(y >= 10) || (x++ > 10) why x=10 here ?

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Suppose x=10 and y=10 what is x after evaluating the expression (y >= 10) || (x++ > 10).

i tried it in c++ compiler and the answer is = 10; but the expected value of x to be 11 .
could you give me the reasons why 10 with simple trace

Posted 6-Jan-14 5:35am

progahmed

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CPallini 6-Jan-14 14:16pm

The manual of the wise C/C++ programmer says: "avoid expressions producing multiple results".
Of course the Klingon developer ignores that.

4 solutions

Solution 4

In your case first condition itself is true so || operator right side expression will not be executed.

Posted 7-Jan-14 3:50am

rammohan503

Solution 5

if || or && operator between expressions then exprss. executed from left to right. so for || operator if first express return true value then it will not execute remaining express. where it got true value return from theire.

so as (y>=10) return true .it will not go for (++x>=10)

Posted 7-Jan-14 19:38pm

Suk@nta

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Manfred Rudolf Bihy · Accepted Answer · 2014-01-06T05:54:00

Solution 2

There is a feature called short circuit evaluation[^] in some languages. So if you are using side effects[^] in boolean conditions you are in for some unexpected fun.

In your case this happens:

Left term of OR-expression: (y >= 10)
Right term of OR-expression: (x++ > 10)

If the left term is already true, one doesn't even have to evaluate the right hand term of the OR-expression since true || true is true the same as true || false. So it doesn't matter and it would be a superfluous operation. Thus x++ is never evaluated in this scenario and x will thus still be ten. :D
The same holds true for AND expressions if the left-hand side of the and expression is false there's no need to evaluate the right-hand side since false && true is false the same as false && false.

So it's quite easy to answer when one knows about short circuit evaluation. One should be aware that mixing side effects with boolean expressions may introduce you to some headscratching later on. ;P

Regards,

— Manfred

Posted 6-Jan-14 5:54am

Manfred Rudolf Bihy

Updated 7-Jan-14 19:28pm

v5

Comments

Ron Beyer 6-Jan-14 11:54am

I missed that part too, +5.

Manfred Rudolf Bihy 6-Jan-14 12:01pm

I got bitten by that bug once and it took me some time to sort it out. So it kind of sticks now. :D
I always try not to mix side effects with boolean expressions, but there's always some exception that makes it easier to solve if you break that rule you otherwise so adamantly adhere to. ;)

CHill60 6-Jan-14 14:13pm

+5. Good spot and excellent advice

CPallini 6-Jan-14 14:14pm

Good catch, 5++.

Maciej Los 6-Jan-14 16:02pm

+5

Manfred Rudolf Bihy 6-Jan-14 18:33pm

Thanks!

[no name] 6-Jan-14 18:28pm

Very good answer. (x++ > 10)||(y >= 10) gives a different result demonstrating again why this approach is a bad idea.

Manfred Rudolf Bihy 6-Jan-14 18:33pm

Thank you!

progahmed 7-Jan-14 4:35am

x will be 11 after the evaluation of (x++ > 10)||(y >= 10)

[no name] 7-Jan-14 4:47am

Correct - and your point is?

progahmed 7-Jan-14 4:49am

the same thing : short circuit evaluation

Stefan_Lang 7-Jan-14 6:50am

Actually short curcuit evaluation is not just an option, it is a rule for C/C++: that is why code like if(p!=0 && p->data!=0) is so widespread and always works.

Manfred Rudolf Bihy 7-Jan-14 7:59am

"Actually short curcuit evaluation is not just an option"
I didn't say it was an option. It's a language feature just like I said in my introductory sentence.
Feel free to elaborate on what you were trying to tell me.

Cheers!

Stefan_Lang 7-Jan-14 9:06am

It may just be my reading or interpretation, but since the link you provided isn't actually referring to the software aspect of short circuit evaluation, and you stated "some languages", I felt it isn't sufficiently clear from your posting alone.

Or maybe you meant to post a different link?

Manfred Rudolf Bihy 7-Jan-14 9:35am

Thanks for the tip off. The link was indeed not the one that I previously entered. The part "evaluation" after the underscore somehow got lost. I fixed it now.
Thanks! :)

Andreas Gieriet 7-Jan-14 16:29pm

Yeah, I experienced the same behaviour: links after the underscore get cut off when pasted in the editor. For me, that resulted in refering to Syntax instead of Syntax_sugar in one of my posts...

Manfred Rudolf Bihy 7-Jan-14 16:39pm

Has this behaviour already been reported in the sugs & bugs forum?

Andreas Gieriet 7-Jan-14 16:43pm

Just done now: see Pasting in the editor a link which contains underscores may cut off the part after the underscore - underscores are very common to wikipedia links....
Cheers
Andi

AlwaysLearningNewStuff 7-Jan-14 11:40am

Well explained, +5!

Best regards.

Andreas Gieriet 7-Jan-14 16:23pm

My 5!
Cheers
Andi

Manfred Rudolf Bihy 7-Jan-14 16:39pm

Thank you!

Ron Beyer · Accepted Answer · 2014-01-06T05:43:00

This is a fun one...

[Edit]
In addition to the below, the main cause is short circuit evaluation. See Solution 2.

What is the difference between:

C++

x++

and

C++

++x

?

The first returns the value FIRST, then increments x. So if I wrote:

C++

int x = 10;
int y = x++;

and after looking at the evaluation, x would be 11, and y would be 10.

Alternatively, if I wrote:

C++

int x = 10;
int y = ++x;

after evaluation, x and y would both be 11.

So in your case:

C++

if ((y >= 10) || (x++ > 10)) //x is 10 during the evaluation and 11 after.
{

}

Since you don't give the whole line, I can only assume that this is in an if statement.

If you wrote something like:

C++

int y = 9;
int x = 10;
if ((y >= 10) || (x++ > 10) || (x > 10))
{

}

The first (x++ > 10) would evaluate to false, but (x > 10) would evaluate to true.