While Loop Examples in CS Thinking

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of While Loop.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in CS Thinking.

Concept Recap

A control structure that repeats a block of code as long as a specified condition remains true. The condition is checked before each iteration, and if it is false from the start, the loop body never executes at all.

A while loop is like 'keep going until...' — keep stirring the soup WHILE it's not boiling. Keep asking for a password WHILE the password is wrong.

Read the full concept explanation →

How to Use These Examples

Read the first worked example with the solution open so the structure is clear.
Try the practice problems before revealing each solution.
Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: While loops check the condition before each iteration. If the condition is false from the start, the body never executes.

Common stuck point: Forgetting to update the loop variable creates an infinite loop. Always make sure the condition will eventually become false.

Sense of Study hint: When writing a while loop, first initialize the variable that the condition tests. Then write the condition that should keep the loop running. Inside the loop body, make sure to update the variable so the condition eventually becomes false. Trace through mentally with a small example.

Worked Examples

Example 1

medium

Trace: SET n = 1. WHILE n < 100: n = n * 2. OUTPUT n. How many iterations?

n=1; while n<100: n*=2; OUTPUT n → 7 iterations, output 128

Answer

Output: 128. 7 iterations.

First step

Step 1: Start at

n = 1

and double each time:

1 \to 2 \to 4 \to 8 \to 16 \to 32 \to 64 \to 128

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Example 2

medium

Write a WHILE loop that asks the user for input until they type 'quit'.

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

medium

Trace: SET total = 0. SET i = 1. WHILE i <= 4: total = total + i. i = i + 1. OUTPUT total.

while i<=4: total+=i; i+=1 — accumulator sums to 10

Example 2

medium

Trace this code and state the final output: SET n = 1 SET result = 1 WHILE n < 5 SET n = n + 1 SET result = result * n END WHILE OUTPUT result

while n<5: n+=1; result*=n — computes 5! = 120

Example 3

easy

Trace: `i=0; while i<3: print(i); i+=1`. What values print?

Trace: i=0; while i<3: print(i); i+=1

Example 4

easy

How many times does the body run? `i=1; while i<=5: i+=1`.

i=1; while i<=5: i+=1 — body runs 5 times

Example 5

easy

Trace: `n=10; while n>0: n-=3; print(n)`. What prints?

Example 6

easy

Does this loop body ever run? `x=5; while x<3: print('hi')`.

x=5; while x<3: print('hi') — zero iterations

Example 7

easy

Trace: `i=0; total=0; while i<4: total+=i; i+=1`. Final total?

i=0; total=0; while i<4: total+=i; i+=1 — final total is 6

Example 8

easy

Spot the bug: `i=0; while i<3: print(i)`. What goes wrong?

Bug: i=0; while i<3: print(i) — missing i+=1 causes infinite loop

Example 9

easy

Trace: `p=1; while p<20: p*=2; print(p)`. What prints?

Example 10

easy

Trace: `s='abc'; i=0; out=''; while i<len(s): out=s[i]+out; i+=1`. Final out?

Example 11

medium

Trace: `i=1; while i<=8: i*=2; print(i)`. What prints and how many iterations?

Example 12

medium

Trace: `a=20; b=12; while b!=0: a,b = b, a%b. print(a)`. Final a?

Example 13

medium

Trace: `n=27; steps=0; while n!=1: n = n//2 if n%2==0 else 3*n+1; steps+=1`. Is termination guaranteed for this code as written, and what is n's stopping value?

Example 14

medium

Trace: `i=0; count=0; while i<10: if i%3==0: count+=1; i+=1`. Final count?

Example 15

medium

Trace: `x=100; c=0; while x>1: x=x//2; c+=1`. Final c?

Example 16

medium

Trace: `i=5; while i>0: print(i); i-=2`. What prints?

i=5; while i>0: print(i); i-=2 — prints 5, 3, 1

Example 17

medium

Trace: `i=0; result=1; while i<5: result*=2; i+=1`. Final result?

