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Tutorial 100: Selection sort

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Tutorial ID100
TitleSelection sort
In computer science, a selection sort is a sorting algorithm, specifically an in-place comparison sort. It has O(n2) time complexity, making it inefficient on large lists, and generally performs worse than the similar insertion sort. Selection sort is noted for its simplicity, and also has performance advantages over more complicated algorithms in certain situations, particularly where auxiliary memory is limited.

 

The algorithm works as follows:

1. Find the minimum value in the list

2. Swap it with the value in the first position

3. Repeat the steps above for the remainder of the list (starting at the second position and advancing each time)

 

Selection SortSelection_Sort

Effectively, the list is divided into two parts: the sublist of items already sorted, which is built up from left to right and is found at the beginning, and the sublist of items remaining to be sorted, occupying the remainder of the array.

 
 
Here is an example of this sort algorithm sorting five elements: 
64 25 12 22 11

11 25 12 22 64

11 12 25 22 64

11 12 22 25 64

11 12 22 25 64
(nothing appears changed on this last line because the last 2 numbers were already in order)Selection sort can also be used on list structures that make add and remove efficient, such as a linked list. In this case it is more common to remove the minimum element from the remainder of the list, and then insert it at the end of the values sorted so far. For example:
64 25 12 22 11

11 64 25 12 22

11 12 64 25 22

11 12 22 64 25

11 12 22 25 64
 
Example Code:
/* a[0] to a[n-1] is the array to sort */
int i,j;
int iMin;
 
/* advance the position through the entire array */
/* (could do j < n-1 because single element is also min element) */
for (j = 0; j < n-1; j++) {
    /* find the min element in the unsorted a[j .. n-1] */
 
    /* assume the min is the first element */
    iMin = j;
    /* test against elements after j to find the smallest */
    for ( i = j+1; i < n; i++) {
        /* if this element is less, then it is the new minimum */  
        if (a[i] < a[iMin]) {
            /* found new minimum; remember its index */
            iMin = i;
        }
    }
 
    /* iMin is the index of the minimum element. Swap it with the current position */
    if ( iMin != j ) {
        swap(a[j], a[iMin]);
    }
}
 
 
 
Reference: Wikipedia

Related Problems

problem_id title description submit accepted difficulty
1084 Sorting I Sorting is a basic problem in Computer Science. Can you write a program to sort integers?   12 9 0

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