Find the index of first 1 in a sorted array of 0's and 1's - GeeksforGeeks

20 May, 2021
Given a sorted array consisting 0’s and 1’s. The problem is to find the index of first ‘1’ in the sorted array. It could be possible that the array consists of only 0’s or only 1’s. If 1’s are not present in the array then print “-1”.Examples :  
Input : arr[] = {0, 0, 0, 0, 0, 0, 1, 1, 1, 1}
Output : 6
The index of first 1 in the array is 6.

Input : arr[] = {0, 0, 0, 0}
Output : -1
1's are not present in the array.
Source: Asked in Amazon Interview 
Naive Approach: Traverse the array from left to right and return the index of first ‘1’. If 1’s are not present in the array, then print “-1”. 
 
 
C++
Java
Python3
C#
PHP
Javascript
// C++ implementation to find the index of// first '1' in a sorted array of 0's and 1's#include <bits/stdc++.h>using namespace std; // function to find the index of first '1'int indexOfFirstOne(int arr[], int n){    // traverse the array from left to right    for (int i = 0; i < n; i++)         // if true, then return i        if (arr[i] == 1)            return i;     // 1's are not present in the array    return -1;} // Driver program to test aboveint main(){    int arr[] = { 0, 0, 0, 0, 0, 0, 1, 1, 1, 1 };    int n = sizeof(arr) / sizeof(arr[0]);    cout << indexOfFirstOne(arr, n);    return 0;}
// Java program to find the index of// first '1' in a sorted array of 0's and 1'sclass GFG {          // function to find the index of first '1'    public static int indexOfFirstOne(int arr[], int n)    {        // traverse the array from left to right        for (int i = 0; i < n; i++)                  // if true, then return i            if (arr[i] == 1)                return i;              // 1's are not present in the array        return -1;    }         /* Driver program to test above function */    public static void main(String[] args)    {        int arr[] = { 0, 0, 0, 0, 0, 0, 1, 1, 1, 1 };        int n = arr.length;        System.out.println(indexOfFirstOne(arr, n));             }  }// This code is contributed by Arnav Kr. Mandal.
# Python3 implementation to find# the index of first '1' in a# sorted array of 0's and 1's # function to find the index of first '1'def indexOfFirstOne(arr, n):     # traverse the array from left to right    for i in range(0, n):                 # if true, then return i        if (arr[i] == 1):            return i     # 1's are not present in the array    return -1 # Driver program to test abovearr = [ 0, 0, 0, 0, 0, 0, 1, 1, 1, 1 ]n = len(arr)ans = indexOfFirstOne(arr, n)print(ans) # This code is contributed by saloni1297
// C# program to find the index of// first '1' in a sorted array// of 0's and 1'susing System; class GFG{    // function to find the index of first '1'    public static int indexOfFirstOne(int []arr, int n)    {        // traverse the array from left to right        for (int i = 0; i < n; i++)                 // if true, then return i            if (arr[i] == 1)                return i;             // 1's are not present in the array        return -1;    }         // Driver program    public static void Main()    {        int []arr = { 0, 0, 0, 0, 0, 0, 1, 1, 1, 1 };        int n = arr.Length;        Console.Write(indexOfFirstOne(arr, n));             }} // This code is contributed by Sam007
<?php// PHP implementation to find the index of// first '1' in a sorted array of 0's and 1's// function to find the index of first '1' function indexOfFirstOne($arr, $n){         // traverse the array from    // left to right    for ($i = 0; $i < $n; $i++)         // if true, then return i        if ($arr[$i] == 1)            return $i;     // 1's are not present    // in the array    return -1;}     // Driver Code    $arr = array(0, 0, 0, 0, 0, 0, 1, 1, 1, 1);    $n = sizeof($arr) / sizeof($arr[0]);    echo indexOfFirstOne($arr, $n);    return 0; // This code is contributed by nitin mittal.?>
<script> // Javascript implementation to find the index of// first '1' in a sorted array of 0's and 1's // function to find the index of first '1'function indexOfFirstOne(arr, n){    // traverse the array from left to right    for (let i = 0; i < n; i++)         // if true, then return i        if (arr[i] == 1)            return i;     // 1's are not present in the array    return -1;} // Driver program to test above     let arr = [ 0, 0, 0, 0, 0, 0, 1, 1, 1, 1 ];    let n = arr.length;    document.write(indexOfFirstOne(arr, n)); //This code is contributed by Mayank Tyagi</script><script> // Javascript implementation to find the index of// first '1' in a sorted array of 0's and 1's // function to find the index of first '1'function indexOfFirstOne(arr, n){    // traverse the array from left to right    for (let i = 0; i < n; i++)         // if true, then return i        if (arr[i] == 1)            return i;     // 1's are not present in the array    return -1;} // Driver program to test above     let arr = [ 0, 0, 0, 0, 0, 0, 1, 1, 1, 1 ];    let n = arr.length;    document.write(indexOfFirstOne(arr, n)); //This code is contributed by Mayank Tyagi</script><script> // Javascript implementation to find the index of// first '1' in a sorted array of 0's and 1's // function to find the index of first '1'function indexOfFirstOne(arr, n){    // traverse the array from left to right    for (let i = 0; i < n; i++)         // if true, then return i        if (arr[i] == 1)            return i;     // 1's are not present in the array    return -1;} // Driver program to test above     let arr = [ 0, 0, 0, 0, 0, 0, 1, 1, 1, 1 ];    let n = arr.length;    document.write(indexOfFirstOne(arr, n)); //This