Strange for loop

Question

Hi,
can somebody please explain this for loop, i.e. the value of int i in individual steps?

int i=0;
for(;i<4;i+=2) {
i = i*i;
cout << i << endl;

}
cout << i << endl;

To me it seems a bit odd that the output is
0
4
6 <-- why was the i incremented?

Answer 1

i was incremented because the increment is done at the end of the loop before checking the condition.

Step by step:
i = 0
0 is below 4, so we enter the for loop
i = 0*0 = 0, and 0 is printed
i is incremented to 2
2 is below 4, we loop again
i = 2*2 = 4, and 4 is printed
i is incremented to 6
6 is not below 4, we exit the loop
6 is printed

Equivalent while loop:
int i=0;
while(i<4) {
i = i*i;
cout << i << endl;

i+=2;
}
cout << i << endl;

In general, it's better to use a while loop over a for loop if you modify the loop variable inside the loop (besides the increment) to avoid confusion.

Answer 2

Thank you! Great answer.