The Counting Sort Algorithm

Counting Sort works by counting the occurrences of each data value. It assumes that there are n data items in the range of 1..k, where k is an integer. The algorithm can then determine, for each input element, the amount of elements less than it. For example if there are 9 elements less than element x, then x belongs in the 10th data position.

The pseudocode Counting Sort algorithm is as follows:

      countingsort(A[], B[], k)
	for i = 1 to k do
	   C[i] = 0
	for j = 1 to length(A) do
	   C[A[j]] = C[A[j]] + 1
	for 2 = 1 to k do
	   C[i] = C[i] + C[i-1]

	for j = 1 to length(A) do
	   B[C[A[j]]] = A[j]
	   C[A[j]] = C[A[j]] - 1


Although this may look complicated, it is actually a very simple and clever algorithm.

Array A[ ] stores the initial data to be sorted.

Array C[ ] is used to count the occurrences of the data values

Array B[ ] is used to store the final, sorted, list.

The first for loop initialises C[ ] top zero.
The second for loop increments the values in C[], according to their frequencies in the data.
The third for loop adds all previous values, making C[] contain a cumulative total.
The fourth for loop writes out the sorted data into array B[].

Two of the for loops take O(k) time, and two take O(n) time. An important point to note is that Counting Sort is stable: All elements of the same value will appear in the same order in the output array array that they do in the input array.

Go to Counting Sort demonstration

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