Thread: incrementation inside for loop

Code:

int i, j = 0;
for (i = 0; i++ < 10; ++i, j++);
j += i;
printf("%d %d", i, j);

Line 2 executes 5 times, but because of the semicolon, the loop only executes the i++, ++i, and j++ expressions. After 5 iterations, i is 11 and j is 5, and the loop terminates.

Line 3 then adds i to j giving 11 and 16, and line 4 prints.

Originally Posted by drawar

Ah I see, thank you all!
So in the first iteration, the original value of i(0) is used to do the comparison, after that it's incremented by 1, and since the condition is true it's incremented by 1 again?

I've also tried changing the position of '++' in the for loop like for(i = 0; ++i < 10; ++i, j++); or for(i = 0; i++ < 10; i++, j++); and still get the same output as before. Why is that so?

There actually is a slight difference internally, but the output is the same.


for(i = 0; ++i < 10; ++i, j++);
for(i = 0; i++ < 10; i++, j++);

The expressions in blue are equivalent - the only time that i++ vs. ++i matters is if the result is being used in another expression, and these are not.

The expressions in red actually are different, but they produce the same final result because each iteration increments twice:

With ++i, you make the following comparisons:
1 < 10 - True
3 < 10 - True
5 < 10 - True
7 < 10 - True
9 < 10 - True
11 < 10 - False

With i++ you make the following:
0 < 10 - True
2 < 10 - True
4 < 10 - True
6 < 10 - True
8 < 10 - True
10 < 10 - False

In both cases, the comparison happens six times and the inner body of the loop happens five times, which is why i is incremented 11 times. You WOULD notice a difference if the number being compared to was odd, not even, or if you used a <= rather than a <