GATE CSE 2008 | Question: 62

Question

The following C function takes a single-linked list of integers as a parameter and rearranges the elements of the list. The function is called with the list containing the integers $1, 2, 3, 4, 5, 6, 7$ in the given order. What will be the contents of the list after function completes execution?

struct node {
    int value;
    struct node *next;
};

void rearrange(struct node *list) {
    struct node *p, *q;
    int temp;
    if (!list || !list -> next) return;
    p = list; q = list -> next;
    while(q) {
        temp = p -> value; p->value =  q -> value;
        q->value = temp; p = q ->next;
        q = p? p ->next : 0;
    }   
}

• A. $1, 2 ,3, 4, 5, 6, 7$
• B. $2, 1, 4 ,3, 6, 5, 7$
• C. $1, 3, 2, 5, 4, 7, 6$
• D. $2, 3, 4, 5, 6, 7, 1$

Comments

Ternary operator having higher precedence executes first , then assignment happens .

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q=p i find little ambigous bcz we dont know whether theyre comparing values or addrress

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Assignment has less priority then ternary operator,  it's not ambiguous.

q is pointer so holds only address at any moment.

Answer 1

The loop is interchanging the adjacent elements of the list. But after each interchange, next interchange starts from the unchanged elements only (due to $p = q \rightarrow  next$;).

$1^{st}$ iteration: $1, 2, 3, 4, 5, 6, 7$
            $\Rightarrow  2, 1, 3, 4, 5, 6, 7$

$2^{nd}$ iteration: $2, 1, 4, 3, 5, 6, 7$

$3^{rd}$ iteration: $2, 1, 4, 3, 6, 5, 7$

$p$ pointing to $7$ and $q$ is pointing to $\textsf{NULL} (0)$, as $p$ is false hence $q=p?$ $p  \rightarrow \textsf{next}$:$0$; will return  $q=0$ ending the loop. (In C language NULL is having the value 0).

Answer is option B.

Comments

how p will point to null?

p will point to 7 and q will point to NULL (q = p->next as p is a non zero value)

Edit:
Program: http://ideone.com/T0yVty

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q = p? p ->next : 0;

Basically it is ternary operator . we can rewrite this expression in simplified form

q = (p!=NULL)? p -> next : 0;

when (p!=NULL ) true then q = p ->next will be assigned when this condition is false then q = 0 will be assigned. 

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the while(q) condition will be false when p=7 & q=q->next i.e null, the ternary operator won't make q=0(null)

I think for even length lists the ternary operator works and makes q=0

Answer 2

It is similiar to https://gateoverflow.in/3815/gate2005-it-54 

Comments

Yes, the only time when question generator algorithm went wrong :P

Answer 3

The confusing ternery operator, some clarity:

Answer 4

original-1,2,3,4,5,6,7

1st iteration-2,1,3,4,5,6,7

2nd iteration- 2,1,4,3,5,6,7

3rd iteration- 2,1,4,3,6,5,7

after this step p pointing to 7  and q pointing to NULL.

at last ternary operator execute -  q = p->next (as p hold address of 7)  i.e q= NULL

while loop terminate as q=NULL
so answer is B

Answer 5

Here is the idea :

Answer 6

.