5.20.3 while

gtroff provides a looping construct using the while request, which is used much like the if (and related) requests.

Request: .while expr anything

Evaluate the expression expr, and repeatedly execute anything (the remainder of the line) until expr evaluates to 0.

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

Some remarks.

• The body of a while request is treated like the body of a de request: gtroff temporarily stores it in a macro that is deleted after the loop has been exited. It can considerably slow down a macro if the body of the while request (within the macro) is large. Each time the macro is executed, the while body is parsed and stored again as a temporary macro.

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

  The traditional and ofter better solution (UNIX troff doesn’t have the while request) is to use a recursive macro instead that is parsed only once during its definition.

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

  Note that the number of available recursion levels is set to 1000 (this is a compile-time constant value of gtroff).

• The closing brace of a while body must end a line.

  .if 1 \{\
  .  nr a 0 1
  .  while (\n[a] < 10) \{\
  .    nop \n+[a]
  .\}\}
      ⇒ unbalanced \{ \}
  

Request: .break

Break out of a while loop. Be sure not to confuse this with the br request (causing a line break).

Request: .continue

Finish the current iteration of a while loop, immediately restarting the next iteration.

See Expressions.