For loop execution model in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of nested for loops with break

What is the output of this Python code?

Python

result = []
for i in range(3):
    for j in range(3):
        if j == 1:
            break
        result.append((i, j))
print(result)

A [(0, 0), (1, 0), (2, 0)]

B [(0, 0), (0, 1), (1, 0), (1, 1), (2, 0), (2, 1)]

C [(0, 0), (0, 1), (0, 2), (1, 0), (1, 1), (1, 2), (2, 0), (2, 1), (2, 2)]

D [(0, 0), (1, 0), (2, 0), (2, 1)]

💡 Hint

Remember that 'break' stops the inner loop only.

The inner loop breaks when j == 1, so only j = 0 is appended for each i. The outer loop continues for i = 0, 1, 2.

❓ Predict Output

intermediate

For loop with else clause output

What will be printed by this code?

Python

for num in range(3):
    print(num)
else:
    print('Done')

A Done

B

0
1
2

C

0
1
2
Done

D

0
1
Done

💡 Hint

The else block runs if the loop completes normally.

The else block runs after the loop finishes all iterations without a break.

❓ Predict Output

advanced

Effect of modifying loop variable inside for loop

What is the output of this code?

Python

nums = [1, 2, 3]
for n in nums:
    n += 10
print(nums)

A [1, 2, 3]

B [11, 12, 13]

C [1, 12, 3]

D TypeError

💡 Hint

Changing the loop variable does not change the list elements.

The variable n is a copy of each element. Modifying n does not affect the original list.

❓ Predict Output

advanced

Loop with continue and else behavior

What will this code print?

Python

for i in range(3):
    if i == 1:
        continue
    print(i)
else:
    print('Finished')

A

0
1
2
Finished

B

0
1
2

C

0
2

D

0
2
Finished

💡 Hint

Continue skips the current iteration but does not stop the loop.

The loop skips printing 1 but continues. The else runs after the loop ends.

🧠 Conceptual

expert

Understanding for loop iteration and variable scope

Consider this code snippet. What is the value of i after the loop finishes?

Python

for i in range(5):
    pass
print(i)

A NameError

B 4

C 0

D 5

💡 Hint

The loop variable keeps the last value it had in the loop.

After the loop ends, i holds the last value from range(5), which is 4.

Practice

1. What does a for loop do in Python?

easy

A. Repeats a block of code for each item in a sequence

B. Runs code only once

C. Stops the program immediately

D. Creates a new variable

Solution

1. Step 1: Understand the purpose of a for loop

   A for loop runs code repeatedly for each item in a list, string, or range. 'Repeats a block of code for each item in a sequence' matches this; others do not.

2. Final Answer:

   Repeats a block of code for each item in a sequence -> Option A

3. Quick Check:

   For loop = repeat for each item [OK]

Hint: For loops repeat actions for every item in a list [OK]

Common Mistakes:

• Thinking for loops run only once
• Confusing for loops with if statements
• Believing for loops stop the program

2. Which of the following is the correct syntax for a for loop in Python?

easy

A. for (i=0; i<5; i++) print(i)

B. for i to range(5) print(i)

C. for i in range(5): print(i)

D. foreach i in range(5): print(i)

Solution

1. Step 1: Recall Python for loop syntax

   Python uses 'for variable in sequence:' followed by indented code. 'for i in range(5): print(i)' matches; others use wrong keywords or styles.

2. Final Answer:

   for i in range(5): print(i) -> Option C

3. Quick Check:

   Python for loop = for variable in sequence: [OK]

Hint: Use 'for variable in sequence:' syntax in Python [OK]

Common Mistakes:

• Using 'to' instead of 'in'
• Using C-style for loop syntax
• Using 'foreach' which is not Python

3. What is the output of this code?

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

medium

A. 2 4 6

B. 0 1 2

C. 1 2 3

D. 0 2 4

Solution

1. Step 1: Trace the loop range and output

   range(3) gives i=0,1,2. Prints i*2: 0, 2, 4.

2. Final Answer:

   0 2 4 -> Option D

3. Quick Check:

   Multiply each i by 2 = 0 2 4 [OK]

Hint: Multiply loop variable by 2 for each iteration [OK]

Common Mistakes:

• Printing i instead of i*2
• Starting count from 1 instead of 0
• Confusing range(3) with range(1,3)

4. Find the error in this code:

for i in range(5)
    print(i)

medium

A. Indentation error on print line

B. Missing colon ':' after range(5)

C. range(5) is invalid syntax

D. Variable i is not defined

Solution

1. Step 1: Check for syntax errors in for loop

   Python requires ':' after the for statement. The code misses ':' after range(5).

2. Final Answer:

   Missing colon ':' after range(5) -> Option B

3. Quick Check:

   For loop needs ':' after header [OK]

Hint: Always add ':' after for loop header [OK]

Common Mistakes:

• Forgetting colon ':' after for statement
• Incorrect indentation
• Assuming range(5) is invalid

5. You want to create a list of squares for numbers 0 to 4 using a for loop. Which code correctly does this?

hard

A. squares = []
for i in range(5):
squares.append(i*i)
print(squares)

B. squares = [i*i for i in range(5)]
print(squares)

C. squares = []
for i in range(5):
squares = i*i
print(squares)

D. for i in range(5):
squares.append(i*i)
print(squares)

Solution

1. Step 1: Identify correct code for list of squares using for loop

   Initialize squares = [], then for i in range(5): append i*i, then print. squares = [] for i in range(5): squares.append(i*i) print(squares) works; others miss init, use comp, or overwrite.

2. Final Answer:

   squares = []\nfor i in range(5):\n squares.append(i*i)\nprint(squares) -> Option A

3. Quick Check:

   Initialize list, append squares in loop [OK]

Hint: Initialize list before loop, append inside loop [OK]

Common Mistakes:

• Not initializing list before appending
• Overwriting list variable inside loop
• Using list comprehension but question asks for for loop