Bucket Sort – Data Structures and Algorithms Tutorials

Bucket sort is a sorting technique that involves dividing elements into various groups, or buckets. These buckets are formed by uniformly distributing the elements. Once the elements are divided into buckets, they can be sorted using any other sorting algorithm. Finally, the sorted elements are gathered together in an ordered fashion.

Bucket Sort Algorithm:

Create n empty buckets (Or lists) and do the following for every array element arr[i].

Insert arr[i] into bucket[n*array[i]]
Sort individual buckets using insertion sort.
Concatenate all sorted buckets.

How does Bucket Sort work?

To apply bucket sort on the input array [0.78, 0.17, 0.39, 0.26, 0.72, 0.94, 0.21, 0.12, 0.23, 0.68], we follow these steps:

Step 1: Create an array of size 10, where each slot represents a bucket.

Creating Buckets for sorting

Step 2: Insert elements into the buckets from the input array based on their range.

Inserting elements into the buckets:

Take each element from the input array.
Multiply the element by the size of the bucket array (10 in this case). For example, for element 0.23, we get 0.23 * 10 = 2.3.
Convert the result to an integer, which gives us the bucket index. In this case, 2.3 is converted to the integer 2.
Insert the element into the bucket corresponding to the calculated index.
Repeat these steps for all elements in the input array.

Inserting Array elements into respective buckets

Step 3: Sort the elements within each bucket. In this example, we use quicksort (or any stable sorting algorithm) to sort the elements within each bucket.

Sorting the elements within each bucket:

Apply a stable sorting algorithm (e.g., quicksort) to sort the elements within each bucket.
The elements within each bucket are now sorted.

Sorting individual bucket

Step 4: Gather the elements from each bucket and put them back into the original array.

Gathering elements from each bucket:

Iterate through each bucket in order.
Insert each individual element from the bucket into the original array.
Once an element is copied, it is removed from the bucket.
Repeat this process for all buckets until all elements have been gathered.

Inserting buckets in ascending order into the resultant array

Step 5: The original array now contains the sorted elements.

The final sorted array using bucket sort for the given input is [0.12, 0.17, 0.21, 0.23, 0.26, 0.39, 0.68, 0.72, 0.78, 0.94].

Return the Sorted Array

Implementation of Bucket Sort Algorithm

Below is the implementation for the Bucket Sort:

C++14

// C++ program to sort an
// array using bucket sort
#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;
 
// Function to sort arr[] of
// size n using bucket sort
void bucketSort(float arr[], int n)
{
 
    // 1) Create n empty buckets
    vector<float> b[n];
 
    // 2) Put array elements
    // in different buckets
    for (int i = 0; i < n; i++) {
 
        // Index in bucket
        int bi = n * arr[i];
        b[bi].push_back(arr[i]);
    }
 
    // 3) Sort individual buckets
    for (int i = 0; i < n; i++)
        sort(b[i].begin(), b[i].end());
 
    // 4) Concatenate all buckets into arr[]
    int index = 0;
    for (int i = 0; i < n; i++)
        for (int j = 0; j < b[i].size(); j++)
            arr[index++] = b[i][j];
}
 
// Driver program to test above function
int main()
{
    float arr[]
        = { 0.897, 0.565, 0.656, 0.1234, 0.665, 0.3434 };
    int n = sizeof(arr) / sizeof(arr[0]);
    bucketSort(arr, n);
 
    cout << "Sorted array is \n";
    for (int i = 0; i < n; i++)
        cout << arr[i] << " ";
    return 0;
}

Java

// Java program to sort an array
// using bucket sort
import java.util.*;
import java.util.Collections;
 
class GFG {
 
    // Function to sort arr[] of size n
    // using bucket sort
    static void bucketSort(float arr[], int n)
    {
        if (n <= 0)
            return;
 
        // 1) Create n empty buckets
        @SuppressWarnings("unchecked")
        Vector<Float>[] buckets = new Vector[n];
 
        for (int i = 0; i < n; i++) {
            buckets[i] = new Vector<Float>();
        }
 
        // 2) Put array elements in different buckets
        for (int i = 0; i < n; i++) {
            float idx = arr[i] * n;
            buckets[(int)idx].add(arr[i]);
        }
 
        // 3) Sort individual buckets
        for (int i = 0; i < n; i++) {
            Collections.sort(buckets[i]);
        }
 
        // 4) Concatenate all buckets into arr[]
        int index = 0;
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < buckets[i].size(); j++) {
                arr[index++] = buckets[i].get(j);
            }
        }
    }
 
    // Driver code
    public static void main(String args[])
    {
        float arr[] = { (float)0.897, (float)0.565,
                        (float)0.656, (float)0.1234,
                        (float)0.665, (float)0.3434 };
 
        int n = arr.length;
        bucketSort(arr, n);
 
