Introduce
Counting sort is a sorting technique based on keys between a specific range. It works by counting the number of objects having distinct key values (kind of hashing). Then doing some arithmetic to calculate the position of each object in the output sequence.
Let us understand it with the help of an example.
For simplicity, consider the data in the range 0 to 9.
Input data: 1, 4, 1, 2, 7, 5, 2
1) Take a count array to store the count of each unique object.
Index: 0 1 2 3 4 5 6 7 8 9
Count: 0 2 2 0 1 1 0 1 0 0
2) Modify the count array such that each element at each index
stores the sum of previous counts.
Index: 0 1 2 3 4 5 6 7 8 9
Count: 0 2 4 4 5 6 6 7 7 7
The modified count array indicates the position of each object in
the output sequence.
3) Output each object from the input sequence followed by
decreasing its count by 1.
Process the input data: 1, 4, 1, 2, 7, 5, 2. Position of 1 is 2.
Put data 1 at index 2 in output. Decrease count by 1 to place
next data 1 at an index 1 smaller than this index.
Examples
C++
// C++ Program for counting sort
#include<iostream>
#include<string.h>
#define RANGE 255
using namespace std;
// The main function that sort
// the given string arr[] in
// alphabatical order
void countSort(char arr[])
{
// The output character array
// that will have sorted arr
char output[strlen(arr)];
// Create a count array to store count of inidividul
// characters and initialize count array as 0
int count[RANGE + 1], i;
memset(count, 0, sizeof(count));
// Store count of each character
for(i = 0; arr[i]; ++i)
++count[arr[i]];
// Change count[i] so that count[i] now contains actual
// position of this character in output array
for (i = 1; i <= RANGE; ++i)
count[i] += count[i-1];
// Build the output character array
for (i = 0; arr[i]; ++i)
{
output[count[arr[i]]-1] = arr[i];
--count[arr[i]];
}
// Copy the output array to arr, so that arr now
// contains sorted characters
for (i = 0; arr[i]; ++i)
arr[i] = output[i];
}
// Driver code
int main()
{
char arr[] = "geeksforgeeks";
countSort(arr);
cout<< "Sorted character array is " << arr;
return 0;
}
// This code is contributed by rathbhupendra
Java
// Counting Sort
class CountingSort {
void sort(char arr[])
{
int n = arr.length;
// The output character array that will have sorted arr
char output[] = new char[n];
// Create a count array to store count of inidividul
// characters and initialize count array as 0
int count[] = new int[256];
for (int i = 0; i < 256; ++i)
count[i] = 0;
// store count of each character
for (int i = 0; i < n; ++i)
++count[arr[i]];
// Change count[i] so that count[i] now contains actual
// position of this character in output array
for (int i = 1; i <= 255; ++i)
count[i] += count[i - 1];
// Build the output character array
for (int i = 0; i < n; ++i) {
output[count[arr[i]] - 1] = arr[i];
--count[arr[i]];
}
// Copy the output array to arr, so that arr now
// contains sorted characters
for (int i = 0; i < n; ++i)
arr[i] = output[i];
}
// Driver method
public static void main(String args[])
{
CountingSort ob = new CountingSort();
char arr[] = {'g', 'e', 'e', 'k', 's', 'f', 'o',
'r', 'g', 'e', 'e', 'k', 's' };
ob.sort(arr);
System.out.print("Sorted character array is ");
for (int i = 0; i < arr.length; ++i)
System.out.print(arr[i]);
}
}
/*This code is contributed by Rajat Mishra */
Python
# Python program for counting sort
# The main function that sort the given string arr[] in
# alphabetical order
def countSort(arr):
# The output character array that will have sorted arr
output = [0 for i in range(256)]
# Create a count array to store count of inidividul
# characters and initialize count array as 0
count = [0 for i in range(256)]
# For storing the resulting answer since the
# string is immutable
ans = ["" for _ in arr]
# Store count of each character
for i in arr:
count[ord(i)] += 1
# Change count[i] so that count[i] now contains actual
# position of this character in output array
for i in range(256):
count[i] += count[i-1]
# Build the output character array
for i in range(len(arr)):
output[count[ord(arr[i])]-1] = arr[i]
count[ord(arr[i])] -= 1
# Copy the output array to arr, so that arr now
# contains sorted characters
for i in range(len(arr)):
ans[i] = output[i]
return ans
# Driver program to test above function
arr = "geeksforgeeks"
ans = countSort(arr)
print "Sorted character array is %s" %("".join(ans))
# This code is contributed by Nikhil Kumar Singh
C#
// C# implementation of Counting Sort
using System;
class GFG {
static void countsort(char []arr)
{
int n = arr.Length;
// The output character array that
// will have sorted arr
char []output = new char[n];
// Create a count array to store
// count of inidividul characters
// and initialize count array as 0
int []count = new int[256];
for (int i=0; i<256; ++i)
count[i] = 0;
// store count of each character
for (int i=0; i<n; ++i)
++count[arr[i]];
// Change count[i] so that count[i]
// now contains actual position of
// this character in output array
for (int i=1; i<=255; ++i)
count[i] += count[i-1];
// Build the output character array
// To make it stable we are operating in reverse order.
for (int i = n-1; i>=0; i--)
{
output[count[arr[i]]-1] = arr[i];
--count[arr[i]];
}
// Copy the output array to arr, so
// that arr now contains sorted
// characters
for (int i = 0; i<n; ++i)
arr[i] = output[i];
}
// Driver method
public static void Main()
{
char []arr = {'g', 'e', 'e', 'k', 's', 'f', 'o',
'r', 'g', 'e', 'e', 'k', 's'
};
countsort(arr);
Console.Write("Sorted character array is ");
for (int i=0; i<arr.Length; ++i)
Console.Write(arr[i]);
}
}
// This code is contributed by Sam007.
PHP
<?php
// PHP Program for counting sort
$RANGE = 255;
// The main function that sort
// the given string arr[] in
// alphabatical order
function countSort($arr)
{
global $RANGE;
// The output character array
// that will have sorted arr
$output = array(strlen($arr));
$len = strlen($arr);
// Create a count array to
// store count of inidividul
// characters and initialize
// count array as 0
$count = array_fill(0, $RANGE + 1, 0);
// Store count of
// each character
for($i = 0; $i < $len; ++$i)
++$count[ord($arr[$i])];
// Change count[i] so that
// count[i] now contains
// actual position of this
// character in output array
for ($i = 1; $i <= $RANGE; ++$i)
$count[$i] += $count[$i - 1];
// Build the output
// character array
// To make it stable we are operating
// in reverse order.
for ($i = $len-1; $i >= 0 ; $i--)
{
$output[$count[ord($arr[$i])] - 1] = $arr[$i];
--$count[ord($arr[$i])];
}
// Copy the output array to
// arr, so that arr now
// contains sorted characters
for ($i = 0; $i < $len; ++$i)
$arr[$i] = $output[$i];
return $arr;
}
// Driver Code
$arr = "geeksforgeeks"; //"applepp";
$arr = countSort($arr);
echo "Sorted character array is " . $arr;
// This code is contributed by mits
?>
El fin!