Iteration — loops

Count-controlled loops

• Iteration means repeating steps.
• A count-controlled loop repeats a fixed number of times.
• In Python that is for i in range(n):.

for i in range(5):
    print("Hello")

Choosing the range

• range(5) gives 0, 1, 2, 3, 4.
• range(1, 6) gives 1, 2, 3, 4, 5 — it stops before 6.
• range(2, 11, 2) counts in steps of 2.

for n in range(1, 6):
    print(n)

Pre-condition loops: while

• A pre-condition loop tests before each pass.
• It may run zero times if the test is false at the start.

count = 0
while count < 5:
    print(count)
    count = count + 1

Post-condition loops: REPEAT UNTIL

• A post-condition loop tests after each pass, so it always runs at least once.
• Python has no repeat, so we use while True with break.

total = 0
while True:
    total = total + 1
    if total >= 3:
        break
print(total)

In the exam: FOR, WHILE, REPEAT

• Pseudocode has all three loop types.

FOR i ← 1 TO 5
    OUTPUT i
NEXT i

Common mistakes

• A for loop repeats a known number of times; a while repeats until something changes.
• range(n) runs 0 to n - 1.
• Change the condition inside a while, or it never ends.

Now you try

• Pick the loop that fits each task.
• Press Check answer to test your code.