StackOverflow2013

Note that there are some explanatory texts on larger screens.

plurals

POHow do I change iterative quicksort with fixed pivot to iterative quicksort with random pivot?
text
Body
copied!<p>I found this code for quicksort with a fixed pivot. It always takes a right-hand side element of the given table as a pivot. I want it to take a random element as a pivot. I think that <code>x</code> is a pivot here, so I thought it was a good idea to change it to a random element from a given list, but it turns out that it is not working.</p> <pre><code>void swap ( int* a, int* b ) { int t = *a; *a = *b; *b = t; } int partition (int arr[], int l, int h) { int x = arr[h]; int i = (l - 1); for (int j = l; j <= h- 1; j++) { if (arr[j] <= x) { i++; swap (&arr[i], &arr[j]); } } swap (&arr[i + 1], &arr[h]); return (i + 1); } void quickSortIterative (int arr[], int l, int h) { int stack[ h - l + 1 ]; int top = -1; stack[ ++top ] = l; stack[ ++top ] = h; while ( top >= 0 ) { h = stack[ top-- ]; l = stack[ top-- ]; int p = partition( arr, l, h ); if ( p-1 > l ) { stack[ ++top ] = l; stack[ ++top ] = p - 1; } if ( p+1 < h ) { stack[ ++top ] = p + 1; stack[ ++top ] = h; } } } </code></pre> <p>I tried changing lines</p> <pre><code>int x = arr[h]; </code></pre> <p>and</p> <pre><code>swap(&arr[i+1], &arr[j]); </code></pre> <p>to</p> <pre><code>int r = l+rand()%(h-l); int x = arr[r]; </code></pre> <p>and then</p> <pre><code>swap(&arr[i+1], &arr[r]); </code></pre> <p>but it is not sorting properly. Obviously I'm doing something wrong here. Please help.</p>

Querying!

Guidance

An individual column

Larger individual text columns get their own page to allow for proper reading.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload