifelse' can take more than four arguments.  If given more
 than four arguments, `ifelse' works like a `case' or `switch' statement
 in traditional programming languages.  If STRING-1 and STRING-2 are
 equal, `ifelse' expands into EQUAL, otherwise the procedure is repeated
 with the first three arguments discarded.  This calls for an example:
 
      ifelse(foo, bar, `third', gnu, gnats, `sixth', `seventh')
      =>seventh
 
    Naturally, the normal case will be slightly more advanced than these
 examples.  A common use of `ifelse' is in macros implementing loops of
 various kinds.
 
    The macro `ifelse' is recognized only with parameters.
 

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