5.23.5 while

GNU troff provides a looping construct: the while request. Its syntax matches the if request.

Request: .while cond-expr input

Evaluate the conditional expression cond-expr, and repeatedly execute input unless and until cond-expr evaluates false. input, which is often a conditional block, is referred to as the while request’s body.

.nr a 0 1
.while (\na < 9) \{\
\n+a,
.\}
\n+a
    ⇒ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10

GNU troff treats the body of a while request similarly to that of a de request (albeit one not read in copy mode¹⁴⁴), but stores it under an internal name and deletes it when the loop finishes. The operation of a macro containing a while request can slow significantly if its body is large. Each time GNU troff interpolates the macro, it parses and stores the while body again.

.de xxx
.  nr num 10
.  while (\\n[num] > 0) \{\
.    \" many lines of code
.    nr num -1
.  \}
..

An often better solution—and one that is more portable, since AT&T troff lacked the while request—is to instead write a recursive macro, which is parsed only once.¹⁴⁵

.de yy
.  if (\\n(nm > 0) \{\
.    \" many lines of code
.    nr nm -1
.    yy
.  \}
..
.
.de xx
.  nr nm 10
.  yy
..

To prevent infinite loops, GNU troff limits the default number of available recursion levels to 1,000 or somewhat less.¹⁴⁶ You can disable this protective measure, or alter the limit, by setting the slimit register. See Debugging.

As noted above, if a while body begins with a conditional block, its closing brace must end an input line.

.if 1 \{\
.  nr a 0 1
.  while (\n[a] < 10) \{\
.    nop \n+[a]
.\}\}
    error→ unbalanced brace escape sequences

Request: .break

Exit a while loop. Do not confuse this request with a typographical break or the br request.

Request: .continue

Skip the remainder of a while loop’s body, immediately retesting its conditional expression.