How to implement loops in Pascal

What are loops?

Loops are programming structures that are used to repeat a block of program statements until a given condition is met.

Loops in Pascal

There are three types of loops in Pascal:

  • Fixed repetition loop: Executes the block a fixed number of times.
  • Pretest loop: Checks if the loop condition is met, then executes the block.
  • Posttest loop: Executes the block, then checks if the loop condition is met.

Fixed repetition loop

The fixed repetition loop is initialized as follows:

for _loopVariable_ := _start_ to _end_ do
begin
  _block_
end;
  • _loopVariable_: The variable name of the loop variable that will iterate from _start to _end_.
  • _start_: The starting value of the loop.
  • _end_: The ending value of the loop.
  • _block_: The program statements that will be executed repeatedly.

Flowchart

Fixed loop - Flowchart

Example

Let’s look at the code snippet below, which demonstrates the implementation of a fixed repetition loop in Pascal.

program fixedRepetetion;
var
   i: integer;
begin
   writeln('Loop in forward direction');
   for i := 0  to 3 do
   begin
      writeln(i, ': Hello world!! This loop is executing in forward direction');
   end;
   
   for i := 3  to 2 do
   begin
      writeln(i, ': Hello world!! This loop is executing in forward direction');
   end;
   
   writeln(sLineBreak + 'Loop in backward direction');
   for i := 3  downto 0 do
   begin
      writeln(i, ': Hello world!! This loop is executing in backward direction');
   end;
end.

Explanation

  • Lines 7-10: Loop iterates in the forward direction from 0 to 3.
  • Lines 12-15: Loop iterates 0 number of times. This is because the starting value of the loop is greater than the ending value of the loop, i.e., 3>2.
  • Lines 18-21: Loop iterates in the backward direction from 3 to 0. This is because of the down keyword used in the for statement.

Pretest loop

The pretest loop is initialized as follows:

while _loopCondition_ do
begin
  _block_
end;
  • _loopCondition_: The condition that is checked every time before executing the block. The block is executed only if this condition is true.
  • _block_: The program statements that will be executed repeatedly.

Flowchart

Pretest loop - Flowchart

Example

Consider the code snippet below, which demonstrates the implementation of a pretest loop in Pascal.

program pretest;
var
   i: integer;
begin
   i := 0;
   while i < 3 do
   begin
      writeln (i, ': Hello World!!');
      i := i + 1;
   end;
end.

Explanation

  • A variable i is initialized with 0 in line 7. The while loop in line 8 checks if i<3 in line 8; this is the loop condition. The loop will iterate until this condition becomes false. As i is incremented in line 11 each time the block executes, the while loop is executed 3 times. The third time the block is executed, i becomes 4. Thus, the condition of while will not be met and the loop will break.

Posttest loop

The posttest loop is initialized as follows:

repeat
begin
  _block_
end;
until _loopCondition_ ;
  • _loopCondition_: The condition that is checked every time after executing the block. The loop breaks if the condition is met.
  • _block_: The program statements that will be executed repeatedly.

Flowchart

Posttest loop - Flowchart

Example

Consider the code snippet below, which demonstrates the implementation of a posttest loop in Pascal.

program posttest;
var
   i: integer;
begin
   i := 0;
   repeat
   begin
      writeln (i, ': Hello World!!');
      i := i + 1;
   end
   until i>3;
end.

Explanation

A variable i is initialized with 0 in line 6. The loop block, which is line 9-10, is executed before checking the loop condition in line 12. The loop block will execute repeatedly until the condition in line 12 is met. As i is incremented in line 10, each time the block executes, the repeat until loop is executed 4 times. The fourth time the block is executed, i becomes 4. Thus, the condition of until will be met and the loop will break.

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