C++ strange behavior of a for cycle initialization

Question

I noticed a strange behavior of a for cycle...

Case 1:

Let us have an internal initialization of i variable

for ( int i=10; i <=10; i++)
{
    std::cout << i;
}
return 0;

Output:

Case 2:

Now we initialize i outside the cycle

int i = 10;

for ( ; i <=10; i++)
{
    std::cout << i;
}
return 0;

Output:

Nothing will be print

Case 3:

Initialization any variable not related to i:

int i = 10;

for ( int k = 0 ; i <=10; i++)
{
    std::cout << i;
}
return 0;

Output:

Question

How can depend a result of a cycle condition on the place where we initialize variable i?

Updated results

I traced a code using degugger and VS 2010 really jumps over a cycle in case 2. However g++ works well. Maybe a bug in compiler?

Fianal results After reboot it works correcly. Something between heaven and earth. Sorry for the useless question....

Second one should print `10`. Are you sure you didn't overlook something? — nullpotent, Aug 17 '12 at 10:41
Please review the output of your case 2: http://ideone.com/alc3j — PlasmaHH, Aug 17 '12 at 10:42
You're doing something else not shown in the code snippets here. — Some programmer dude, Aug 17 '12 at 10:43
I think you did not run your compiler properly or you did not show us the full source, see [this program](http://ideone.com/kcIqA), it seems to be your 'case 2' but it prints `10`. — mpu, Aug 17 '12 at 10:44
VS 2010 has a mature compiler, it should print `10`. Perhaps you should follow Andreas' advice and flush the output. — nullpotent, Aug 17 '12 at 10:45
@iccthedral And indeed it does. Just tested in VC++2010 ("only" Express version though) and it works just fine. With or without newlines or manual flushes. — Some programmer dude, Aug 17 '12 at 11:08
@justik Can you post a complete program (including main) that reproduces this problem? — Andreas Brinck, Aug 17 '12 at 11:11
@ Andreas Brinck: This is a complete program, such a simple construction :-) — justik, Aug 17 '12 at 11:13
Would you just c/p it, as it is, so we could get this done. This doesn't make any sense if the outputs are consistent. Then again, I've seen trolls wandering near `c++` woods. — nullpotent, Aug 17 '12 at 11:18
@justik I want the complete code that can be pasted into a single translation unit and compiled into an executable which reproduces this problem, the above doesn't. — Andreas Brinck, Aug 17 '12 at 11:22

score 0 · Accepted Answer · answered Aug 17 '12 at 10:42

0

If case 2 doesn't print 10 you have a bug in your compiler or the output isn't being flushed. Add a << "\n" to the output as well.

answered Aug 17 '12 at 10:42

Andreas Brinck

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Actually, just adding `'\n'` doesn't flush the output, as I discovered in [this question](http://stackoverflow.com/q/8311058/440558). However, the program terminating should _definitely_ flush the output. – Some programmer dude Aug 17 '12 at 10:48
But If it flushes when `i` is initilized at loop and it doesn't flush when i is initilized before loop is also a bug in compiler. But I don't think VS will do that. – Neel Basu Aug 17 '12 at 10:52
@ Andreas it looks like a VS 2010 compiler bug, g++ works well – justik Aug 17 '12 at 10:53
I would be very surprised if this was a VS 2010 compiler bug, it's not like you're at the frontier of C++ features :) – Andreas Brinck Aug 17 '12 at 11:02
1

@justik VS2010 works as expected on my windows 7 x64 machine. – WiSaGaN Aug 17 '12 at 11:10

score 0 · Answer 2 · answered Aug 17 '12 at 10:48

0

The second one prints 10 too. If it doesn't there is a bug in your compiler or you are using a wrong compiler.

I've run the same code in http://ideone.com/UB0V0. you can have a see it it. it prints 10

answered Aug 17 '12 at 10:48

Neel Basu

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C++ strange behavior of a for cycle initialization

2 Answers2