C - while (something || something)

I have a quick question about while loop.

I want to read ASCII char code and if it's not some particular ascii code I don't want to continue and ask user to enter it again.

This works

    while (yn != 89)
开发者_StackOverflow社区    {
        printf("\nEnter");
        flushall();
        scanf("\n%c", &yn);
    }

This does not

    while (yn != 89 || yn != 121)
    {
        printf("\nEnter");
        flushall();
        scanf("\n%c", &yn);
    }

Gosh, I think I need to get some sleep. This was rather... stupid. :) Thanks guys BTW.

The expression yn != 89 || yn != 12 is always true. Any value is either not equal to 89 or not equal to 12. So, this expression makes no practical sense. The cycle will loop forever, since the repetition condition is always true.

According to your description, the condition you need is yn != 89 && yn != 12. &&, not ||.

Your boolean logic is a bit off:

while (yn != 89 && yn != 121)

Another way to write this, which may be more logical as a "reader":

while (!(yn == 89 || yn == 121))

Replace || with &&.

The reason:

!(yn == 89 || yn == 121) <==> yn != 89 && yn != 121 (DeMorgan)

When yn is 89 it is not 121; similarly when yn is 121, it is not 89; thus, the condition is always satisfied.

Perhaps you mean while yn is not either 89 or 121, that's phrased with an and:

while(yn != 89 && yn != 121)

or alternatively you can say "when yn is neither 89 or 121":

while( !(yn == 89 || yn == 121) )

To expand a little on the other replies - you will never get to your second condition. The first "short-circuits"

if (yn != 89

And stop right there. If it's not 89, we're done. We don't care about anything after that in the if because || (OR) is "If either of these is the case". And of course, if it IS 89 ... then it's certainly not 121.

You want while (yn != 89 && yn != 121)

The inverse of (A || B) is (!A && !B) and the inverse of (A && B) is (!A || !B). Your thought process in constructing the original loop was something like, "I want yn == 89, so I should invert that to loop while yn != 89." Then you decided to change it to, "I want yn == 89 || yn == 121, so I need to invert that." That's where you went wrong: From the forms I presented originally you can see the inverse (to use as your while condition) is yn != 89 && yn != 121. You simply extended your original, simple inversion with || inappropriately.