Recursive program to find all Indices of a Number

30 Nov, 2021
Given an array arr of size N and an integer X. The task is to find all the indices of the integer X in the arrayExamples:  
Input: arr = {1, 2, 3, 2, 2, 5}, X = 2 Output: 1 3 4 Element 2 is present at indices 1, 3, 4 (0 based indexing)Input: arr[] = {7, 7, 7}, X = 7 Output: 0 1 2  
 
The iterative approach is simple, just traverse the given array and keep on storing the indices of the element in the another array.Recursive approach:  
If the start index reaches the length of the array, then return empty array
Else keep the first element of the array with yourself and pass the rest of the array to recursion. If the element at start index is not equal to x then just simply return the answer which came from recursion.  Else if the element at start index is equal to x then shift the elements of the array (which is the answer of recursion) one step to the right and then put the start index in the front of the array (which came through recursion)  
If the element at start index is not equal to x then just simply return the answer which came from recursion.  
Else if the element at start index is equal to x then shift the elements of the array (which is the answer of recursion) one step to the right and then put the start index in the front of the array (which came through recursion)  
Below is the implementation of the above approach: 
C++
Java
Python3
C#
Javascript
// CPP program to find all indices of a number#include <bits/stdc++.h>using namespace std; // A recursive function to find all// indices of a numberint AllIndexesRecursive(int input[], int size,                    int x, int output[]){         // If an empty array comes    // to the function, then    // return zero    if (size == 0) {        return 0;    }     // Getting the recursive answer    int smallAns = AllIndexesRecursive(input + 1,                                    size - 1, x, output);     // If the element at index 0 is equal    // to x then add 1 to the array values    // and shift them right by 1 step    if (input[0] == x) {        for (int i = smallAns - 1; i >= 0; i--) {            output[i + 1] = output[i] + 1;        }         // Put the start index in front        // of the array        output[0] = 0;        smallAns++;    }    else {                 // If the element at index 0 is not equal        // to x then add 1 to the array values        for (int i = smallAns - 1; i >= 0; i--) {            output[i] = output[i] + 1;        }    }    return smallAns;} // Function to find all indices of a numbervoid AllIndexes(int input[], int n, int x){    int output[n];    int size = AllIndexesRecursive(input, n,                                x, output);    for (int i = 0; i < size; i++) {        cout << output[i] << " ";    }} // Driver Codeint main(){    int arr[] = { 1, 2, 3, 2, 2, 5 }, x = 2;         int n = sizeof(arr) / sizeof(arr[0]);         // Function call    AllIndexes(arr, n, x);         return 0;}
// Java program to find all// indices of a numberpublic class GFG {     public int[] AllIndexesRecursive(int input[],                                int x, int start)    {        // If the start index reaches the        // length of the array, then        // return empty array        if (start == input.length) {            int[] ans = new int[0]; // empty array            return ans;        }         // Getting the recursive answer in        // smallIndex array        int[] smallIndex = AllIndexesRecursive(input, x,                                              start + 1);         // If the element at start index is equal        // to x then        // (which is the answer of recursion) and then        // (which came through recursion)        if (input[start] == x) {            int[] myAns = new int[smallIndex.length + 1];             // Put the start index in front            // of the array            myAns[0] = start;            for (int i = 0; i < smallIndex.length; i++) {                                 // Shift the elements of the array                // one step to the right                // and putting them in                // myAns array                myAns[i + 1] = smallIndex[i];            }            return myAns;        }        else {                         // If the element at start index is not            // equal to x then just simply return the            // answer which came from recursion.            return smallIndex;        }    }     public int[] AllIndexes(int input[], int x)    {         return AllIndexesRecursive(input, x, 0);    }         // Driver Code    public static void main(String args[])    {        GFG g = new GFG();        int arr[] = { 1, 2, 3, 2, 2, 5 }, x = 2;                 int output[] = g.AllIndexes(arr, x);                 // Printing the output array        for (int i = 0; i < output.length; i++) {            System.out.print(output[i] + " ");        }    }}
