对于循环执行，增加混乱

Question

I don't get it why i in fillArray method ends up being equal to 10 even though the array score is filled only up to index 9 .

As per my understanding i has to be smaller than 10 ,so how can it end up being 10 at the end, it should have incremented.

I tried another loop to test if for loop executes incrementation at the end if condition is true.

In test loop i ends up being only 10 which makes sense but the two for loop contradict.

public class GoldScores {
        public static final int MAX_NUMBER_SCORES = 10;
        public static void main(String[] args) {
            double[] score = new double[MAX_NUMBER_SCORES];
            int numberUsed = 0;

            System.out.println("This program reads gold scores and shows");
            System.out.println("how much each differs from the average.");
            System.out.println("Enter gold scores:");
            //numberUsed = fillArray(score);
        //  showdifference(score,numberUsed);
             for(int i=1; i<11; i++){                   //Test loop
                 System.out.println("Count is: " + i);
            }
        }
        private static void showdifference(double[] score, int numberUsed) {
            // TODO Auto-generated method stub

        }
        public static int fillArray(double[] a){
            System.out.println("Enter up to " + a.length + " nonnegative numbers.");
            System.out.println("Mark the end of the list with a negative number.");
            Scanner keyboard = new Scanner(System.in);

            double next = keyboard.nextDouble();
            int i = 0;
            for(i = 0;(next>=0 && i<a.length);i++){     //HELP!!!!
                a[i] = next;
                next = keyboard.nextDouble();
            }
            return i;
        }

Answer 1

You must understand exactly how the for loop works to understand what is going on, and why i is 10 after the for loop in fillArray .

1. Perform initialization, before the first semicolon.
2. Test the condition in between the first and second semicolons. If the condition is false , break out of the loop.
3. Execute the body of the loop.
4. Perform the statement after the second semicolon (the increment).
5. Go back to Step 2.

In the last iteration of the i for loop, i is 9 , and index 9 is assigned in the array, step 3. Step 4 performs the increment, and i is now 10 . Then the condition is tested, which is false , and the loop is exited. i is now 10 .

However, in your main for loop, you print the value in the body instead of examining the looping variable afterwards. The last iteration is when i is 10 , because the condition is different: i < 11 . If you were to print i after that for loop, you'll see it's 11 .

Answer 2

in For loop, increment occurs after testing loop condition, not before. So in last iteration when your condition is checked, I is already equal to 10 and that is exactly what is being returned. Consider this, if your I would still be 9 in last iteration, your condition would still be true which would mean one more execution in loop.

Answer 3

While others have already explained in detail , to eliminate confusion you could modify your code to something like :

        double next = keyboard.nextDouble();
        int i = 0;
        int current_i = i;
        for( i = 0; ( next >= 0 && i < a.length ); i++ )
        {
            current_i = i;
            a[i] = next;
            next = keyboard.nextDouble();
        }
        return current_i;

instead of

        double next = keyboard.nextDouble();
        int i = 0;
        for(i = 0;(next>=0 && i<a.length);i++){     //HELP!!!!
            a[i] = next;
            next = keyboard.nextDouble();
        }
        return i;