Delete DynamicArray about algorithms101

[jrasmusson]

Hi André! Thank you so much for commenting on this code.

I think the solution I have works as is.

public void delete(int index) {
        // Copy down
        for (int j = index; j < size - 1; j++) {
            data[j] = data[j + 1];
        }
        size--;

        // Clear if last element in array
        if (index == size) {
            data[index] = null;
        }
    }

Because I decrement size-- before doing the check, I can delete the last element OK.

So if we look at this example.

0 1 2 size = 3
a b c

delete(2)
copy down

0 1 2 size = 3
b c c index = 2

We now have an extra c at index = 2 that we want to delete.
If we decrement the size--

0 1 2 size = 2
b c c index = 2

We can delete the last element if the index == the new size or our array.

if (index == size) {   // 2 = 2
  data[index] = null;
}

0 1 2 size = 2
b c null index = 2

Does this make sense? I may have missed something, but I think it works.

If it doesn't, can your write a unit test that shows me where the bug is? I can then try to fix.

Thank you so much for raising this question and asking.

Cheers - Jonathan

[andrelugomes]

Hi Jonathan, tanks fos answer :)

I'm actually writing unit tests for the exercises

https://github.com/andrelugomes/data-structures-and-algorithms/blob/master/java/src/test/java/datastructures/DynamicArrayTest.java

I'll follow your suggestion and check by the tests.

[andrelugomes]

It doesn't work.

public void delete(int index) {
        // Copy down
        for (int j = index; j < size - 1; j++) {
            data[j] = data[j + 1];
        }
        size--;

        // Clear if last element in array
        if (index == size) {
            data[index] = null;
        }
    }

@Test
    public void shouldDeleteFromArray(){
        var array = new DynamicArray<>(2);
        array.add("object1");
        array.add("object2");
        array.add("object3");

        array.delete(1);

        assertThat(array.size(), is(2));
        assertThat(array.get(0), is("object1"));
    }

By my sugestion:

[jrasmusson]

Which assertion in your unit test is failing?

[andrelugomes]

The unit tests are passing. But the final array still holding dirty data in last index.

[jrasmusson]

Apologies :) I understand now. Bad unit test on my part.
Yes you are correct! And you solution works! Thank you.

I am going to tweak it a bit and add some comments.

Thanks again for pointing this out. You are really getting this stuff!

Cheers - Jonathan

[jrasmusson]

Note: I gave you credit for finding the but on the page that explains what the bug is. Let me know if you if that is OK.

Cheers - Jonathan

[andrelugomes]

Hi @jrasmusson

I'm pretty happy to discuss with you that solution.
Congratulations! The course is awesome.