FAQ: Quicksort: Javascript - Partitioning Part III - Swapping

This community-built FAQ covers the “Partitioning Part III - Swapping” exercise from the lesson “Quicksort: Javascript”.

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This exercise can be found in the following Codecademy content:

Pass the Technical Interview with JavaScript

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I don’t understand the reason of putting up this last ‘if’ condition :-

if(leftIndex <= rightIndex) {
      swap(array, leftIndex, rightIndex);
      leftIndex ++;
      rightIndex --;
    }

Since the outer ‘while’ condition is same as the ‘if’ condition, I don’t see any point in using the extra ‘if’ condition. Correct me if I am wrong and why do we need this ‘if’ condition?

1 Like

I was wondering the same thing here…

I believe it’s because the while loops and “if” condition are all performed sequentially.

If we picture the code working, first the leftIndex’s are checked while array[leftIndex] < pivot. If the array[leftIndex] > pivot, the first loop stops and the 2nd while loop begins checking the elements at rightIndex. If array[rightIndex] < pivot, that while loop also breaks.

Thus, the last “if” condition is in case both of these while loops have been exited without checking all the elements in the array.

If such is the case, we know that:
array[leftIndex] > pivot
AND
array[leftIndex] < pivot

We can therefore assume with certainty that if we swap the elements at leftIndex and rightIndex, it will then re-satisfy our initial while loops and the process continues until eventually the outer while loop breaks at which point all elements will have been checked.

This is my best guess as to how it works–I definitely think the lesson could have been explained a bit better.