While loops

The repeat in a loop can be modified by adding one or more while clauses. Each clause contains a predicate immediately following the while keyword. The predicate is tested before the evaluation of the body of the loop. The loop body is evaluated whenever the predicate in a while clause is true.

The syntax for a simple loop using while is

while predicate repeat loopbody

The predicate is evaluated before loopbody is evaluated. A while loop terminates immediately when predicate evaluates to false or when a leave or return expression is evaluted. See )help repeat for more information on leave and return.

Here is a simple example of using while in a loop. We first initialize the counter.

i := 1
 1
                    Type: PositiveInteger

while i < 1 repeat
  output "hello"
  i := i + 1
                    Type: Void

The steps involved in computing this example are

(1) set i to 1
(2) test the condition i < 1 and determine that it is not true
(3) do not evaluate the loop body and therefore do not display "hello"

 (x, y) := (1, 1)
  1
                     Type: PositiveInteger

If you have multiple predicates to be tested use the logical and operation to separate them. FriCAS evaluates these predicates from left to right.

while x < 4 and y < 10 repeat
  output [x,y]
  x := x + 1
  y := y + 2
 [1,1]
 [2,3]
 [3,5]
                    Type: Void

A leave expression can be included in a loop body to terminate a loop even if the predicate in any while clauses are not false.

(x, y) := (1, 1)
 1
                    Type: PositiveInteger

while x < 4 and y < 10 repeat
  if x + y > 7 then leave
  output [x,y]
  x := x + 1
  y := y + 2
 [1,1]
 [2,3]
                    Type: Void