Nested loops in MATLAB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of nested loops with matrix assignment

What is the output of the following MATLAB code?

result = zeros(2,3);
for i = 1:2
    for j = 1:3
        result(i,j) = i + j;
    end
end
result

MATLAB

result = zeros(2,3);
for i = 1:2
    for j = 1:3
        result(i,j) = i + j;
    end
end
result

A [2 3 4; 3 4 5]

B [1 2 3; 2 3 4]

C [3 4 5; 4 5 6]

D [1 1 1; 1 1 1]

💡 Hint

Remember that MATLAB indices start at 1 and the loops assign i + j to each element.

The outer loop runs i from 1 to 2, inner loop runs j from 1 to 3. Each element result(i,j) is assigned i + j. So first row is [1+1, 1+2, 1+3] = [2 3 4], second row is [2+1, 2+2, 2+3] = [3 4 5].

❓ Predict Output

intermediate

Nested loops with conditional break

What is the value of variable count after running this MATLAB code?

count = 0;
for i = 1:5
    for j = 1:5
        count = count + 1;
        if i * j > 6
            break;
        end
    end
end
count

MATLAB

count = 0;
for i = 1:5
    for j = 1:5
        count = count + 1;
        if i * j > 6
            break;
        end
    end
end
count

A 20

B 25

C 16

D 10

💡 Hint

The inner loop breaks when i*j > 6, so not all inner iterations run fully. Note that count increments before the check.

count increments before the break check. i=1: j=1-5 (5*1=5<=6), 5 times. i=2: j=1-4 (2*4=8>6 after inc), 4 times. i=3: j=1-3 (3*3=9>6), 3 times. i=4: j=1-2 (4*2=8>6), 2 times. i=5: j=1-2 (5*2=10>6), 2 times. Total: 5+4+3+2+2=16.

🔧 Debug

advanced

Identify the error in nested loops

What error does this MATLAB code produce when run?

for i = 1:3
    for j = 1:3
        disp(i + j
    end
end

MATLAB

for i = 1:3
    for j = 1:3
        disp(i + j
    end
end

A SyntaxError: Missing closing parenthesis

B RuntimeError: Variable j undefined

C IndexError: Index exceeds matrix dimensions

D No error, outputs sums

💡 Hint

Check the parentheses in the disp function call.

The disp function call is missing a closing parenthesis, causing a syntax error.

🧠 Conceptual

advanced

Number of iterations in nested loops

How many times will the innermost statement run in this MATLAB code?

count = 0;
for i = 1:4
    for j = i:4
        count = count + 1;
    end
end
count

MATLAB

count = 0;
for i = 1:4
    for j = i:4
        count = count + 1;
    end
end
count

A 16

B 10

C 8

D 6

💡 Hint

The inner loop starts at i and goes to 4, so the number of iterations decreases as i increases.

For i=1, j=1..4 (4 iterations); i=2, j=2..4 (3 iterations); i=3, j=3..4 (2 iterations); i=4, j=4..4 (1 iteration). Total = 4+3+2+1=10.

🚀 Application

expert

Create a multiplication table matrix

Which MATLAB code correctly creates a 5x5 multiplication table matrix M where M(i,j) = i*j using nested loops?

A

M = zeros(5,5);
for i = 1:5
    for j = 1:5
        M(i,j) = i + j;
    end
end

B

M = zeros(5,5);
for i = 1:5
    for j = 1:5
        M(j,i) = i * j;
    end
end

C

M = zeros(5,5);
for i = 1:5
    for j = 1:5
        M(i+j) = i * j;
    end
end

D

M = zeros(5);
for i = 1:5
    for j = 1:5
        M(i,j) = i * j;
    end
end

💡 Hint

Remember MATLAB matrices are indexed by row and column, and multiplication table means product of indices.

Option D correctly initializes a 5x5 matrix and assigns M(i,j) = i*j. Option D swaps indices but produces the same matrix since multiplication is commutative. However, strictly matching M(i,j)=i*j is D. Option D uses invalid indexing M(i+j). Option D sums indices instead of multiplying.

Practice

1. What does a nested loop in MATLAB do?
for i = 1:2
for j = 1:3
disp([i, j])
end
end

easy

A. Runs one loop inside another to repeat tasks multiple times

B. Runs two loops one after another without overlap

C. Creates a single loop with combined ranges

D. Stops execution after the first loop finishes

Solution

1. Step 1: Understand the loop structure

   The outer loop runs with i from 1 to 2, and inside it, the inner loop runs with j from 1 to 3.

