Nested loops in MATLAB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a nested loop that prints numbers 1 to 3 for both loops.

MATLAB

for i = 1:3
    for j = 1:[1]
        disp([i, j])
    end
end

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

A4

B2

C3

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different range for the inner loop causing fewer or more iterations.

The inner loop must run from 1 to 3 to match the outer loop and print pairs from 1 to 3.

2fill in blank

medium

Complete the code to sum all elements in a 2D matrix using nested loops.

MATLAB

total = 0;
for row = 1:size(matrix, 1)
    for col = 1:size(matrix, [1])
        total = total + matrix(row, col);
    end
end

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

A4

B1

C3

D2

Attempts:

3 left

💡 Hint

Common Mistakes

Using size(matrix, 1) for columns which is actually rows.

The second dimension of the matrix is the number of columns, which is size(matrix, 2).

3fill in blank

hard

Fix the error in the nested loops that print the multiplication table from 1 to 5.

MATLAB

for i = 1:5
    for j = 1:[1]
        fprintf('%d x %d = %d\n', i, j, i*j);
    end
end

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

A5

B3

C6

D4

Attempts:

3 left

💡 Hint

Common Mistakes

Using a smaller range in the inner loop causing incomplete output.

The inner loop must run from 1 to 5 to complete the multiplication table for numbers 1 to 5.

4fill in blank

hard

Fill both blanks to create a nested loop that fills a matrix with the product of its indices.

MATLAB

rows = 3;
cols = 4;
result = zeros(rows, cols);
for r = 1:[1]
    for c = 1:[2]
        result(r, c) = r * c;
    end
end

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

Arows

Bcols

C4

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping rows and columns in the loops.

The outer loop runs over rows, the inner loop over columns to fill the matrix correctly.

5fill in blank

hard

Fill all three blanks to create a nested loop that counts how many elements in a matrix are greater than 10.

MATLAB

count = 0;
for i = 1:[1]
    for j = 1:[2]
        if matrix(i, j) [3] 10
            count = count + 1;
        end
    end
end

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

Asize(matrix, 1)

Bsize(matrix, 2)

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong size dimensions or wrong comparison operator.

The loops iterate over all rows and columns. The condition checks if elements are greater than 10.

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