3.5 · Iteration (for and while)

Goal: use for to count, while to wait for a condition. Remember nested loops are 2C material.

for — counting loops

for i in range(5):
    print(i)           # 0, 1, 2, 3, 4

range(start, stop, step):

Call	Sequence
range(5)	0, 1, 2, 3, 4
range(1, 6)	1, 2, 3, 4, 5
range(0, 10, 2)	0, 2, 4, 6, 8
range(10, 0, -1)	10, 9, 8, …, 1

Iterating over a list

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

You can also enumerate to get index + value:

for i, fruit in enumerate(fruits):
    print(i, fruit)

while — conditional loops

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

Use while when you don't know in advance how many iterations are needed.

Sentinel-controlled while

total = 0
n = int(input("Number (0 to stop): "))
while n != 0:
    total += n
    n = int(input("Number (0 to stop): "))
print("Total:", total)

The user types 0 to stop. 0 is the sentinel value.

break and continue

Keyword	Effect
break	Exit the loop immediately
continue	Skip the rest of this iteration, go to the next

for i in range(10):
    if i == 5:
        break          # stop loop at 5
    print(i)           # 0 1 2 3 4

for i in range(10):
    if i % 2 == 0:
        continue       # skip even numbers
    print(i)           # 1 3 5 7 9

Avoiding infinite loops

# BAD — count never changes
count = 0
while count < 10:
    print(count)

# GOOD
count = 0
while count < 10:
    print(count)
    count += 1

Worked example · Sum 1..n

n = int(input("n? "))
total = 0
for i in range(1, n + 1):
    total += i
print("Sum:", total)

Worked example · Guess the number

import random
secret = random.randint(1, 100)
guess = -1
while guess != secret:
    guess = int(input("Guess: "))
    if guess < secret:
        print("Too low")
    elif guess > secret:
        print("Too high")
print("Correct!")

Common student mistakes

• Off-by-one in range: range(1, 10) gives 1..9, not 1..10.
• Forgetting to update the loop variable in while → infinite loop.
• Writing nested loops in compulsory pseudocode (it's a 2C topic).

Exam-style question

Q (5 marks): Write a Python program that reads numbers until the user enters -1, then prints the average of the entered numbers (excluding -1). If no numbers are entered, print "no input".

Sample answer:

total = 0
count = 0
while True:
    n = int(input("Number (-1 to stop): "))
    if n == -1:
        break
    total += n
    count += 1

if count == 0:
    print("no input")
else:
    print("Average:", total / count)

Key takeaways

• for for counting; while for "until a condition".
• range(start, stop, step) is exclusive of stop.
• break exits; continue skips.
• Avoid nested loops in compulsory part.

➡️ Next: 3.6 Lists (1D Arrays)