groff: Expressions
5.3 Expressions
===============
'gtroff' has most arithmetic operators common to other languages:
* Arithmetic: '+' (addition), '-' (subtraction), '/' (division), '*'
(multiplication), '%' (modulo).
'gtroff' only provides integer arithmetic. The internal type used
for computing results is 'int', which is usually a 32-bit signed
integer.
* Comparison: '<' (less than), '>' (greater than), '<=' (less than or
equal), '>=' (greater than or equal), '=' (equal), '==' (the same
as '=').
* Logical: '&' (logical and), ':' (logical or).
* Unary operators: '-' (negating, i.e. changing the sign), '+' (just
for completeness; does nothing in expressions), '!' (logical not;
Expressions-Footnote-1::) See below for the use of unary operators
in motion requests.
The logical not operator, as described above, works only within
'if' and 'while' requests. Furthermore, it may appear only at the
beginning of an expression, and negates the entire expression.
Attempting to insert the '!' operator within the expression results
in a 'numeric expression expected' warning. This maintains
compatibility with old versions of 'troff'.
Example:
.nr X 1
.nr Y 0
.\" This does not work as expected
.if (\n[X])&(!\n[Y]) .nop X only
.
.\" Use this construct instead
.if (\n[X]=1)&(\n[Y]=0) .nop X only
* Extrema: '>?' (maximum), '<?' (minimum).
Example:
.nr x 5
.nr y 3
.nr z (\n[x] >? \n[y])
The register 'z' now contains 5.
* Scaling: '(C;E)'. Evaluate E using C as the default scaling
indicator. If C is missing, ignore scaling indicators in the
evaluation of E.
Parentheses may be used as in any other language. However, in
'gtroff' they are necessary to ensure order of evaluation. 'gtroff' has
no operator precedence; expressions are evaluated left to right. This
means that 'gtroff' evaluates '3+5*4' as if it were parenthesized like
'(3+5)*4', not as '3+(5*4)', as might be expected.
For many requests that cause a motion on the page, the unary
operators '+' and '-' work differently if leading an expression. They
then indicate a motion relative to the current position (down or up,
respectively).
Similarly, a leading '|' operator indicates an absolute position.
For vertical movements, it specifies the distance from the top of the
page; for horizontal movements, it gives the distance from the beginning
of the _input_ line.
'+' and '-' are also treated differently by the following requests
and escapes: 'bp', 'in', 'll', 'lt', 'nm', 'nr', 'pl', 'pn', 'po', 'ps',
'pvs', 'rt', 'ti', '\H', '\R', and '\s'. Here, leading plus and minus
signs indicate increments and decrements.
⇒Setting Registers, for some examples.
-- Escape: \B'anything'
Return 1 if ANYTHING is a valid numeric expression; or 0 if
ANYTHING is empty or not a valid numeric expression.
Due to the way arguments are parsed, spaces are not allowed in
expressions, unless the entire expression is surrounded by parentheses.
DONTPRINTYET ⇒Request and Macro Arguments, and *noteConditionals and
DONTPRINTYET ⇒Request and Macro Arguments, and ⇒Conditionals and
Loops.