Logic / Precedence Question...

[tazatek]

Does C# evaluate the second half of a logic equation if the first half is false in a question like:


if ( A==B) && (C==D) )
{

}

If A !=B , would C==D even be evaluated?


I'm guessing that this would be like other languages, but thought I'd ask just to be sure...

Thanks

Matt

[eDanny]

Both must be true in your example.

[tazatek]

Originally Posted by eDanny

Both must be true in your example.

yes

but will they both be *evaluated*

better example...

bool isfalse() { return false;}
bool istrue() { return true; }

if ( isfalse() && istrue() )
{ .... }

The question is - will istrue() ever get called/evaluated since isfalse() kills the whole logic set....

Matt

[eDanny]

To not possibly evaluate the second half you would need to use this instead:

if ( A==B) || (C==D) )

If you are using &&, both sides need to be evaluated to determine if both are true.

Dan

[tazatek]

Originally Posted by eDanny

To not possibly evaluate the second half you would need to use this instead:

if ( A==B) || (C==D) )

If you are using &&, both sides need to be evaluated to determine if both are true.

Dan

Now you have me second guessing myself, so let me talk outloud for a minute...


Because && is a logical AND it means that both sides must be true for the whole thing to be true... if one side is false, then the whole thing is false, so only one half has to be false for the entire equation to be false, right?

And for || it is a logical OR (not to be confused with an exclusive OR) that needs only one (or both) of the sides to evaluate to true to have a true expression

So I think my logic is right that it should be an AND and not an OR -- don't confuse the english and math operators in that

But what I really didn't know was if C# was smart enough to know when to stop evaluation.... or if it would compute both halves and figure it out later.


BTW - I just re-read your statement, and realized that you were speaking of the TRUE condition, and I was speaking of the FALSE

I think we're on the same page - but in the FALSE condition, will it stop when the first argument is FALSE in an AND statement?

[eDanny]

Yes you are correct (at least I believe so) and I was speaking of the 'true' conditions.

[mountainclimber]

I think you have the idea, but the syntax is a little off...

if((A==B) || (C==D))
{
//do action 1
}

So if A=B or C=D then you will //do action 1. You could do the same thing if you did this:

if(A==B)
{
//do action 1
}

if(C==D)
{
//do action 1
}

Of course the first one is better coding...just showing you this to illustrate.

However, if the code at the very top had && instead of || then it would be the same as:

if(A==B)
{
if(C==D)
{
//do action 1
}
}

Hope that helps...