2. Step 2: Identify nested loop behavior

   For each i, the inner loop runs fully for all j values, repeating tasks multiple times.

3. Final Answer:

   Runs one loop inside another to repeat tasks multiple times -> Option A

4. Quick Check:

   Nested loops repeat inner loop for each outer loop iteration [OK]

Hint: Nested loops repeat inner loop fully for each outer loop step [OK]

Common Mistakes:

• Thinking loops run sequentially without nesting
• Confusing nested loops with single combined loops
• Assuming loops stop early without completing inner loop

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

easy

A. for i = 1:3
disp(i)
for j = 1:2
disp(j)
end

B. for i = 1:3
for j = 1:2
disp(i+j)
end

C. for i = 1:3
for j = 1:2
disp(i+j)
end

D. for i = 1:3
for j = 1:2
disp(i+j)
end
end

Solution

1. Step 1: Check proper loop nesting and end statements

   for i = 1:3 for j = 1:2 disp(i+j) end end correctly nests the inner loop inside the outer loop and closes both with end.

2. Step 2: Identify syntax errors in other options

   The other options miss proper indentation or have missing end statements causing syntax errors.

3. Final Answer:

   for i = 1:3 for j = 1:2 disp(i+j) end end -> Option D

4. Quick Check:

   Proper nested loops need matching end for each for [OK]

Hint: Each for loop must have its own end statement [OK]

Common Mistakes:

• Missing end for inner or outer loop
• Incorrect indentation causing confusion
• Starting loops without proper nesting

3. What is the output of this MATLAB code?

for i = 1:2
  for j = 1:2
    fprintf('%d ', i*j)
  end
  fprintf('\n')
end

medium

A. 1 2 2 4

B. 1 1
2 2

C. 1 2
2 4

D. 2 4
1 2

Solution

1. Step 1: Calculate inner loop outputs for each outer loop value

   When i=1, j=1:2, outputs are 1*1=1 and 1*2=2. When i=2, j=1:2, outputs are 2*1=2 and 2*2=4.

2. Step 2: Format output with new lines

   Each inner loop prints values with space, then outer loop prints newline. So output lines are '1 2 ' and '2 4 '.

3. Final Answer:

   1 2 2 4 -> Option C

4. Quick Check:

   Multiply i and j in nested loops for output [OK]

Hint: Multiply outer and inner loop vars for each output [OK]

Common Mistakes:

• Ignoring newline after inner loop
• Mixing up i and j values
• Printing all values in one line

4. Find the error in this nested loop code:

for i = 1:3
  for j = 1:2
    disp(i+j)
end

medium

A. Missing end for outer loop

B. Inner loop range is incorrect

C. disp function is used incorrectly

D. Loop variables i and j are not initialized

Solution

1. Step 1: Check loop endings

   The inner loop has an end, but the outer loop does not have its closing end.

2. Step 2: Identify syntax error cause

   Missing the outer loop's end causes MATLAB to throw an error.

3. Final Answer:

   Missing end for outer loop -> Option A

4. Quick Check:

   Each for loop needs its own end [OK]

Hint: Count ends: one per for loop [OK]

Common Mistakes:

• Forgetting to close outer loop
• Assuming one end closes both loops
• Misreading error messages

5. You want to create a 3x3 multiplication table using nested loops in MATLAB. Which code correctly fills matrix M with products of row and column indices?

hard

A. M = zeros(3);
for i = 1:3
for j = 1:3
M(j,i) = i+j;
end
end

B. M = zeros(3);
for i = 1:3
for j = 1:3
M(i,j) = i*j;
end
end

C. M = zeros(3);
for i = 1:3
for j = 1:3
M(i,j) = i+j;
end
end

D. M = zeros(3);
for i = 1:3
for j = 1:3
M(i,j) = i-j;
end
end

Solution

1. Step 1: Understand the multiplication table goal

   The table should store products of row and column indices, so M(i,j) = i*j.

2. Step 2: Check each option's assignment

   M = zeros(3); for i = 1:3 for j = 1:3 M(i,j) = i*j; end end correctly assigns M(i,j) = i*j. The other options assign sums or differences, not products.

3. Final Answer:

   M = zeros(3); for i = 1:3 for j = 1:3 M(i,j) = i*j; end end -> Option B

4. Quick Check:

   Multiplication table uses product of indices [OK]

Hint: Assign M(i,j) = i*j for multiplication table [OK]

Common Mistakes:

• Using addition or subtraction instead of multiplication
• Swapping indices in assignment
• Not initializing matrix before loops