Info Catalog groff: Conditionals and Loops groff: Conditionals and Loops groff: if-else

groff: Operators in Conditionals

 
 5.20.1 Operators in Conditionals
 --------------------------------
 
 In 'if', 'ie', and 'while' requests, in addition to ordinary ⇒
 Expressions, 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).  ⇒Troff and Nroff Mode.
 
 't'
      True if the document is being processed in troff mode (i.e., the
      '.troff' command has been issued).  ⇒Troff and Nroff Mode.
 
 'v'
      Always false.  This condition is for compatibility with other
      'troff' versions only (identifying a '-Tversatec' device).
 
 ''XXX'YYY''
      True if the output produced by XXX is equal to the output produced
      by YYY.  Other characters can be used in place of the single
      quotes; the same set of delimiters as for the '\D' escape is used
      (⇒Escapes).  'gtroff' formats XXX and YYY in separate
      environments; after the comparison the resulting data is discarded.
 
           .ie "|"\fR|\fP" \
           true
           .el \
           false
               => true
 
      The resulting motions, glyph sizes, and fonts have to match,(1)
      (⇒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 '|' glyph 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.
 
      To compare strings without processing, surround the data with '\?'.
 
           .ie "\?|\?"\?\fR|\fP\?" \
           true
           .el \
           false
               => false
 
      Since data protected with '\?' is read in copy-in mode it is even
      possible to use incomplete input without causing an error.
 
           .ds a \[
           .ds b \[
           .ie '\?\*a\?'\?\*b\?' \
           true
           .el \
           false
               => true
 
 '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.
 
 'm XXX'
      True if there is a color named XXX.
 
 'c G'
      Conditionals-Footnote-2::); G is either an ASCII character or a
      special character ('\N'XXX'', '\(GG' or '\[GGG]'); the condition is
      also true if G has been defined by the 'char' request.
 
 'F FONT'
      True if a font named FONT exists.  FONT is handled as if it was
      opened with the 'ft' request (that is, font translation and styles
      are applied), without actually mounting it.
 
      This test doesn't load the complete font but only its header to
      verify its validity.
 
 'S STYLE'
      True if style STYLE has been registered.  Font translation is
      applied.
 
    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.
 
    ⇒Expressions.
 

Info Catalog groff: Conditionals and Loops groff: Conditionals and Loops groff: if-else