Loops are used for repetition of certain statements for a set number of times or according to a certain condition.

FOR loop (unconditional loop)

• Repeat instructions for a set number of times.
• Uses a counter (must be ordinal: integer or char)
• Uses a start and ending value that determines repetitions.
• Can count/move up (e.g. 1 TO 10) or down (10 DOWNTO 1).
  

Structure:

      FOR <counter> := <start value> TO <ending value> DO
      BEGIN
       <statements;>
      END;
      

WHILE loop (conditional loop)

• Repeats depending on a certain condition(s).
• Repeats while a certain condition is TRUE.
• ITC:
  • Variable tested at beginning of loop must have initial value.
  • Variable is tested at the beginning of loop.
  • Variable must change in order for loop to stop.

Structure:

      WHILE <condition(s)> DO
      BEGIN
       <statement(s)>
       <change condition to stop loop>
      END;


REPEAT loop (conditional loop)

• Repeats depending on a certain condition(s).
• Unlike the WHILE loop the REPEAT loop tests for a condition at the end of executing certain statements rather than before.
• Instructions in a REPEAT loop will always be executed at least once.
• Does not need a BEGIN and END.
• The condition(s) indicate when to stop not when to go on as with the WHILE.
• ICT
  • Variables to be tested must be initialized before execution.
  • Variable must be changed at the end of loop to avoid an infinite loop (loop will never stop repeating)
  • Variable is tested at the end of the loop. If the statement is FALSE then the loop will be repeated again.
    

Structure:

      REPEAT
       <statement(s)>
       <change condition to stop loop>
      UNTIL <conditions to stop>;