Nested for loop execution in Python - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to print numbers from 1 to 3 using a for loop.

Python

for i in [1]:
    print(i)

Drag options to blanks, or click blank then click option'

A[1, 2]

Blist(3)

C3

Drange(1, 4)

Attempts:

3 left

💡 Hint

Common Mistakes

Using a single number like 3 instead of a range.

Using a list with fewer numbers than needed.

The range(1, 4) function generates numbers 1, 2, and 3, which the loop prints.

2fill in blank

medium

Complete the code to print all pairs of numbers from 1 to 2 using nested loops.

Python

for i in range(1, 3):
    for j in [1]:
        print(i, j)

Drag options to blanks, or click blank then click option'

A[1, 2, 3]

Brange(0, 2)

Crange(1, 3)

Drange(3, 5)

Attempts:

3 left

💡 Hint

Common Mistakes

Using a range that starts at 0 or goes beyond 2.

Using a list with incorrect numbers.

The inner loop uses range(1, 3) to match the outer loop's range, printing pairs (1,1), (1,2), (2,1), and (2,2).

3fill in blank

hard

Fix the error in the nested loop to print all pairs of letters and numbers.

Python

letters = ['a', 'b']
numbers = [1, 2]
for letter in letters:
    for num in [1]:
        print(letter, num)

Drag options to blanks, or click blank then click option'

Arange(1, 3)

Bnumbers

Cletters

Dletter

Attempts:

3 left

💡 Hint

Common Mistakes

Using the outer loop variable letter inside the inner loop.

Using the wrong list like letters again.

The inner loop should iterate over the numbers list to pair each letter with each number.

4fill in blank

hard

Fill both blanks to create a nested loop that prints all combinations of colors and sizes.

Python

colors = ['red', 'blue']
sizes = ['S', 'M']
for [1] in colors:
    for [2] in sizes:
        print(color, size)

Drag options to blanks, or click blank then click option'

Acolor

Bsize

Citem

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same variable name for both loops.

Using generic names like item or value that don't match the print statement.

The outer loop variable is color and the inner loop variable is size to match the lists and print correctly.

5fill in blank

hard

Fill all three blanks to create a nested loop that builds a dictionary with keys as colors and values as sizes.

Python

colors = ['red', 'green']
sizes = ['L', 'XL']
combo_dict = {}
for [1] in colors:
    for [2] in sizes:
        combo_dict[[3]] = size
print(combo_dict)

Drag options to blanks, or click blank then click option'

Acolor

Bsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong variable as dictionary key.

Mixing up variable names in the assignment.

The outer loop variable is color, the inner loop variable is size, and the dictionary key is color to store the last size for each color.

Practice

1. What does a nested for loop do in Python?
for i in range(2):
for j in range(3):
print(i, j)

easy

A. Runs the inner loop fully for each outer loop step

B. Runs both loops only once

C. Runs the outer loop fully for each inner loop step

D. Runs only the outer loop

Solution

1. Step 1: Understand nested loop structure

   The outer loop runs 2 times (i=0,1). For each i, the inner loop runs 3 times (j=0,1,2).

2. Step 2: Observe loop execution order

   For each i, the inner loop completes all its iterations before the outer loop moves to next i.

3. Final Answer:

   Runs the inner loop fully for each outer loop step -> Option A

4. Quick Check:

   Inner loop runs fully per outer loop [OK]

Hint: Inner loop runs completely inside each outer loop step [OK]

Common Mistakes:

• Thinking outer loop runs inside inner loop
• Assuming loops run only once
• Confusing loop order

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

easy

A. for i in range(3): for j in range(2): print(i, j)

B. for i in range(3):
for j in range(2):
print(i, j)

C. for i in range(3):
for j in range(2):
print(i, j)

D. for i in range(3):
for j in range(2):
print(i, j)

Solution

1. Step 1: Check indentation rules

   Python requires consistent indentation to define nested blocks. Inner loop must be indented inside outer loop.

2. Step 2: Identify correct indentation

   for i in range(3): for j in range(2): print(i, j) uses 4 spaces indentation for inner loop and print statement, which is correct.

3. Final Answer:

   for i in range(3):\n for j in range(2):\n print(i, j) -> Option B

4. Quick Check:

   Proper indentation = for i in range(3): for j in range(2): print(i, j) [OK]

Hint: Indent inner loop inside outer loop with spaces [OK]

Common Mistakes:

• Missing indentation for inner loop
• Incorrect indentation for print
• Writing loops on same line without colon

3. What is the output of this code?

for i in range(2):
    for j in range(2):
        print(i + j, end=' ')
    print()

medium

A. 0 1 \n0 1

B. 1 2 \n2 3

C. 0 1 1 2

D. 0 1 \n1 2

Solution

1. Step 1: Calculate inner loop sums for i=0

   j=0: 0+0=0, j=1: 0+1=1 -> prints '0 1 '

2. Step 2: Calculate inner loop sums for i=1

   j=0: 1+0=1, j=1: 1+1=2 -> prints '1 2 '

3. Final Answer:

   0 1 \n1 2 -> Option D

4. Quick Check:

   Sum of i and j per line = 0 1 \n1 2 [OK]

Hint: Add i and j for each inner loop iteration [OK]

Common Mistakes:

• Ignoring print() creates new line
• Adding values incorrectly
• Confusing end=' ' usage

4. Find the error in this nested loop code:

for i in range(3):
for j in range(2):
    print(i, j)

medium

A. print statement should be outside loops

B. Missing colon after inner for loop

C. Inner loop not indented inside outer loop

D. range(2) should be range(3)

Solution

1. Step 1: Check indentation of inner loop

   Inner for loop must be indented inside outer loop to be nested properly.

2. Step 2: Identify correct indentation

   Here, inner loop is at same level as outer loop, causing IndentationError.

3. Final Answer:

   Inner loop not indented inside outer loop -> Option C

4. Quick Check:

   Indent inner loop inside outer loop [OK]

Hint: Indent inner loop under outer loop to fix error [OK]

Common Mistakes:

• Not indenting inner loop
• Misplacing colons
• Wrong range values

5. How many times will the print statement execute in this nested loop?

for i in range(4):
    for j in range(3):
        for k in range(2):
            print(i, j, k)

hard

A. 24

B. 12

C. 36

D. 9

Solution

1. Step 1: Calculate iterations of each loop

   Outer loop: 4 times (i=0..3), middle loop: 3 times (j=0..2), inner loop: 2 times (k=0..1).

2. Step 2: Multiply iterations for total prints

   Total = 4 * 3 * 2 = 24 print executions.

3. Step 3: Re-check options

   36 says 36, but calculation is 24, so correct is 24.

4. Final Answer:

   24 -> Option A

5. Quick Check:

   4*3*2 = 24 prints [OK]

Hint: Multiply all loop ranges for total executions [OK]

Common Mistakes:

• Adding instead of multiplying loop counts
• Counting only outer loops
• Misreading loop ranges