Pseudocode and Flowcharts

CIE pseudocode supports three loop constructs. Each one fits a different situation; getting the right loop for the job makes algorithms cleaner and easier to read.

FOR loop

A FOR loop runs a block of code a fixed number of times, with a counter that takes each value in a stated range.

Syntax:

FOR i ← 1 TO 10
   OUTPUT i
NEXT i

Use a FOR loop when:

• You know in advance how many times the loop should run.
• A counter is naturally needed for the work (e.g. processing each element in an array).

Variations:

• FOR i ← 1 TO 10 STEP 2 advances the counter by 2 each iteration (1, 3, 5, 7, 9).
• FOR i ← 10 TO 1 STEP -1 counts down.

WHILE loop

A WHILE loop runs a block of code as long as a condition is true. The condition is checked at the top before each iteration.

Syntax:

WHILE Answer <> "stop" DO
   INPUT Answer
ENDWHILE

Use a WHILE loop when:

• You do not know in advance how many iterations will be needed.
• It may be appropriate to run the loop zero times if the condition is already false at the start.

REPEAT-UNTIL loop

A REPEAT-UNTIL loop runs a block of code repeatedly until a condition becomes true. The condition is checked at the bottom after each iteration.

Syntax:

REPEAT
   INPUT Number
UNTIL Number > 0

Use a REPEAT-UNTIL loop when:

• You do not know in advance how many iterations will be needed.
• The loop body must run at least once.

Side-by-side comparison

Choosing the right loop

Two questions decide which loop fits a given task:

1. Is the number of iterations known before the loop starts? Yes → use FOR. No → continue to question 2.
2. Should the loop body always run at least once? Yes → use REPEAT-UNTIL (the condition is checked at the bottom). No → use WHILE (the condition is checked at the top, and zero iterations are allowed).

Two quick examples of the pattern in everyday programming. Printing the first twenty multiples of three has a known iteration count, so a FOR loop is the right pick. Asking the user to type a positive number and re-prompting if they type a negative one needs the input to be read at least once before any check happens, so REPEAT-UNTIL fits. Waiting for a sensor reading to drop below a threshold has unknown duration and may already be below at the start, so WHILE fits.

Avoiding common loop pitfalls

• Off-by-one errors: forgetting that FOR i ← 1 TO 10 runs 10 times, not 9 or 11.
• Infinite loops: forgetting to update the variable in the loop's condition (e.g. forgetting to read fresh input inside a WHILE Answer <> "stop" loop).
• Wrong loop choice: using a REPEAT-UNTIL when zero iterations should be possible, or a WHILE when at least one iteration is mandatory.
• Counter inside the loop body: in CIE pseudocode, you generally should not modify the FOR loop's counter from inside the loop body.