GATE IT 2008 | Question: 50

Question

Consider the C program below. What does it print?

# include <stdio.h>
# define swapl (a, b) tmp = a; a = b; b = tmp
void swap2 ( int a, int b)
{
        int tmp;
        tmp = a; a = b; b = tmp;
 }
void swap3 (int*a, int*b)
{
        int tmp;
        tmp = *a; *a = *b; *b = tmp;
}
int main ()
{
        int num1 = 5, num2 = 4, tmp;
        if (num1 < num2) {swap1 (num1, num2);}
        if (num1 < num2) {swap2 (num1 + 1, num2);}
        if (num1 > = num2) {swap3 (&num1, &num2);}
        printf ("%d, %d", num1, num2);
}

• A. $5, 5$
• B. $5, 4$
• C. $4, 5$
• D. $4, 4$

Answer 1

Answer is C.

Only:

if (num1 > = num2) {swap3 (&num1, &num2);}

Statement works, which in turn swaps num1 and num2.

Comments

The following code might help someone understand the swapping using macro expansion

https://onlinegdb.com/si4b0N8Ec

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Hi, could you kindly give a detailed explanation ? I could not understand the solution. Not even from the comments am I clear.

5>4. So 1^st condition is true !. Macro swaps so the 2^nd cond is also true ! swap2 only changes adress so no change. 3^rd cond is true and finally this gives us (5,5)…..

I understand my answer is wrong.
Could you kindly correct my mistakes?

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Bhai,I am not getting this swapping by macro

How it is been performed???

Answer 2

In main program num1=5, num2=4

first condition true. num1=4, num2=5

Now, second condition become true but no affect on values still num1=4, num2=5

third condition fail.

Ans: 4,5

Comments

What? please see the question again.

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yeah, it's right. the swap1( ) function does not create new variables for a and b. so it changes the same a and b. so they become 4 and 5.

swap2( ) function cant change the value of a and b because it is called by value and create different a and b.

the third condition does not get satisfied. so values remain as 4 and 5 for a and 5 respectively.

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This is wrong!

First if-condition is not true. Only last one is.

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You are wrong bro, only swap3 will be satisfied not swap1 or swap2 if statement. You can run this code on gcc c compiler with/without swap3 if statement..

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Can you please explain why 1st condition is not true ?

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Is 5<4???? @KunuSwavik

Answer 3

the swap1( ) function does not create new variables for a and b and temp. so it changes the same a and b. so they become 4 and 5.

swap2( ) function cant change the value of a and b because it is called by value and create different a and b.

the third condition does not get satisfied. so swap3( ) is not called and values remain as 4 and 5 for a and 5 respectively.

Comments

Swap 1 is executing before the birth of a,b ... Then how it changes the *same a and b* ?

Answer 4

Here is my approach: