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.