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(groff)Operators in Conditionals

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Operators in Conditionals
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   In `if' and `while' requests, there are several more operators
available:

`e'
`o'
     True if the current page is even or odd numbered (respectively).

`n'
     True if the document is being processed in nroff mode (i.e., the
     `.nroff' command has been issued).

`t'
     True if the document is being processed in troff mode (i.e., the
     `.troff' command has been issued).

`v'
     Always false.

`'XXX'YYY''
     True if the string XXX is equal to the string YYY.  Other
     characters can be used in place of the single quotes; the same set
     of delimiters as for the `\D' escape is used (Note: Escapes).
     `gtroff' formats the strings before being compared:


          .ie "|"\fR|\fP" \
          true
          .el \
          false
              => true

     The resulting motions, character sizes, and fonts have to
     match,(1) (Note: Operators in Conditionals-Footnote-1) and not
     the individual motion, size, and font requests.  In the previous
     example, `|' and `\fR|\fP' both result in a roman `|' character
     with the same point size and at the same location on the page, so
     the strings are equal.  If `.ft I' had been added before the
     `.ie', the result would be "false" because (the first) `|'
     produces an italic `|' rather than a roman one.

`r XXX'
     True if there is a number register named XXX.

`d XXX'
     True if there is a string, macro, diversion, or request named XXX.

`c CH'
     True if there is a character CH available; CH is either an ASCII
     character or a special character (`\(CH' or `\[CH]'); the
     condition is also true if CH has been defined by the `char'
     request.

   Note that these operators can't be combined with other operators like
`:' or `&'; only a leading `!' (without whitespace between the
exclamation mark and the operator) can be used to negate the result.


     .nr xxx 1
     .ie !r xxx \
     true
     .el \
     false
         => false

   A whitespace after `!' always evaluates to zero (this bizarre
behaviour is due to compatibility with UNIX `troff').


     .nr xxx 1
     .ie ! r xxx \
     true
     .el \
     false
         => r xxx true

   It is possible to omit the whitespace before the argument to the
`r', `d', and `c' operators.

   Note: Expressions.