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GO Classes Test Series 2024 | Mock GATE | Test 12 | Question: 27

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Consider the following C program:-

#include <stdio.h>
void ubswap(int **a, int **b) {
    int* temp = *a;
    *a = *b;
    *b = temp;
}
int main() {
    int x = 1, y = 9;
    int* u = &x;
    int* v = &y;
    int** a = &u;
    int** b = &v;
    
    ubswap(a, b);
    return 0;
}

Which pair of variables in the main function are swapped immediately after the function call  $\textsf{“ubswap(a, b)”}$ and before $\text{“return 0”}?$

  1. $x$ and $y$
  2. $u$ and $v$
  3. $a$ and $b$
  4. None of the above
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1 Answer

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7 votes

See this is easy but tricky as well.

x is a variable which contains value 1 let say the address of x is 100 .

y is a variable which contains value 9 let say the address of y is 200 .

Now u and v are single pointer pointing to addresses 100 and 200 respectively. And assume the address of u is 300 and v is 400.

And a and b are double pointer pointing to addresses 300 and 400 respectively.And assume the address of a is 500 and b is 600.

Now ubswap() function is called which passes the two double pointers a and b.

Note: a is double pointer , so *a will be a single pointer.

int *temp = *a ;

This line mean temp is a single pointer which pointing to that location for which *a is pointing to .

And we know that *a or just say u , which is single pointer pointing to address location 100 which is nothing but address of x.

Similarly, *b or just say v , which is single pointer  pointing to address location 200 which is nothing but address of y.

So basically temp is pointing to x.

*a = *b ;

Means now *a or u starts pointing to y.

*b = temp;

Now *b or v is pointing to x;

And now return.

If we see carefully u and v pairs of variables got swapped as initially u was pointing to x but now it is pointing to y and v intially was pointing to y but now to x.

So, Option B is correct.

Hope, this helps.

 

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