Counting Sort Demo

src/counting_sort_demo.cpp

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+/*
+Author : Aswin Sampath 
+Topic : Counting Sort
+
+                            Counting Sort
+    When to use?
+        Vizualise the array/vector elements 
+        Amount of space needed by the algorithm = Range of the vector
+        Range of vector = Max(vector)- Min(vector)
+        If Range is of the order 10^5 then we can use counting sort subjet to the compiler 
+
+    How to Implement 
+        Sort the numbers according to their frequency/ number of occurences
+        Place the element in their respective position according to the ranking 
+
+    Time Complexity : O(n)
+    Space COmplexity : O(Max(arr)-Min(arr));
+*/
+
+
+#include<iostream>
+#include<vector>
+using namespace std;
+
+void printVector(vector<int>v){
+    cout<<"\n Vector elements \n ";
+    for(int element:v){
+        cout<<element<<" ";
+    }
+    cout<<"\n";
+}
+
+int findMin(vector<int>arr){
+    int minele = arr[0];
+    for(int ele:arr){
+        if(ele<=minele)minele=ele;
+    }
+    return minele;
+}
+
+int findMax(vector<int>arr){
+    int maxele = arr[0];
+    for(int ele:arr){
+        if(ele>=maxele)maxele=ele;
+    }
+    return maxele;
+}
+
+
+int findRange(vector<int>arr){
+    int maxele = findMax(arr);
+    int minele = findMin(arr);
+    return maxele - minele;
+}
+
+vector<int>freq;
+vector<int>output;
+
+void countingSort(vector<int> &arr,int range){
+
+    // cout<<"Size of arr = "<<arr.size()<<"\n";
+    output.resize(arr.size());
+    freq.resize(range+1);
+
+    // Find the frequency for each element in the array
+    for(int ele : arr){
+        freq[ele]++;
+    }
+    //printVector(freq);
+
+    // Find the ranking for each element by using prefix sum 
+    for(int i=1;i<freq.size();i++){
+        freq[i]+=freq[i-1];
+    }
+    //printVector(freq);
+    /* Ranking  table
+    number  rank
+    0   -   1
+    1   -   3
+    2   -   7
+    3   -   10
+    4   -   14
+    5   -   17
+    6   -   21
+    7   -   25
+    8   -   28
+    9   -   30
+
+    */
+
+    // Arrange the Elements according their rank
+    for(int i=0;i<arr.size();i++){
+        int num = arr[i]; //The number in the array
+        int rank = freq[num]; // The rank of the number
+        output[rank-1]= num; // Since indexing starts from and rank from 1 we need to adjust the difference
+        freq[num]--; // Decrease the freq count of the number once it has been written to output
+    }
+    //printVector(output);
+
+    // Rewrite the output to the original array
+    for(int i=0;i<arr.size();i++){
+        arr[i]=output[i];
+    }
+    output.resize(0); // Free the output array space
+    cout<<"\n";
+}
+
+int main(){
+
+    // TODO : Sort the below array
+    vector<int>arr = {2,3,7,4,6,8,9,2,5,4,8,3,1,6,7,2,5,7,6,1,4,2,3,9,8,7,4,5,6,0};
+
+    /* Frequency Table
+    ele     freq
+    0   -   1   
+    1   -   2
+    2   -   4
+    3   -   3
+    4   -   4
+    5   -   3
+    6   -   4
+    7   -   4
+    8   -   3
+    9   -   2
+    */
+
+    // Vector Range = max(arr)-min(arr) = 9 - 0 = 9 Hence we require of a 9 space in frequency array
+    int range = findRange(arr);
+    cout<<"Before Sorting \n";
+    printVector(arr);
+    countingSort(arr,range); // Sorting algorithm
+    cout<<"After Sorting \n";
+    printVector(arr);
+}
+