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TIFR CSE 2017 | Part A | Question: 12

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9 9 votes

Consider the following program modifying an $n \times n$ square matrix $A$:

for i=1 to n:
    for j=1 to n:
        temp=A[i][j]+10
        A[i][j]=A[j][i]
        A[j][i]=temp-10
    end for
end for

Which of the following statements about the contents of matrix $A$ at the end of this program must be TRUE?

  1. the new $A$ is the transpose of the old $A$
  2. all elements above the diagonal have their values increased by $10$ and all the values below have their values decreased by $10$
  3. all elements above the diagonal have their values decreased by $10$ and all the values below have their values increased by $10$
  4. the new matrix $A$ is symmetric, that is, $A[i][j]=A[j][i]$ for all $1 \leq i, j \leq n$
  5. $A$ remains unchanged

1 Answer

Best answer
13 13 votes 
1.for i=1 to n:
2.    for j=1 to n:
3.        temp=A[i][j]+10
4.        A[i][j]=A[j][i]
5.        A[j][i]=temp-10
6.    end for
7.end for

The $3,4,5$ lines swap $A [ j ] [ i ]$ and $A [ i ] [ j ]$.

The same variables are swapped twice. For eg when: $i =5$, $j = 10$. $A [10] [5]$ and $A [5] [10]$ will be swapped. They will be swapped again when $i = 10$, $j =5$.

Two times swap of same elements will lead to $A$ remaining unchanged.

Hence, E is correct.

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