Radix Sort Algorithm

Radix sort is a sorting algorithm that sorts the elements by first grouping the individual digits of the same place value. Then, sort the elements according to their increasing/decreasing order.

Suppose, we have an array of 8 elements. First, we will sort elements based on the value of the unit place. Then, we will sort elements based on the value of the tenth place. This process goes on until the last significant place.

Let the initial array be [121, 432, 564, 23, 1, 45, 788]. It is sorted according to radix sort as shown in the figure below.

Radix Sort Working
Working of Radix Sort

Please go through the counting sort before reading this article because counting sort is used as an intermediate sort in radix sort.


Working of Radix Sort

  1. Find the largest element in the array, i.e. max. Let X be the number of digits in max. X is calculated because we have to go through all the significant places of all elements.

    In this array [121, 432, 564, 23, 1, 45, 788], we have the largest number 788. It has 3 digits. Therefore, the loop should go up to hundreds place (3 times).
  2. Now, go through each significant place one by one.

    Use any stable sorting technique to sort the digits at each significant place. We have used counting sort for this.

    Sort the elements based on the unit place digits (X=0).
    Radix Sort working with Counting Sort as intermediate step
    Using counting sort to sort elements based on unit place
  3. Now, sort the elements based on digits at tens place.
    Radix Sort Step
    Sort elements based on tens place
  4. Finally, sort the elements based on the digits at hundreds place.
    Selection Sort Step
    Sort elements based on hundreds place

Radix Sort Algorithm

radixSort(array)
  d <- maximum number of digits in the largest element
  create d buckets of size 0-9
  for i <- 0 to d
    sort the elements according to ith place digits using countingSort

countingSort(array, d)
  max <- find largest element among dth place elements
  initialize count array with all zeros
  for j <- 0 to size
    find the total count of each unique digit in dth place of elements and
    store the count at jth index in count array
  for i <- 1 to max
    find the cumulative sum and store it in count array itself
  for j <- size down to 1
    restore the elements to array
    decrease count of each element restored by 1

Radix Sort Code in Python, Java, and C/C++

# Radix sort in Python
# Using counting sort to sort the elements in the basis of significant places
def countingSort(array, place):
    size = len(array)
    output = [0] * size
    count = [0] * 10

    # Calculate count of elements
    for i in range(0, size):
        index = array[i] // place
        count[index % 10] += 1

    # Calculate cumulative count
    for i in range(1, 10):
        count[i] += count[i - 1]

    # Place the elements in sorted order
    i = size - 1
    while i >= 0:
        index = array[i] // place
        output[count[index % 10] - 1] = array[i]
        count[index % 10] -= 1
        i -= 1

    for i in range(0, size):
        array[i] = output[i]


# Main function to implement radix sort
def radixSort(array):
    # Get maximum element
    max_element = max(array)

    # Apply counting sort to sort elements based on place value.
    place = 1
    while max_element // place > 0:
        countingSort(array, place)
        place *= 10


data = [121, 432, 564, 23, 1, 45, 788]
radixSort(data)
print(data)
// Radix Sort in Java Programming
public class RadixSort {
    // Using counting sort to sort the elements based on significant places
    public static void countingSort(int[] array, int place) {
        int size = array.length;
        int[] output = new int[size];
        int[] count = new int[10];
        
