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.

The best way to see this is to simply get out pen and paper and follow thru the logic. Just remember that sometimes, future indices point to values that have been altered. So always reference the current state of the array. — WJS
– WJS, Commented Sep 17, 2019 at 13:01
Thanks for the tip, I do enjoy working on paper so will try this in future when stuck. — Lorcan
– Lorcan, Commented Sep 18, 2019 at 18:01

Eran · Accepted Answer · 2019-09-17 12:53:20Z

10

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.

edited Sep 17, 2019 at 12:53

answered Sep 17, 2019 at 12:34

Eran

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3 Comments

Lorcan Over a year ago

I've just copied and pasted the above code and my IDE has evaluated a[3] to equal 10 and it's evaluating a[a[3]] to equal 9?

Eran Over a year ago

@Lorcan thanks. I was misreading the first condition. For some reason my head translated a[i] % 2 == 0 as i % 2 == 0, which made me think that condition doesn't modify a[1]. Fixed now.

Lorcan Over a year ago

Apologies, I was looking at the array as if it wasn't being updated as it was looping! Thanks!

Ronan Boiteau · Accepted Answer · 2019-09-17 21:31:16Z

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

Qiqke · Accepted Answer · 2019-09-17 12:45:48Z

1

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!.

answered Sep 17, 2019 at 12:45

Qiqke

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Nested array references

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I thought that `a[3]` would equal `1`

and then `a[1]` would equal `8`

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I thought that a[3] would equal 1

and then a[1] would equal 8

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I thought that `a[3]` would equal `1`

and then `a[1]` would equal `8`