Converting a for-loop with continue statement to while-loop

Question

I am working on an exercise where a small piece of code based on a for-loop is converted to preform the same operation with a while loop. The conversion is wrong by purpose, and looks like this:

int sum = 0;

    for (int i = 0; i < 4; i++) {
        if (i % 3 == 0) continue;
        sum += i;   
    }
System.out.println(sum); // prints 3

This is converted into:

int i = 0, sum = 0;

        while (i < 4) {
            if (i % 3 == 0) continue;
            sum += i;   
            i++;    
        }
System.out.println(sum); // will not print

In the exercise, I am asked to explain why the conversion is wrong and then fix it. My thoughts are:

1. With the initial value of i = 0, this will trigger continue instantly after entering the will loop, since (0 % 3 == 0) will make the if-statement true. As long as the initial value is 0, this will execute the loop, for so to skip it an endless amount of times. I tried changing the initial value of i = 1, but noe sum is printed. Then I tried to increment i before executing the if-statement, and the program now prints the sum 7. The question here is; why won't the program print if i incremented after the if statement, even if the initial value of i = 1 suggests (in my head) that program should run properly?

2. I made a table for each program to compare the summing.

The for-loop version:

i = 0, sum += i not preformed (continue), i++

i = 1, sum = 1, i++

i = 2, sum = 3, i++

i = 3, sum += i not preformed (continue), i++

i = 4, i < 4 false, loop stopped

The while-loop version:

i = 0, i++, sum = 1

i = 1, i++, sum = 3

i = 2, i++, sum += i not preformed (continue)

i = 3, i++, sum = 7

i = 4, i < 4 false, loop stopped

In the while-loop, sum += i is preformed once more than in the for-loop. Is this the right way to convert the for-loop into a while-loop?

int i = 0, sum = 0;

        while (i < 3) {
            i++;
            if (i % 3 == 0) continue;
            sum += i;           
        }

        System.out.println(sum);

Answer 1

Your 1 is focussing on it being the initial value, but that's not the point. The point is that i is never incremented when i % 3 == 0 is true, not that 0 is the initial value. So the while loop loops forever.

Your 2 doesn't make any sense: The while version will loop forever, never summing anything.

Is this the right way to convert the for-loop into a while-loop?

No, you're incrementing i at the wrong time.

Think bout how a for loop works:

1. Initialization - First it sets the variables to the values in the first expression.

2. Test - Then it tests the values using the second expression.

3. Execute - If the value is true, it executes the loop body.

4. Increment - When the loop body is complete, it executes the third (increment) expression. Note that this is after the loop body.

Make your while loop do what the for loop is doing. (I'm intentionally not doing that for you; this is a learning exercise. I'll note that the best way to convert that for loop will not use continue, however. "Best," of course, is subjective.)

Answer 2

In the exercise, I am asked to explain why the conversion is wrong and then fix it

The conversion is wrong simply because when you will reach a i value that modulo 3 equals 0 (the first iteration in that case), you will skip to the next iteration and re-validate. However, since you skipped directly without incrementing i , you will re-validate the same condition and re-validate ad-infinitum.

You could fix it easily by getting rid of the continue and negating the condition :

while (i < 4) {
    if (i % 3 != 0)
        sum += i;

    i++;    
}