Nested array references

Question

I'm currently learning Java online and am confused about the following code and what one of the elements in the array is evaluating to:

int[] a = new int[]{9, 8, 3, 1, 5, 4};

for (int i = 0; i < a.length; i++) {
    if (a[i] % 2 == 0) {
        a[i] += 1;
    } else if (a[i] < a.length) {
        a[i] += a[a[i]];
    }
}

I am looking at a[3] and the number that this evaluates to, and when I am debugging the code, my IDE is showing that a[a[i]] is evaluating to 9 , which is where I'm a bit confused.

I thought that a[3] would equal 1 and then a[1] would equal 8 , however this doesn't seem to be the case. Could anyone provide clarity as the JetBrains Academy course doesn't refer to this.

Answer 1

Note the first condition - if (a[i] % 2 == 0) {a[i] += 1;} - this causes even values to be incremented. Therefore a[1] is incremented from 8 to 9 .

Now, when i==3 , a[a[i]] is evaluated to a[1] which is equal to 9 . Then you are adding it to the original value of a[3] (note the operator is += , not = ), so a[3] becomes 1 + 9 , which is 10 .

Answer 2

int[] a = new int[] {9, 8, 3, 1, 5, 4};

for (int i = 0; i < a.length; i++){
    if (a[i] % 2 == 0)
        a[i] += 1;
    else if(a[i] < a.length)
        a[i] += a[a[i]];
}

---

|i | a[i] old | a[i] % 2 | a[i] < a.length | a[a[i]] | a[i] new |
|--|----------|----------|-----------------|---------|----------|
|0 |  9       |    -     |   -             |  NaN    |    9     |
|1 |  8       |    +     |   -             |  NaN    |    9     |
|2 |  3       |    -     |   +             |  1      |    4     |
|3 |  1       |    -     |   +             |  9      |    10    |
|4 |  5       |    -     |   +             |  4      |    9     |
|5 |  4       |    +     |   -             |  9      |    5     |

---

// a = {9, 9, 4, 10, 9, 5};

I thought that a[3] would equal 1

That's correct, at step 3 , a[3] = 1

and then a[1] would equal 8

That's not correct, because at step 1 it was incremented to 9

Answer 3

Its okey. Best way to see what you are doing is debuggin (painting in this case) your code:

int[] a = new int[] {9, 8, 3, 1, 5, 4};

    for (int i = 0; i < a.length; i++){
        if (a[i] % 2 == 0){
            System.out.printf(a[i]);
            a[i] += 1;
            System.out.printf(a[i]);
        }else if(a[i] < a.length){
            System.out.printf(a[i]);
            a[i] += a[a[i]];
            System.out.printf(a[i]);
}

And you will see more clearly.Sometimes its better to face the wall by ourselves because one best practices is learn about you. Good luck!.