while(pointer!=NULL && pointer->next!=NULL) vs while(pointer->next!=NULL && pointer!=NULL) in C++

Question

I was trying to detect a cycle in a linked list. When I put the condition while(pointer!=NULL && pointer->next!=NULL), it worked all fine, gave all the desirable outputs. But when I inverted the condition, it stopped showing output when no cycle exists. Like when a cycle exists its giving true as output, but when it doesn't exist its giving no outputs. So, I'm wondering what could the reason be behind it.

Answer 1

Because the conditional is evaluated left-to-right. In your statement, 'pointer' and 'pointer->next' are values that you check against 'NULL'.

However, notice that accessing 'next' from 'pointer' requires that pointer itself be non-null. Trying to access an attribute from a null value results in undefined behaviour, which is what happened in your program

Thus, 'pointer' has to come before 'pointer->next' in the conditional statement. And, due to the use of the '&&' operator, the conditional is exited immediately if 'pointer' is null-checked first.

Answer 2

The benefit of && operator is that, wherever the condition gives first false value, the condition is not checked any further.

#include <iostream>
#include <stdlib.h>
using namespace std;

int main() {
    struct ll{
        int value;
        struct ll* next;
    };
    struct ll* pointer;
    
    while(pointer != NULL && pointer->next != NULL){
        cout<< "pointer value:" << pointer <<endl;
    }

    return 0;
}

Note:In the above program, I have not allocated space to the pointer variable.

while(pointer!=NULL && pointer->next!=NULL)

Since the first condition returned false so further part of condition is not checked.

#include <iostream>
#include <stdlib.h>
using namespace std;

int main() {
    struct ll{
        int value;
        struct ll* next;
    };
    struct ll* pointer;
    
    while(pointer->next != NULL && pointer != NULL){
        cout<< pointer->next << endl;
    }

    return 0;
}

Note:In the above program, I have not allocated space to the pointer variable.

while(pointer->next != NULL && pointer != NULL)

Here first condition is Invalid for comparision. So the program dumps here causing mostly segmentation fault.