or problem

[Weighbridge]

Hi

I am trying to get around the use of "||".
In the below code, the "while" line is giving me problems.

#include <iostream>

using namespace std;

int main()
{
    int input_1 = 0;

    do
    {
        cout << "List 1 \n List 2 \n List 3 \n";
        cin >> input_1;
    }
     
     while (input_1 != 1 || input_1 != 2 || input_1 != 3);

        cout << "Complete \n";


return 0;

}

[Peter87]

|| returns false iff both operands are false.

If input_1 != 1 is true the whole condition will be true. We don't even have to look at the other two.

If input_1 != 1 is false we can be sure that input_1 != 2 is true so the whole expression will be true here too.

So, the loop condition will always be true.

Take a look at &&. It is probably what you want to use here.

[vlad from moscow]

Sometimes it is useful to apply the mathematical logic. Let consider inverse condition

! (input_1 != 1 || input_1 != 2 || input_1 != 3)

It is equivalent to

(input_1 == 1 && input_1 == 2 && input_1 == 3)

It is obvious that this condition is always equal to false that means that the original condition is always equal to true.

I think this is not what you wanted. Maybe you wanted another condition namely

(input_1 != 1 && input_1 != 2 && input_1 != 3)

It can be rewritten as

( input_1 < 1 || input_1 > 3 )

[Weighbridge]

Thanks Peter87 and Vlad from moscow.

I can see I am not using it right.

I will have to mull this over for a while.

Dan