GATE CSE 2004 | Question: 2,ISRO2017-54

Question

Consider the following function

void swap(int a, int b)  
{       
    int temp;
    temp = a;
    a = b;
    b = temp;  
}  

In order to exchange the values of two variables $x$ and $y$.

• A. call $swap(x, y)$

• B. call $swap(\&x, \&y)$

• C. $swap (x, y)$ cannot be used as it does not return any value

• D. $swap (x, y)$ cannot be used as the parameters are passed by value

Comments

One straight point is to be able to swap the  original values in the function, It should receive them as pointer variables but here we are just receiving them directly as values so its not possible to swap

Answer 1

ans (D).

Option A will not exchange the values of $x$ and $y$ because parameters are passed by value in C. i.e., the code is exchanging $x'$ and $y'$ which are having the values of $x$ and $y$ respectively. 

Option B will not swap the value

 void swap(int a, int b) 

Here, it is wrong to pass in address (int*) as the parameters are of int type, even sizeof int and int* varies depending on the compiler. Now, even if ignoring this error, the given code would not exchange the values of $x$ and $y$ as it is merely exchanging $p_1'$ and $p_2'$ where $p_1'$ and $p_2'$ are containing the copies of the addresses of $x$ and $y$ respectively. (Even addresses are passed by value in C language).

Option C is false, return value is not required for exchanging the variables.

Option D is correct. We cannot use $swap(x,y)$ because parameters are passed by value. Only way now to exchange the variables are by passing their addresses and then modifying the contents using the de-referencing operator $(*).$

Comments

This Might Give Clear Understanding

Answer 2

I think, option D will be answer.

Because in the given function, parameters are used as passed by value.So there will be no change in actual parameters which is defined under the main function..

Comments

Yes option D should be correct.

Answer 3

Correct answer: Option D, swap(x,y)swap(x,y) cannot be used as the parameters are passed by value.

a) call swap (x, y) will not cause any effect on x and y as parameters are passed by value.
b) call swap (&x, &y) will no work as function swap() expects values, not addresses (or pointers).
c) swap (x, y) cannot be used but the reason given is not correct.

Answer 4

Pass by value: In this approach, we pass the copy of actual variables in function as a parameter. Hence any modification on parameters inside the function will not reflect in the actual variable.

Therefore to exchange the values of x and y, we cannot use pass by value( given code is for pass by value). So option D is correct

Answer 5

Ans: D swap (x, y) cannot be used as the parameters are passed by value