# Python3 program to find all# indices of a numberdef AllIndexesRecursive(input, x, start):         # If the start index reaches the    # length of the array, then    # return empty array    if (start == len(input)):        ans = [] # empty array        return ans     # Getting the recursive answer in    # smallIndex array    smallIndex = AllIndexesRecursive(input, x,                                     start + 1)     # If the element at start index is equal    # to x then    # (which is the answer of recursion) and then    # (which came through recursion)    if (input[start] == x):        myAns = [0 for i in range(len(smallIndex) + 1)]         # Put the start index in front        # of the array        myAns[0] = start        for i in range(len(smallIndex)):             # Shift the elements of the array            # one step to the right            # and putting them in            # myAns array            myAns[i + 1] = smallIndex[i]         return myAns    else:         # If the element at start index is not        # equal to x then just simply return the        # answer which came from recursion.        return smallIndex # Function to find all indices of a numberdef AllIndexes(input, x):     return AllIndexesRecursive(input, x, 0) # Driver Codearr = [ 1, 2, 3, 2, 2, 5 ]x = 2 output=AllIndexes(arr, x) # Printing the output arrayfor i in output:    print(i, end = " ") # This code is contributed by Mohit Kumar
// C# program to find all// indices of a numberusing System;class GFG{    public int[] AllIndexesRecursive(int []input,                                        int x, int start)    {        // If the start index reaches the        // length of the array, then        // return empty array        if (start == input.Length)        {            int[] ans = new int[0]; // empty array            return ans;        }         // Getting the recursive answer in        // smallIndex array        int[] smallIndex = AllIndexesRecursive(input, x,                                               start + 1);         // If the element at start index is equal        // to x then        // (which is the answer of recursion) and        // then (which came through recursion)        if (input[start] == x)        {            int[] myAns = new int[smallIndex.Length + 1];             // Put the start index in front            // of the array            myAns[0] = start;            for (int i = 0; i < smallIndex.Length; i++)            {                                 // Shift the elements of the array                // one step to the right                // and putting them in                // myAns array                myAns[i + 1] = smallIndex[i];            }            return myAns;        }        else        {                         // If the element at start index is not            // equal to x then just simply return the            // answer which came from recursion.            return smallIndex;        }    }     public int[] AllIndexes(int []input, int x)    {        return AllIndexesRecursive(input, x, 0);    }         // Driver Code    public static void Main()    {        GFG g = new GFG();        int []arr = { 1, 2, 3, 2, 2, 5 };        int x = 2;                 int []output = g.AllIndexes(arr, x);                 // Printing the output array        for (int i = 0; i < output.Length; i++)        {            Console.Write(output[i] + " ");        }    }} // This code is contributed by anuj_67..
<script>    // Javascript program to find all indices of a number         function AllIndexesRecursive(input, x, start)    {        // If the start index reaches the        // length of the array, then        // return empty array        if (start == input.length)        {            let ans = new Array(0); // empty array            return ans;        }           // Getting the recursive answer in        // smallIndex array        let smallIndex = AllIndexesRecursive(input, x, start + 1);           // If the element at start index is equal        // to x then        // (which is the answer of recursion) and        // then (which came through recursion)        if (input[start] == x)        {            let myAns = new Array(smallIndex.length + 1);               // Put the start index in front            // of the array            myAns[0] = start;            for (let i = 0; i < smallIndex.length; i++)            {                                   // Shift the elements of the array                // one step to the right                // and putting them in                // myAns array                myAns[i + 1] = smallIndex[i];            }            return myAns;        }        else        {                           // If the element at start index is not            // equal to x then just simply return the            // answer which came from recursion.            return smallIndex;        }    }       function AllIndexes(input, x)    {        return AllIndexesRecursive(input, x, 0);    }         let arr = [ 1, 2, 3, 2, 2, 5 ];    let x = 2;     let output = AllIndexes(arr, x);     // Printing the output array    for(let i = 0; i < output.length; i++)    {      document.write(output[i] + " ");    }         // This code is contributed by mukesh07.</script>
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Simpler Recursive approach:  (Traversing from the End)
If the last index reaches the length of the array, then return empty array
Else reduce the last index and pass the whole array to recursion.
If the element at the last index is not equal to x then just simply return the answer which came from recursion.
Else if the element at the last index is equal to x then simply insert the last element at the last index into index pointed by ans.
This avoids the complication of shifting all the elements by one index and also incrementing all the elements in the output array.
