I'm finding the whole objective c Looping principal confusing

While loop:

A while loop runs as long as a condition is true. Say you had

i = 0;
sum = 0;
while ( i < 3) {

sum +=2;
i++;
}

First time the loop runs: I is 0, which is less than 3, perform the loop. Add 2 to sum (it is now 2, since it was 0 before) increment i by 1 (it is now 1)

Second time the loop runs: I is 1, which is less than 3, perform the loop. Add 2 to sum (it is now 4, since it was 2 before) increment i by 1 (it is now 2)

Third time the loop runs: I is 2, which is less than 3, perform the loop. Add 2 to sum (it is now 6, since it was 4before) increment i by 1 (it is now 3)

4th time the loop runs: i is 3, which is not less than 3. Dont perform the loop

The do while loop is the same as a while loop except for one thing: A do while loop will always run AT LEAST once

If you had

i = 2;
while (i < 2) {
i++;
}

The loop would not be performed because i is not less than 2.

The do while version of this loop would be run once, since first it performs whatever is in the do block regardless of the while condition. it would do whats in the do block, then check the condition to determine if it needs to continue looping or stop

i = 2;
do {
i++;
} while (i > 2);