        System.out.println("Sorted array is ");
        for (float el : arr) {
            System.out.print(el + " ");
        }
    }
}
 
// This code is contributed by Himangshu Shekhar Jha

Python3

# Python3 program to sort an array
# using bucket sort
 
 
def insertionSort(b):
    for i in range(1, len(b)):
        up = b[i]
        j = i - 1
        while j >= 0 and b[j] > up:
            b[j + 1] = b[j]
            j -= 1
        b[j + 1] = up
    return b
 
 
def bucketSort(x):
    arr = []
    slot_num = 10  # 10 means 10 slots, each
    # slot's size is 0.1
    for i in range(slot_num):
        arr.append([])
 
    # Put array elements in different buckets
    for j in x:
        index_b = int(slot_num * j)
        arr[index_b].append(j)
 
    # Sort individual buckets
    for i in range(slot_num):
        arr[i] = insertionSort(arr[i])
 
    # concatenate the result
    k = 0
    for i in range(slot_num):
        for j in range(len(arr[i])):
            x[k] = arr[i][j]
            k += 1
    return x
 
 
# Driver Code
x = [0.897, 0.565, 0.656,
     0.1234, 0.665, 0.3434]
print("Sorted Array is")
print(bucketSort(x))
 
# This code is contributed by
# Oneil Hsiao

C#

// C# program to sort an array
// using bucket sort
using System;
using System.Collections;
using System.Collections.Generic;
class GFG {
 
    // Function to sort arr[] of size n
    // using bucket sort
    static void bucketSort(float[] arr, int n)
    {
        if (n <= 0)
            return;
 
        // 1) Create n empty buckets
        List<float>[] buckets = new List<float>[ n ];
 
        for (int i = 0; i < n; i++) {
            buckets[i] = new List<float>();
        }
 
        // 2) Put array elements in different buckets
        for (int i = 0; i < n; i++) {
            float idx = arr[i] * n;
            buckets[(int)idx].Add(arr[i]);
        }
 
        // 3) Sort individual buckets
        for (int i = 0; i < n; i++) {
            buckets[i].Sort();
        }
 
        // 4) Concatenate all buckets into arr[]
        int index = 0;
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < buckets[i].Count; j++) {
                arr[index++] = buckets[i][j];
            }
        }
    }
 
    // Driver code
    public static void Main()
    {
        float[] arr = { (float)0.897, (float)0.565,
                        (float)0.656, (float)0.1234,
                        (float)0.665, (float)0.3434 };
 
        int n = arr.Length;
        bucketSort(arr, n);
 
        Console.WriteLine("Sorted array is ");
        foreach(float el in arr)
        {
            Console.Write(el + " ");
        }
    }
}
 
// This code is contributed by rutvik_56

Javascript

<script>
// Javascript program to sort an array
// using bucket sort
 
// Function to sort arr[] of size n
// using bucket sort
function bucketSort(arr,n)
{
    if (n <= 0)
            return;
  
        // 1) Create n empty buckets      
        let buckets = new Array(n);
  
        for (let i = 0; i < n; i++)
        {
            buckets[i] = [];
        }
  
        // 2) Put array elements in different buckets
        for (let i = 0; i < n; i++) {
            let idx = arr[i] * n;
            let flr = Math.floor(idx);
            buckets[flr].push(arr[i]);
        }
  
        // 3) Sort individual buckets
        for (let i = 0; i < n; i++) {
            buckets[i].sort(function(a,b){return a-b;});
        }
  
        // 4) Concatenate all buckets into arr[]
        let index = 0;
        for (let i = 0; i < n; i++) {
            for (let j = 0; j < buckets[i].length; j++) {
                arr[index++] = buckets[i][j];
            }
        }
}
 
// Driver code
let arr = [0.897, 0.565,
         0.656, 0.1234,
         0.665, 0.3434];
let n = arr.length;
bucketSort(arr, n);
 
document.write("Sorted array is <br>");
for (let el of arr.values()) {
    document.write(el + " ");
}
 
// This code is contributed by unknown2108
</script>

Output

Sorted array is 
0.1234 0.3434 0.565 0.656 0.665 0.897

Complexity Analysis of Bucket Sort Algorithm

Time Complexity: O(n²),

If we assume that insertion in a bucket takes O(1) time then steps 1 and 2 of the above algorithm clearly take O(n) time.
The O(1) is easily possible if we use a linked list to represent a bucket.
Step 4 also takes O(n) time as there will be n items in all buckets.
The main step to analyze is step 3. This step also takes O(n) time on average if all numbers are uniformly distributed.

Auxiliary Space: O(n+k)

Recommended

Solve DSA problems on GfG Practice.

Solve Problems

Last Updated : 08 Sep, 2023