Loops (repeating actions) in Intro to Computing - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ trace

intermediate

Trace the output of a simple for loop

What will be printed by the following code?

Intro to Computing

for i in range(3):
    print(i * 2)

A 0 2 4

B 0 1 2

C 2 4 6

D 1 3 5

💡 Hint

Remember, range(3) produces 0, 1, 2.

The loop runs with i = 0, 1, 2. Multiplying each by 2 gives 0, 2, 4.

🧠 Conceptual

intermediate

Understanding while loop behavior

What will happen when this code runs?

Intro to Computing

count = 3
while count > 0:
    print(count)
    count -= 1

A Prints 3 2 1 0 then stops

B Prints 3 2 1 then stops

C Prints 1 2 3 then stops

D Runs forever printing 3

💡 Hint

Check the condition count > 0 and how count changes.

The loop prints 3, then 2, then 1. When count becomes 0, the condition fails and loop stops.

❓ Comparison

advanced

Compare outputs of two loops

Which option correctly describes the difference between these two loops?

Loop 1:
for i in range(1, 5):
    print(i)

Loop 2:
i = 1
while i < 5:
    print(i)
    i += 1

A Both print 1 2 3 4 but Loop 1 uses for, Loop 2 uses while

B Loop 1 prints 1 2 3 4, Loop 2 prints 1 2 3 4 5

C Loop 1 prints 0 1 2 3, Loop 2 prints 1 2 3 4

D Loop 1 runs forever, Loop 2 stops after printing 4

💡 Hint

Check the range and condition carefully.

Both loops print numbers 1 to 4. Loop 1 uses for, Loop 2 uses while but output is same.

❓ identification

advanced

Identify the error in loop code

What error will this code cause?

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

A No error, prints 0 1 2

B TypeError because i is not a number

C SyntaxError because while is missing a colon

D IndentationError because print and increment are not indented

💡 Hint

Check the indentation of lines inside the loop.

Python requires code inside loops to be indented. Here print and i += 1 are not indented, causing IndentationError.

🚀 Application

expert

Determine final value after nested loops

What is the final value of total after running this code?

total = 0
for x in range(3):
    for y in range(2):
        total += x + y

A 18

B 12

C 9

D 15

💡 Hint

Calculate sums for each pair (x, y) and add carefully.

Pairs and sums: (0+0)=0, (0+1)=1, (1+0)=1, (1+1)=2, (2+0)=2, (2+1)=3. Sum = 0+1+1+2+2+3 = 9.

Practice

1. What is the main purpose of a loop in programming?

easy

A. To display text on the screen

B. To store data permanently

C. To repeat a set of actions multiple times

D. To create a new variable

Solution

1. Step 1: Understand the role of loops

   Loops are used to repeat actions without writing the same code again and again.

2. Step 2: Identify the correct purpose

   Repeating a set of actions multiple times matches the purpose of loops.

3. Final Answer:

   To repeat a set of actions multiple times -> Option C

4. Quick Check:

   Loops = Repeat actions [OK]

Hint: Loops repeat tasks to save writing code again [OK]

Common Mistakes:

• Confusing loops with data storage
• Thinking loops create variables
• Believing loops only display text

2. Which of the following is the correct syntax for a for loop in Python to repeat 5 times?

easy

A. for i to 5:

B. for i in range(5):

C. loop i from 1 to 5:

D. repeat 5 times:

Solution

1. Step 1: Recall Python for loop syntax

   Python uses for variable in range(number): to repeat actions a set number of times.

2. Step 2: Match the correct syntax

   for i in range(5): correctly repeats 5 times from 0 to 4.

3. Final Answer:

   for i in range(5): -> Option B

4. Quick Check:

   Python for loop = for i in range(5): [OK]

Hint: Python for loops use 'for variable in range():' [OK]

Common Mistakes:

• Using 'for i to 5:' which is invalid syntax
• Confusing loop syntax with other languages
• Missing colon at the end of the loop line

3. What will be the output of this Python code?

sum = 0
for i in range(3):
    sum += i
print(sum)

medium

A. 6

B. 3

C. 0

D. SyntaxError

Solution

1. Step 1: Trace the loop iterations

   The loop runs with i = 0, 1, 2. sum starts at 0.

2. Step 2: Calculate sum after each iteration

   Iteration 1: sum = 0 + 0 = 0
   Iteration 2: sum = 0 + 1 = 1
   Iteration 3: sum = 1 + 2 = 3

3. Step 3: Sum all values

   Sum of 0 + 1 + 2 = 3, but the code adds i each iteration, so final sum is 3.

4. Final Answer:

   3 -> Option A

5. Quick Check:

   0+1+2 = 3 [OK]

Hint: Add all numbers from 0 up to 2 [OK]

Common Mistakes:

• Adding 3 instead of stopping at 2
• Confusing range(3) with range(1,3)
• Expecting SyntaxError due to indentation

4. Identify the error in this loop code:

i = 1
while i < 4
    print(i)
    i += 1

medium

A. Missing colon after while condition

B. Variable i should start at 0

C. print() should be outside the loop

D. i += 1 should be before print()

Solution

1. Step 1: Check while loop syntax

   Python requires a colon ':' after the while condition to start the loop block.

2. Step 2: Identify the missing colon

   The code misses ':' after while i < 4, causing a syntax error.

3. Final Answer:

   Missing colon after while condition -> Option A

4. Quick Check:

   while condition needs ':' [OK]

Hint: Always put ':' after while condition [OK]

Common Mistakes:

• Ignoring missing colon error
• Changing variable start unnecessarily
• Moving print() outside loop incorrectly

5. You want to print all even numbers from 2 to 10 using a loop. Which code snippet correctly does this?

hard

A. for i in range(2, 11, 2):
print(i)

B. for i in range(1, 11):
if i % 2 == 0:
print(i)

C. i = 2
while i <= 10:
print(i)
i += 2

D. All of the above

Solution

1. Step 1: Analyze each code snippet

   for i in range(2, 11, 2): print(i) uses for loop with step 2 from 2 to 10.
   for i in range(1, 11): if i % 2 == 0: print(i) uses for loop with condition to print even numbers.
   i = 2 while i <= 10: print(i) i += 2 uses while loop increasing by 2 starting at 2.

2. Step 2: Confirm all snippets print even numbers 2 to 10

   All three snippets correctly print 2, 4, 6, 8, 10.

3. Final Answer:

   All of the above -> Option D

4. Quick Check:

   All methods print even numbers 2 to 10 [OK]

Hint: Multiple loop types can print even numbers correctly [OK]

Common Mistakes:

• Using wrong range end value
• Forgetting to increment in while loop
• Not checking condition for even numbers