        // Calculate count of elements
        for (int i = 0; i < size; i++) {
            int index = (array[i] / place) % 10;
            count[index]++;
        }
        // Calculate cumulative count
        for (int i = 1; i < 10; i++) {
            count[i] += count[i - 1];
        }
        // Place the elements in sorted order
        for (int i = size - 1; i >= 0; i--) {
            int index = (array[i] / place) % 10;
            output[count[index] - 1] = array[i];
            count[index]--;
        }
        // Copy the sorted elements into original array
        for (int i = 0; i < size; i++) {
            array[i] = output[i];
        }
    }
    // Main function to implement radix sort
    public static void radixSort(int[] array) {
        // Get maximum element
        int maxElement = getMax(array);
        // Apply counting sort to sort elements based on place value
        for (int place = 1; maxElement / place > 0; place *= 10) {
            countingSort(array, place);
        }
    }
    // A utility function to get the maximum value in the array
    public static int getMax(int[] array) {
        int max = array[0];
        for (int i = 1; i < array.length; i++) {
            if (array[i] > max) {
                max = array[i];
            }
        }
        return max;
    }
    public static void main(String[] args) {
        int[] data = {121, 432, 564, 23, 1, 45, 788};
        radixSort(data);
        System.out.println("Sorted Array in Ascending Order: ");
        for (int num : data) {
            System.out.print(num + " ");
        }
    }
}
// Radix Sort in C Programming
#include <stdio.h>
// Function to get the maximum value in the array
int getMax(int array[], int n) {
    int max = array[0];
    for (int i = 1; i < n; i++) {
        if (array[i] > max) {
            max = array[i];
        }
    }
    return max;
}
// Using counting sort to sort the elements based on significant places
void countingSort(int array[], int n, int place) {
    int output[n];
    int count[10] = {0};
    // Calculate count of elements
    for (int i = 0; i < n; i++) {
        int index = (array[i] / place) % 10;
        count[index]++;
    }
    // Calculate cumulative count
    for (int i = 1; i < 10; i++) {
        count[i] += count[i - 1];
    }
    // Place the elements in sorted order
    for (int i = n - 1; i >= 0; i--) {
        int index = (array[i] / place) % 10;
        output[count[index] - 1] = array[i];
        count[index]--;
    }
    // Copy the sorted elements into original array
    for (int i = 0; i < n; i++) {
        array[i] = output[i];
    }
}
// Main function to implement radix sort
void radixSort(int array[], int n) {
    // Get maximum element
    int maxElement = getMax(array, n);
    // Apply counting sort to sort elements based on place value
    for (int place = 1; maxElement / place > 0; place *= 10) {
        countingSort(array, n, place);
    }
}
int main() {
    int data[] = {121, 432, 564, 23, 1, 45, 788};
    int n = sizeof(data) / sizeof(data[0]);
    radixSort(data, n);
    printf("Sorted array in ascending order:\n");
    for (int i = 0; i < n; i++) {
        printf("%d ", data[i]);
    }
    return 0;
}

// Radix Sort in C++ Programming

#include <iostream>
using namespace std;

// Function to get the largest element from an array
int getMax(int array[], int n) {
  int max = array[0];
  for (int i = 1; i < n; i++)
    if (array[i] > max)
      max = array[i];
  return max;
}

// Using counting sort to sort the elements in the basis of significant places
void countingSort(int array[], int size, int place) {
  const int max = 10;
  int output[size];
  int count[max];

  for (int i = 0; i < max; ++i)
    count[i] = 0;

  // Calculate count of elements
  for (int i = 0; i < size; i++)
    count[(array[i] / place) % 10]++;

  // Calculate cumulative count
  for (int i = 1; i < max; i++)
    count[i] += count[i - 1];

  // Place the elements in sorted order
  for (int i = size - 1; i >= 0; i--) {
    output[count[(array[i] / place) % 10] - 1] = array[i];
    count[(array[i] / place) % 10]--;
  }

  for (int i = 0; i < size; i++)
    array[i] = output[i];
}

// Main function to implement radix sort
void radixsort(int array[], int size) {
  // Get maximum element
  int max = getMax(array, size);

  // Apply counting sort to sort elements based on place value.
  for (int place = 1; max / place > 0; place *= 10)
    countingSort(array, size, place);
}

// Print an array
void printArray(int array[], int size) {
  int i;
  for (i = 0; i < size; i++)
    cout << array[i] << " ";
  cout << endl;
}

// Driver code
int main() {
  int array[] = {121, 432, 564, 23, 1, 45, 788};
  int n = sizeof(array) / sizeof(array[0]);
  radixsort(array, n);
  printArray(array, n);
}

Radix Sort Complexity

Time Complexity  
Best O(n+k)
Worst O(n+k)
Average O(n+k)
Space Complexity O(max)
Stability Yes

Since radix sort is a non-comparative algorithm, it has advantages over comparative sorting algorithms.

For the radix sort that uses counting sort as an intermediate stable sort, the time complexity is O(d(n+k)).

Here, d is the number cycle and O(n+k) is the time complexity of counting sort.

Thus, radix sort has linear time complexity which is better than O(nlog n) of comparative sorting algorithms.

If we take very large digit numbers or the number of other bases like 32-bit and 64-bit numbers then it can perform in linear time however the intermediate sort takes large space.

This makes radix sort space inefficient. This is the reason why this sort is not used in software libraries.


Radix Sort Applications

Radix sort is implemented in

  • DC3 algorithm (Kärkkäinen-Sanders-Burkhardt) while making a suffix array.
  • places where there are numbers in large ranges.

Similar Sorting Algorithms

  1. Quicksort
  2. Merge Sort
  3. Bucket Sort
  4. Counting Sort
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