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Bucket Sort
• Difficulty Level : Easy
• Last Updated : 10 Mar, 2021

Bucket sort is mainly useful when input is uniformly distributed over a range. For example, consider the following problem.
Sort a large set of floating point numbers which are in range from 0.0 to 1.0 and are uniformly distributed across the range. How do we sort the numbers efficiently?
A simple way is to apply a comparison based sorting algorithm. The lower bound for Comparison based sorting algorithm (Merge Sort, Heap Sort, Quick-Sort .. etc) is Ω(n Log n), i.e., they cannot do better than nLogn.
Can we sort the array in linear time? Counting sort can not be applied here as we use keys as index in counting sort. Here keys are floating point numbers.
The idea is to use bucket sort. Following is bucket algorithm.

```bucketSort(arr[], n)
1) Create n empty buckets (Or lists).
2) Do following for every array element arr[i].
.......a) Insert arr[i] into bucket[n*array[i]]
3) Sort individual buckets using insertion sort.
4) Concatenate all sorted buckets.```

Time Complexity: 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 (In the following code, C++ vector is used for simplicity). 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 (please refer CLRS book for more details)
Following is the implementation of the above algorithm.

C++

 `// C++ program to sort an``// array using bucket sort``#include ``#include ``#include ``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++) {``        ``int` `bi = n * arr[i]; ``// Index in bucket``        ``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[] buckets = ``new` `Vector[n];` `        ``for` `(``int` `i = ``0``; i < n; i++) {``            ``buckets[i] = ``new` `Vector();``        ``}` `        ``// 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`
Output
```Sorted array is
0.1234 0.3434 0.565 0.656 0.665 0.897 ```

Bucket Sort for numbers having integer part:

Algorithm

1. Find maximum element and minimum of the array
2. Calculate the range of each bucket
```          range = (max - min) / n
n is the number of buckets```

3. Create n buckets of calculated range

4. Scatter the array elements to these buckets

`          BucketIndex = ( arr[i] - min ) / range`

6. Now sort each bucket individually

7. Gather the sorted elements from buckets to original array

```Input :
Unsorted array:  [ 9.8 , 0.6 , 10.1 , 1.9 , 3.07 , 3.04 , 5.0 , 8.0 , 4.8 , 7.68 ]
No of buckets :  5

Output :
Sorted array:   [ 0.6 , 1.9 , 3.04 , 3.07 , 4.8 , 5.0 , 7.68 , 8.0 , 9.8 , 10.1 ]```

```Input :
Unsorted array:  [0.49 , 5.9 , 3.4 , 1.11 , 4.5 , 6.6 , 2.0]
No of buckets: 3

Output :
Sorted array:   [0.49 , 1.11 , 2.0 , 3.4 , 4.5 , 5.9 , 6.6]```

Code :

Python3

 `# Python program for the above approach` `# Bucket sort for numbers``# having interger part``def` `bucketSort(arr, noOfBuckets):``    ``max_ele ``=` `max``(arr)``    ``min_ele ``=` `min``(arr)` `    ``# range(for buckets)``    ``rnge ``=` `(max_ele ``-` `min_ele) ``/` `noOfBuckets` `    ``temp ``=` `[]` `    ``# create empty buckets``    ``for` `i ``in` `range``(noOfBuckets):``        ``temp.append([])` `    ``# scatter the array elements``    ``# into the correct bucket``    ``for` `i ``in` `range``(``len``(arr)):``        ``diff ``=` `(arr[i] ``-` `min_ele) ``/` `rnge ``-``              ``int``((arr[i] ``-` `min_ele) ``/` `rnge)` `        ``# append the boundary elements to the lower array``        ``if``(diff ``=``=` `0` `and` `arr[i] !``=` `min_ele):``            ``temp[``int``((arr[i] ``-` `min_ele) ``/` `rnge) ``-` `1``].append(arr[i])` `        ``else``:``            ``temp[``int``((arr[i] ``-` `min_ele) ``/` `rnge)].append(arr[i])` `    ``# Sort each bucket individually``    ``for` `i ``in` `range``(``len``(temp)):``        ``if` `len``(temp[i]) !``=` `0``:``            ``temp[i].sort()` `    ``# Gather sorted elements``    ``# to the original array``    ``k ``=` `0``    ``for` `lst ``in` `temp:``        ``if` `lst:``            ``for` `i ``in` `lst:``                ``arr[k] ``=` `i``                ``k ``=` `k``+``1`  `# Driver Code``arr ``=` `[``9.8``, ``0.6``, ``10.1``, ``1.9``, ``3.07``, ``3.04``, ``5.0``, ``8.0``, ``4.8``, ``7.68``]``noOfBuckets ``=` `5``bucketSort(arr, noOfBuckets)``print``(``"Sorted array: "``, arr)` `# This code is contributed by``# Vinita Yadav`
Output
`Sorted array:  [0.6, 1.9, 3.04, 3.07, 4.8, 5.0, 7.68, 8.0, 9.8, 10.1]`

Quiz on Bucket Sort

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