5.23.1 Operators in Conditionals

The if, ie, and while requests test the truth values of numeric expressions. They also support several additional Boolean operators; the members of this expanded class are termed conditional expressions; their truth values are as shown below.

c chr

True if a character chr is available; chr is an ordinary, special or indexed character, whether defined by a font description file or a request.

d name

True if a string, macro, diversion, or request called name exists.

e

True if the current page is even-numbered.

F font

True if font exists. font is handled as if it were an argument to the ft request (that is, the default family is combined with an abstract style and font translation is applied), but font cannot be a mounting position, and no font is mounted.

m color

True if color is defined.

n

True if the document is being processed in nroff mode.

o

True if the current page is odd-numbered.

r register

True if register exists.

S style

True if style is available for the current font family. Font translation is applied.

t

True if the document is being processed in troff mode.

v

Always false. This condition exists for compatibility with certain other troff implementations.¹⁴¹

If the first argument to an if, ie, or while request begins with a non-alphanumeric character apart from ! (see below) and is not a numeric expression, the formatter performs an output comparison test. ¹⁴²

'xxx'yyy'

This output comparison operator interpolates a true value if formatting the comparands xxx and yyy produces the same output commands. The delimiter need not be a neutral apostrophe: the output comparison operator accepts the same delimiters as most escape sequences; see Delimiters. troff formats xxx and yyy in separate scratch buffers; after comparison, it discards the resulting data.

.ie "|"\fR|\fP" true
.el false
    ⇒ true

The resulting glyph properties, including font family, style, size, and slant, must match, but not necessarily the requests and/or escape sequences used to obtain them. In the previous example, ‘|’ and ‘\fR|\fP’ result in ‘|’ glyphs in the same typefaces at the same positions, so the comparands are equal. If ‘.ft I’ had been added before the ‘.ie’, they would differ: the first ‘|’ would produce an italic ‘|’, not a roman one. Motions must match in orientation and magnitude to within the applicable horizontal and vertical motion quanta of the device, after rounding. ‘.if "\u\d"\v'0'"’ is false even though both comparands result in zero net motion, because motions are not interpreted or optimized but sent as-is to the output.¹⁴³ On the other hand, ‘.if "\d"\v'0.5m'"’ is true, because \d is defined as a downward motion of one-half em.¹⁴⁴

Surround the comparands with \? to avoid formatting them; this causes them to be compared character by character, as with string comparisons in other programming languages.

.ie "\?|\?"\?\fR|\fP\?" true
.el false
    ⇒ false

Since GNU troff reads comparands protected with \? in copy mode,¹⁴⁵ they need not even be syntactically valid. The escape character is still lexically recognized, however, and consumes the next character.

.ds a \[
.ds b \[
.if '\?\*a\?'\?\*b\?' a and b equivalent
.if '\?\\?'\?\\?' backslashes equivalent
.if '\?\P\?'\?P\?' backslash-P and P equivalent
    ⇒ a and b equivalent

The above operators can’t be combined with most others, but a leading ‘!’, not followed immediately by spaces or tabs, complements an expression.

.nr x 1
.ie !r x register x is not defined
.el      register x is defined
    ⇒ register x is defined

Spaces and tabs are optional immediately after the ‘c’, ‘d’, ‘F’, ‘m’, ‘r’, and ‘S’ operators, but right after ‘!’, they end the predicate and the conditional evaluates true.¹⁴⁶

.nr x 1
.ie ! r x register x is not defined
.el       register x is defined
    ⇒ r x register x is not defined

The unexpected ‘r x’ in the output is a clue that our conditional was not interpreted as we planned, but matters may not always be so obvious.

Conditional operators do not create roff language objects as interpolations with \n and \* escape sequences do.