0.6 Sorting  (Page 13/20)

function siftUp(a, start, end) is

input: start represents the limit of how far up the heap to sift.

end is the node to sift up.

child := end

while child>start

parent := ⌊(child - 1) ÷ 2⌋

if a[parent]<a[child] then (out of max-heap order)

swap(a[parent], a[child])

child := parent (repeat to continue sifting up the parent now)

else

return

It can be shown that both variants of heapify run in O(n) time.[ citation needed ]

C-code

Below is an implementation of the "standard" heapsort (also called bottom-up-heapsort). It is faster on average (see Knuth. Sec. 5.2.3, Ex. 18) and even better in worst-case behavior (1.5n log n) than the simple heapsort (2n log n). The sift_in routine is first a sift_up of the free position followed by a sift_down of the new item. The needed data-comparison is only in the macro data_i_LESS_THAN_ for easy adaption.

This code is flawed - see talk page

/* Heapsort based on ideas of J.W.Williams/R.W.Floyd/S.Carlsson */

#define data_i_LESS_THAN_(other) (data[i]<other)

#define MOVE_i_TO_free { data[free]=data[i]; free=i; }

void sift_in(unsigned count, SORTTYPE *data, unsigned free_in, SORTTYPE next)

{

unsigned i;

unsigned free = free_in;

// sift up the free node

for (i=2*free;i<count;i+=i)

{ if (data_i_LESS_THAN_(data[i+1])) i++;

MOVE_i_TO_free

}

// special case in sift up if the last inner node has only 1 child

if (i==count)

MOVE_i_TO_free

// sift down the new item next

while( ((i=free/2)>=free_in)&&data_i_LESS_THAN_(next))

MOVE_i_TO_free

data[free] = next;

}

void heapsort(unsigned count, SORTTYPE *data)

{

unsigned j;

if (count<= 1) return;

data-=1; // map addresses to indices 1 til count

// build the heap structure

for(j=count / 2; j>=1; j--) {

SORTTYPE next = data[j];

sift_in(count, data, j, next);

}

// search next by next remaining extremal element

for(j= count - 1; j>=1; j--) {

SORTTYPE next = data[j + 1];

data[j + 1] = data[1]; // extract extremal element from the heap

sift_in(j, data, 1, next);

}

}

6.2.3. quicksort

(From Wikipedia, the free encyclopedia)

Quicksort is a well-known sorting algorithm developed by C. A. R. Hoare that, on average , makes ( big O notation ) comparisons to sort n items. However, in the worst case , it makes Θ(n2) comparisons. Typically, quicksort is significantly faster in practice than other algorithms, because its inner loop can be efficiently implemented on most architectures, and in most real-world data it is possible to make design choices which minimize the possibility of requiring quadratic time.

Quicksort is a comparison sort and is not a stable sort .

The algorithm

Quicksort sorts by employing a divide and conquer strategy to divide a list into two sub-lists.

The steps are:

1. Pick an element, called a pivot , from the list.
2. Reorder the list so that all elements which are less than the pivot come before the pivot and so that all elements greater than the pivot come after it (equal values can go either way). After this partitioning, the pivot is in its final position. This is called the partition operation.
3. Recursively sort the sub-list of lesser elements and the sub-list of greater elements.