Matrix concatenation in MATLAB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of horizontal concatenation?

Consider the following MATLAB code:

A = [1 2; 3 4];
B = [5 6; 7 8];
C = [A B];
disp(C);

What will be displayed?

MATLAB

A = [1 2; 3 4];
B = [5 6; 7 8];
C = [A B];
disp(C);

A [1 2 5 6; 3 4 7 8]

B [1 2 3 4; 5 6 7 8]

C [6 8; 10 12]

D [1 2; 3 4; 5 6; 7 8]

💡 Hint

Horizontal concatenation joins matrices side by side, so columns add up.

The code concatenates matrices A and B horizontally, so the columns of B are appended to the columns of A, resulting in a 2x4 matrix.

❓ Predict Output

intermediate

What is the output of vertical concatenation?

Given the MATLAB code:

A = [1 2; 3 4];
B = [5 6; 7 8];
C = [A; B];
disp(C);

What will be displayed?

MATLAB

A = [1 2; 3 4];
B = [5 6; 7 8];
C = [A; B];
disp(C);

A [1 2 5 6; 3 4 7 8]

B [1 2; 3 4; 5 6; 7 8]

C [6 8; 10 12]

D [1 2; 3 4; 1 2; 3 4]

💡 Hint

Vertical concatenation stacks matrices on top of each other, so rows add up.

The code concatenates matrices A and B vertically, so the rows of B are appended below the rows of A, resulting in a 4x2 matrix.

❓ Predict Output

advanced

What error occurs with mismatched dimensions in horizontal concatenation?

What happens when you run this MATLAB code?

A = [1 2; 3 4];
B = [5 6 7];
C = [A B];

MATLAB

A = [1 2; 3 4];
B = [5 6 7];
C = [A B];

A C = [1 2 5 6 7; 3 4 5 6 7]

B C = [6 8 7; 10 12 7]

C Error: Dimensions of matrices being concatenated are not consistent.

D C = [1 2; 3 4; 5 6; 7]

💡 Hint

Horizontal concatenation requires the same number of rows.

Matrix A has 2 rows, but B has 1 row. MATLAB cannot concatenate horizontally matrices with different row counts, so it throws a dimension mismatch error.

❓ Predict Output

advanced

What is the output of nested concatenation?

What does this MATLAB code output?

A = [1 2];
B = [3 4];
C = [5 6];
D = [A; B C];
disp(D);

MATLAB

A = [1 2];
B = [3 4];
C = [5 6];
D = [A; B C];
disp(D);

A Error: Dimensions of matrices being concatenated are not consistent.

B [1 2; 3 4; 5 6]

C [1 2; 3 4 5 6]

D [1 2 3 4 5 6]

💡 Hint

Check the sizes of B and C before concatenation.

B is 1x2 and C is 1x2, so [B C] is 1x4. A is 1x2. Vertical concatenation [A; B C] requires same number of columns, but 2 != 4, so MATLAB throws an error.

🧠 Conceptual

expert

How many elements are in the resulting matrix after concatenation?

Given these matrices:

A = ones(3,4);
B = zeros(3,2);
C = [A B];

How many elements does matrix C contain?

MATLAB

A = ones(3,4);
B = zeros(3,2);
C = [A B];

A 24

B 20

C 12

D 18

💡 Hint

Count rows and columns after concatenation, then multiply.

A is 3x4, B is 3x2. Horizontal concatenation results in 3x(4+2) = 3x6 matrix. Number of elements = 3*6 = 18.