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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(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')
ifelse(foo, bar, `true', `false')
ifelse(foo, foo, `true', `false')

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')

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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