9.3 Loop statements

A loop statement specifies that one or more statements are executed a number of times and as mentioned, cycle and for:to:repeat are examples of loop statements.

Defining for:to:repeat

The for:to:repeat is defined as follows:

   for(first: var integer):to(last: var integer):repeat{repeat:< object}:
      inx: var integer
      inx := first
      doIt: do
         if (inx <= last) :then
            repeat
            inx := inx + 1
            restart(doIt)

A for:to:repeat has the following structure:

• The name of the control method is for:to:repeat.
• It has three parameters: first, last and repeat.
• It has a local integer variable inx.
• It has a local do-statement +++ do-objekter skal forklares og måske skal do som keyword udgå, men det kræver et keyword for metoder (og patterns)

As explained in section X on statements, a ffor:to:repeat iterates over a range of integers and executes a set of items for each iteration:

• The parameters first and last defines the range, where the first value is first and the last value is last.
• The variable inx has the value of the current element in the range. This means that inx goes through the values start, start + 1, start + 1, … , last.
• For each value in the range, the virtual method parameter repeat is executed.
• If start > last, then repeat is never executed.

Here is a simple example of using a for:to:repeat:

A,B: obj Array(10,Integer)
for (1):to(10):repeat
    A.put(inx * inx):at[inx]
for (1):to(10):repeat
    B.put(A.get[inx] * A.get[inx])

Defining while:repeat

Another common control structure is the while-loop, which repeats executing a list of statements as long as a given condition is true. The following example shows a program fragment that computes 2⁴.

a,i: var integer
a := 2
i := 1
while (i < 5):repeat
   a := a * 2
   i := i + 1

The while:repeat-loop executes a := a * 2; i := i + 1 as long as i < 5.

The while:repeat is defined as follows:

while(cond:< Condition):repeat{doPart:< Object}:
   loop: do
      if (cond):then
         doPart
         restart(loop)

The first parameter cond, is a virtual method qualified by the supermethod Condition. Condition returns a boolean value and it may be bound to boolean expressions like i < 5 as in the above example.

The second parameter doPart, may be bound to a list of items that will be executed as long as cond evaluates to true.

Note the difference of the first parameter of if:then (and if:then:else) and while:repeat. For if:then, the first parameter is a boolean value since it is only evaluated at the point of invocation of if:then.

The cond of while:repeat is, however, a virtual method since it must be evaluated for each iteration in the execution of the while-loop.