CODEWITHSUNDEEP
c++
user3211537
user3211537

Reputation: 3

No checking condition in for loop C++

char i;

for (i = 1; i < 10, i++;) 
{
    cout << (i+1) << endl;
}

return 0;

I understand that for loop has the following syntax: 

for(initialization; condition; increment)
{

}

As I run the debug, why it never checks the condition and it eventually stops at i = 0?

(Thank you Damien and Andars, I don't know why the "on purpose" statement was removed, but you interpret my question correctly. Could someone explain why the complier skips the condition before the comma, and why the loop stop at i = 0 instead of looping forever? Thanks!)

Upvotes: 0

Views: 119

Answers (5)

Damien Black
Damien Black

Reputation: 5647

I believe he is indicating that he wrote the code that way on purpose.

To answer, i++ will always return true. The last value is the value that matters with comma separated statements, so the loop will not stop.

Edit for elaborations:

It isn't simply using a logical or, it disregards what is before the comma and only takes the last value. The value of anything non-zero is considered true, and since i starts at 1, and goes up, it will always be non-zero (until it overflows and wraps back around, which explains why i ends at 0).

If you say:

x = 4, 5, 6;

x will be equal to 6, the last value.

Upvotes: 2

andars
andars

Reputation: 1404

It seems to me also that the code was written incorrectly on purpose. As others have mentioned, the comma operator will discard the value of the i<10 and only i++ will be evaluated as condition. This will return true until i overflows (values only from -127 to 127) and ends up at -1, when i++ will return 0 and the loop exits. Thus the final value for i will be 0.

Upvotes: 1

Digital_Reality
Digital_Reality

Reputation: 4738

Change to 

for (i = 1; i < 10; i++) //Notice correct placement of ;

Upvotes: 1

herohuyongtao
herohuyongtao

Reputation: 50667

Change 

for (i = 1; i < 10, i++;)

to

for (i = 1; i < 10; i++)

Upvotes: 1

leshy84
leshy84

Reputation: 700

Because you used the comma operator instead of a semi-colon

Upvotes: 0

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