Chapter 3: The while Loop — Keep Going While True¶

Why Do We Need Loops?¶

When you need to run the same code multiple times, use a loop.

# Without a loop: print 1~5
print(1)
print(2)
print(3)
print(4)
print(5)

What if you needed 100 times? Use a loop.

while Basic Structure¶

while condition:
    code to execute

If the condition is True, the block runs, then the condition is checked again. When the condition becomes False, the loop exits.

Check condition
  ↓ True
Execute code
  ↓
Check condition  ← go back
  ↓ False
Loop ends

Understanding Through Examples¶

Example 1: Counter¶

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

i keeps increasing by 1 inside the loop. When i < 5 becomes False (i = 5), the loop ends.

Example 2: Printing Inside the Loop¶

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

print(i) comes before i += 1, so it starts from 0.

Example 3: Running Total¶

i = 0
total = 0
while i < 5:
    print(i * '#')    # print i '#' characters
    total += i
    i += 1
print(total)

(blank line)
#
##
###
####
10

0+1+2+3+4 = 10

break — Immediate Exit¶

Ends the loop immediately without waiting for the condition.

i = 0
while i < 10:
    if i == 5:
        break
    print(i)
    i += 1

When i reaches 5, break exits the entire loop.

continue — Skip This Iteration¶

Skips the rest of the current iteration and jumps back to the condition check.

i = 0
while i < 10:
    i += 1
    if i % 2 == 0:
        continue
    print(i)

When i is even, continue skips print(i).

while True — Infinite Loop¶

Using True as the condition makes the loop run forever. Use break to escape.

while True:
    cmd = input("Enter command (q=quit): ")
    if cmd == "q":
        break
    print("Input:", cmd)

Enter command (q=quit): hello
Input: hello
Enter command (q=quit): q

Useful when you don't know the exit condition in advance or need continuous user input.

Fibonacci Sequence¶

x, y = 0, 1
while x < 100:
    print(x, end=" ")
    x, y = y, x + y

0 1 1 2 3 5 8 13 21 34 55 89

x, y = y, x + y simultaneously updates both variables.

Step	x	y
Start	0	1
1st	1	1
2nd	1	2
3rd	2	3
4th	3	5
5th	5	8

Practice Missions¶

Mission 1: Multiplication Table¶

Take a number as input and print its multiplication table.

dan = int(input("Table for: "))
i = 1
# Use a while loop

# Table for: 3
# 3 × 1 = 3
# 3 × 2 = 6
# ...
# 3 × 9 = 27

Mission 2: Number Guessing Game¶

The computer picks a random number between 1 and 100. Give hints until the user guesses it.

import random
secret = random.randint(1, 100)
# Use a while loop
# Hints: "Higher", "Lower", "Correct!"

# Guess the number: 50
# Higher!
# Guess the number: 75
# Lower!
# Guess the number: 63
# Correct! You got it in 3 tries

Mission 3: Prime Number Checker¶

Take a number and determine if it's prime. (Use a while loop)

Primes: 2, 3, 5, 7, 11, 13, ...
If divisible by any number from 2 to n-1, it's not prime

n = int(input("Number: "))
# Write your code here

# Number: 17
# 17 is prime

# Number: 15
# 15 is not prime

Mission 4: Running Total¶

Starting from 1, find the number when the sum first exceeds 100.

total = 0
n = 0
# Use a while loop

# Sum exceeded 100 at: 14  (1+2+...+13=91, 1+2+...+14=105)

Mission 5: Fibonacci Extended¶

Print all Fibonacci numbers up to 1000 and count them.

x, y = 0, 1
count = 0
# Write your code here

# 0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987
# Total: 17 numbers

Key Summary¶

Concept	Description
`while condition:`	Repeat while condition is True
`break`	Exit loop immediately
`continue`	Skip rest of iteration, return to condition
`while True:`	Infinite loop (escape with `break`)
Counter variable	Increment inside loop to change condition
`x, y = y, x+y`	Simultaneous variable update