Example 18

medium

Trace: `q=[3,1,2]; out=[]; while q: out.append(q.pop(0))`. Final out?

Example 19

medium

Trace: `i=0; while True: if i>=3: break; print(i); i+=1`. What prints?

while True with break: prints 0, 1, 2 then breaks

Example 20

challenge

Bug hunt: `i=0; while i<5: print(i)` vs `i=0; while i<5: print(i); i=i+1`. Which terminates and why, and how many lines does the terminating one print?

Example 21

challenge

Trace: `i=1; j=10; while i<j: i+=2; j-=1; print(i,j)` give the final i and j and iteration count.

Example 22

challenge

Trace: `n=5; result=0; i=1; while i<=n: result+=i*i; i+=1`. Final result, and what formula does it compute?

Example 23

easy

Trace: `i=0; while i<2: print(i); i+=1`. What prints?

Example 24

easy

How many times does the body run? `i=0; while i<6: i+=1`.

Example 25

easy

Does this loop body execute even once? `x=10; while x<5: x-=1`.

Example 26

easy

Trace: `n=5; while n>0: n-=1; print(n)`. What prints?

Example 27

easy

Spot the bug: `n=3; while n>0: print('hi')`. What goes wrong?

Example 28

easy

Trace: `i=1; total=0; while i<=3: total+=i; i+=1`. Final total?

Example 29

medium

Trace: `i=2; while i<=16: i*=2`. Final

i

and iteration count?

Example 30

medium

Trace: `n=100; c=0; while n>1: n=n//2; c+=1`. Final

c

Example 31

medium

Trace: `i=0; while i<5: i+=2; print(i)`. What prints?

Example 32

medium

Trace: `a=48; b=18; while b!=0: a,b = b, a%b`. Final

a

Example 33

medium

Trace: `i=1; p=1; while i<=4: p*=i; i+=1`. Final

p

Example 34

medium

Trace: `s='hello'; i=0; out=''; while i<len(s): out+=s[i].upper(); i+=1`. Final out?

Example 35

medium

Trace: `i=10; while i>1: i//=2; print(i)`. What prints?

Example 36

medium

Trace: `n=7; sum=0; while n>0: sum+=n%10; n=n//10`. Final sum?

Example 37

medium

Trace: `n=123; sum=0; while n>0: sum+=n%10; n=n//10`. Final sum?

Example 38

medium

Trace: `a=0; b=1; while a<10: a,b = b, a+b`. Final

a

Example 39

medium

Trace: `i=0; while i<5: if i==3: break; print(i); i+=1`. What prints?

Example 40

medium

Trace: `i=0; while i<5: i+=1; if i%2==0: continue; print(i)`. What prints?

Example 41

medium

Trace: `x=1; y=0; while x<=4: y+=x*x; x+=1`. Final

y

Example 42

hard

Trace: `n=20; r=0; while n>0: if n%2==1: r+=1; n=n//2`. Final

r

Example 43

hard

Trace: `n=16; r=0; while (1<<r) < n: r+=1`. Final

r

Example 44

hard

Trace: `lo=0; hi=10; target=7; arr=[1,3,5,7,9,11,13,15,17,19]; while lo<hi: m=(lo+hi)//2; if arr[m]<target: lo=m+1; else: hi=m`. Final lo?

Example 45

hard

Trace: `n=6; a=0; b=1; i=2; while i<=n: a,b = b, a+b; i+=1`. Final

b

Example 46

challenge

Trace: `n=100; r=0; while n: r = r*10 + n%10; n//=10`. Final

r

Example 47

challenge

Bug analysis: `i=0; while i<5: print(i); i+=1; i-=1`. Does this terminate? If not, why?

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Related Concepts

Iteration For Loop

Background Knowledge

These ideas may be useful before you work through the harder examples.

iteration