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 32bit 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; this works only within if and while requests).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 which 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.

See Setting Registers, for some examples.

Due to the way arguments are parsed, spaces are not allowed in expressions, unless the entire expression is surrounded by parentheses.

See Request and Macro Arguments, and Conditionals and Loops.