# MATLAB Questions and Answers – Sparse Matrices – 2

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This set of MATLAB Multiple Choice Questions & Answers focuses on “Sparse Matrices – 2”.

1. What is the output of the following code?

`A=[0 0 0; 0 9 0; 1 2 3]; nnz[A]`

a) 4
b) 5
c) 3
d) Error

Explanation: There is syntactical error while giving input to the nnz command. The input to the nnz command should be within parentheses and here the input is within []. If the input was given within (), the output would’ve been 4.
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2. What is the output of the following code?

`A=[1 2 3; 32 23 26; 0 0 0]; spones(A)`

a) Returns a sparse matrix with the non-zeros replaced by normally distributed random numbers
b) Returns a sparse matrix with the zeros replaced by ones
c) Returns a sparse matrix with the non-zeros replaced by fractions
d) Returns a sparse matrix with the non-zeros replaced by random numbers

Explanation: The spones command returns a sparse matrix with ones replaced by zeros. Hence, Returns a sparse matrix with the zeros replaced by ones is correct while rest of the options are incorrect.
Output:
ans =
(1,1) 1
(2,1) 1
(1,2) 1
(2,2) 1
(1,3) 1
(2,3) 1
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3. The space located for the matrix generated from the spones command is _______
a) Same as a sparse matrix
b) Same as the original matrix
c) Same as an identity matrix
d) Double that of the sparse matrix

Explanation: The spones command also creates a sparse matrix. Hence, the newly generated matrix will have the same size as that of a sparse matrix.

4. What is the output of the following code?

`A=[1 2 3; 4 5 6; 7 8 9];if( nzmax(A)==nzmax(spones(A) ) disp(‘Yeah !’)`

a) No output
b) Error
c) Yeah !
d) Output suppressed

Explanation: There is no error in the above code. Since, the if command is not ended, no output will be generated. If the if command was ended, the output would’ve been Yeah !.

5. What is the output of the following code?

`nnz(spconvert([1 2 3; 4 5 6; 7 8 9])`

a) 3
b) 2
c) 1
d) 6

Explanation: The spconvert command converts the original matrix into a sparse matrix by only keeping memory for the elements in the columns. Since in the above matrix, there are no non-zero elements- the total space occupied by the new matrix is 3.
Output: 3

6. What is the output of the following code?

`nzmax(spconvert([1 2 3; 4 5 6; 7 8 9])`

a) 2
b) 3
c) 1
d) Error

Explanation: The matrix generated by the spconvert command consists of non-zero elements from the columns of the original matrix. Hence, 3 is correct.
Output: 3

7. A memory for sparse matrix is dedicated by the ______ command.
a) spalloc
b) sparsealloc
c) allocspar
d) no such command

Explanation: The spalloc command is used to allocate memory for a sparse matrix. The rest of the commands are incorrect.

8. What is the output of the following command?

`spalloc(2,3, 7)`

a) A 2*3 sparse matrix
b) Memory is allocated for a 2*3 sparse matrix
c) A 3*2 sparse matrix
d) Error

Explanation: The spalloc command will allocate memory for 2*3 matrix due to the above code. There will be no error.

9. The default number of non-zero elements which can be put into the memory allocated by the spalloc command is > 1.
a) True
b) False

Explanation: Even only 1 element can be put into the memory which has been already allocated by the spalloc command. Hence, the above statement is false.

10. The pattern generated by the spy command is a measure of the number of zeros in the input matrix.
a) True
b) False

Explanation: It is actually a pattern of the number of non-zero elements in the input matrix. Hence, the above statement is false.

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