Below is the implementation of the above approach:
C++
Java
Python3
C#
Javascript
// CPP program to find all indices of a number#include <bits/stdc++.h>using namespace std; // A recursive function to find all// indices of a numberint AllIndexesRecursive(int input[], int size,                    int x, int output[]){         // If the input array becomes empty      if(size == 0)        return 0;              // Getting the recursive answer      int smallAns = AllIndexesRecursive(input, size - 1, x, output);           // If the last element of input array is equal to x    if(input[size - 1] == x)    {      // Insert the index of last element of the input array into the output array      // And increment ans      output[smallAns++] = size - 1;    }    return smallAns;} // Function to find all indices of a numbervoid AllIndexes(int input[], int n, int x){    int output[n];    int size = AllIndexesRecursive(input, n,                                x, output);    for (int i = 0; i < size; i++) {        cout << output[i] << " ";    }} // Driver Codeint main(){    int arr[] = { 1, 2, 3, 2, 2, 5 }, x = 2;         int n = sizeof(arr) / sizeof(arr[0]);         // Function call    AllIndexes(arr, n, x);         return 0;}
// Java program to find all indices of a numberpublic class Main{    // A recursive function to find all    // indices of a number    static int AllIndexesRecursive(int[] input, int size, int x, int[] output)    {                  // If the input array becomes empty          if(size == 0)            return 0;                            // Getting the recursive answer        int smallAns = AllIndexesRecursive(input, size - 1, x, output);                    // If the last element of input array is equal to x        if(input[size - 1] == x)        {          // Insert the index of last element of the input array into the output array          // And increment ans          output[smallAns++] = size - 1;        }        return smallAns;    }          // Function to find all indices of a number    static void AllIndexes(int[] input, int n, int x)    {        int[] output = new int[n];        int size = AllIndexesRecursive(input, n, x, output);        for (int i = 0; i < size; i++) {            System.out.print(output[i] + " ");        }    }         public static void main(String[] args) {        int[] arr = { 1, 2, 3, 2, 2, 5 };        int x = 2;                  int n = arr.length;                  // Function call        AllIndexes(arr, n, x);    }} // This code is contributed by suresh07.
# Python3 program to find all indices of a number # A recursive function to find all# indices of a numberdef AllIndexesRecursive(Input, size, x, output):    # If the input array becomes empty    if size == 0:        return 0        # Getting the recursive answer    smallAns = AllIndexesRecursive(Input, size - 1, x, output)            # If the last element of input array is equal to x    if Input[size - 1] == x:             # Insert the index of last element of the input array into the output array      # And increment ans      output[smallAns] = size - 1      smallAns+=1    return smallAns    # Function to find all indices of a numberdef AllIndexes(Input, n, x):    output = [0]*n    size = AllIndexesRecursive(Input, n, x, output)    for i in range(size):        print(output[i], "", end = "") arr = [ 1, 2, 3, 2, 2, 5 ]x = 2    n = len(arr)    # Function callAllIndexes(arr, n, x) # This code is contributed by rameshtravel07
// C# program to find all indices of a numberusing System;using System.Collections.Generic;class GFG {     // A recursive function to find all    // indices of a number    static int AllIndexesRecursive(int[] input, int size, int x, int[] output)    {                  // If the input array becomes empty          if(size == 0)            return 0;                            // Getting the recursive answer        int smallAns = AllIndexesRecursive(input, size - 1, x, output);                    // If the last element of input array is equal to x        if(input[size - 1] == x)        {          // Insert the index of last element of the input array into the output array          // And increment ans          output[smallAns++] = size - 1;        }        return smallAns;    }          // Function to find all indices of a number    static void AllIndexes(int[] input, int n, int x)    {        int[] output = new int[n];        int size = AllIndexesRecursive(input, n, x, output);        for (int i = 0; i < size; i++) {            Console.Write(output[i] + " ");        }    }   static void Main() {    int[] arr = { 1, 2, 3, 2, 2, 5 };    int x = 2;          int n = arr.Length;          // Function call    AllIndexes(arr, n, x);  }} // This code is contributed by decode22007.
<script>    // Javascript program to find all indices of a number         // A recursive function to find all    // indices of a number    function AllIndexesRecursive(input, size, x, output)    {                   // If the input array becomes empty          if(size == 0)            return 0;                              // Getting the recursive answer        let smallAns = AllIndexesRecursive(input, size - 1, x, output);                     // If the last element of input array is equal to x        if(input[size - 1] == x)        {          // Insert the index of last element of the input array into the output array          // And increment ans          output[smallAns++] = size - 1;        }        return smallAns;    }           // Function to find all indices of a number    function AllIndexes(input, n, x)    {        let output = new Array(n);        let size = AllIndexesRecursive(input, n, x, output);        for (let i = 0; i < size; i++) {            document.write(output[i] + " ");        }    }         let arr = [ 1, 2, 3, 2, 2, 5 ];    let x = 2;           let n = arr.length;           // Function call    AllIndexes(arr, n, x); // This code is contributed by divyeshrabadiya07.</script>
1 3 4 
mohit kumar 29
vt_m
Akanksha_Rai
mukesh07
anirudhkharish
decode2207
suresh07
divyeshrabadiya07
rameshtravel07
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