World Rally

Counting sort is a sorting algorithm which (like bucket sort) takes advantage of knowing the range of the numbers in the array to be sorted (array A). It uses this range to create an array C of this length. Each index i in array C is then used to count how many elements in A that have a value less than i. The counts stored in C can then be used to put the elements in A into their right position in the resulting sorted array. It is less efficient than pigeonhole sort. Characteristics of counting sort Counting sort is stable (see sorting algorithm) and has a running time of T(n+k), where n and k are the lengths of the arrays A (the input array) and C (the counting array), respectively. In order for this algorithm to be efficient, k must not be too large compared to n. As long as k is O(n) the running time of the algorithm is T(n). The indices of C must run from the minimum to the maximum value in A to be able to index C directly with the values of A. Otherwise, the values of A will need to be translated, so that the minimum value of A matches the smallest index of C. If the minimum and maximum values of A are not known, an initial pass of the data will be necessary to find these (this pass will take time T(n); see selection algorithm). The length of the counting array C must be at least equal to the range of the numbers to be sorted (that is, the maximum value minus the minimum value plus 1). This makes counting sort impractical for large ranges in terms of time and memory needed. Counting sort may for example be the best algorithm for sorting numbers whose range is between 0 and 100, but it is probably unsuitable for sorting a list of names alphabetically. However counting sort can be used in radix sort to sort a list of numbers whose range is too large for counting sort to be suitable alone. Because counting sort uses key values as indexes into an array, it is not a comparison sort, allowing it to break the O(n log n) lower-bound on those sorts.