code is contributed by Mayank Tyagi</script><script> // Javascript implementation to find the index of// first '1' in a sorted array of 0's and 1's // function to find the index of first '1'function indexOfFirstOne(arr, n){    // traverse the array from left to right    for (let i = 0; i < n; i++)         // if true, then return i        if (arr[i] == 1)            return i;     // 1's are not present in the array    return -1;} // Driver program to test above     let arr = [ 0, 0, 0, 0, 0, 0, 1, 1, 1, 1 ];    let n = arr.length;    document.write(indexOfFirstOne(arr, n)); //This code is contributed by Mayank Tyagi</script><script> // Javascript implementation to find the index of// first '1' in a sorted array of 0's and 1's // function to find the index of first '1'function indexOfFirstOne(arr, n){    // traverse the array from left to right    for (let i = 0; i < n; i++)         // if true, then return i        if (arr[i] == 1)            return i;     // 1's are not present in the array    return -1;} // Driver program to test above     let arr = [ 0, 0, 0, 0, 0, 0, 1, 1, 1, 1 ];    let n = arr.length;    document.write(indexOfFirstOne(arr, n)); //This code is contributed by Mayank Tyagi</script><script> // Javascript implementation to find the index of// first '1' in a sorted array of 0's and 1's // function to find the index of first '1'function indexOfFirstOne(arr, n){    // traverse the array from left to right    for (let i = 0; i < n; i++)         // if true, then return i        if (arr[i] == 1)            return i;     // 1's are not present in the array    return -1;} // Driver program to test above     let arr = [ 0, 0, 0, 0, 0, 0, 1, 1, 1, 1 ];    let n = arr.length;    document.write(indexOfFirstOne(arr, n)); //This code is contributed by Mayank Tyagi</script><script> // Javascript implementation to find the index of// first '1' in a sorted array of 0's and 1's // function to find the index of first '1'function indexOfFirstOne(arr, n){    // traverse the array from left to right    for (let i = 0; i < n; i++)         // if true, then return i        if (arr[i] == 1)            return i;     // 1's are not present in the array    return -1;} // Driver program to test above     let arr = [ 0, 0, 0, 0, 0, 0, 1, 1, 1, 1 ];    let n = arr.length;    document.write(indexOfFirstOne(arr, n)); //This code is contributed by Mayank Tyagi</script><script> // Javascript implementation to find the index of// first '1' in a sorted array of 0's and 1's // function to find the index of first '1'function indexOfFirstOne(arr, n){    // traverse the array from left to right    for (let i = 0; i < n; i++)         // if true, then return i        if (arr[i] == 1)            return i;     // 1's are not present in the array    return -1;} // Driver program to test above     let arr = [ 0, 0, 0, 0, 0, 0, 1, 1, 1, 1 ];    let n = arr.length;    document.write(indexOfFirstOne(arr, n)); //This code is contributed by Mayank Tyagi</script><script> // Javascript implementation to find the index of// first '1' in a sorted array of 0's and 1's // function to find the index of first '1'function indexOfFirstOne(arr, n){    // traverse the array from left to right    for (let i = 0; i < n; i++)         // if true, then return i        if (arr[i] == 1)            return i;     // 1's are not present in the array    return -1;} // Driver program to test above     let arr = [ 0, 0, 0, 0, 0, 0, 1, 1, 1, 1 ];    let n = arr.length;    document.write(indexOfFirstOne(arr, n)); //This code is contributed by Mayank Tyagi</script><script> // Javascript implementation to find the index of// first '1' in a sorted array of 0's and 1's // function to find the index of first '1'function indexOfFirstOne(arr, n){    // traverse the array from left to right    for (let i = 0; i < n; i++)         // if true, then return i        if (arr[i] == 1)            return i;     // 1's are not present in the array    return -1;} // Driver program to test above     let arr = [ 0, 0, 0, 0, 0, 0, 1, 1, 1, 1 ];    let n = arr.length;    document.write(indexOfFirstOne(arr, n)); //This code is contributed by Mayank Tyagi</script>
Output :  
6
Time Complexity: O(n)Efficient Approach(Binary Search): Use the technique of binary search on the sorted array, so as to find the index of first ‘1’. 
C++
Java
Python3
C#
PHP
Javascript
// C++ implementation to find the index of first// '1' in a sorted array of 0's and 1's#include <bits/stdc++.h>using namespace std; // function to find the index of first '1'// binary search technique is appliedint indexOfFirstOne(int arr[], int low, int high){    while (low <= high) {        int mid = (low + high) / 2;         // if true, then 'mid' is the index of first '1'        if (arr[mid] == 1 && (mid == 0 || arr[mid - 1] == 0))            return mid;         // first '1' lies to the left of 'mid'        else if (arr[mid] == 1)            high = mid - 1;         // first '1' lies to the right of 'mid'        else            low = mid + 1;    }     // 1's are not present in the array    return -1;} // Driver program to test aboveint main(){    int arr[] = { 0, 0, 0, 0, 0, 0, 1, 1, 1, 1 };    int n = sizeof(arr) / sizeof(arr[0]);    cout << indexOfFirstOne(arr, 0, n - 1);    return 0;}
// Java program to find the index of// first '1' in a sorted array of 0's and 1'sclass GFG {         // function to find the index of first '1'    // binary search technique is applied    public static int indexOfFirstOne(int arr[], int low,                                                int high)    {        while (low <= high) {            int mid = (low + high) / 2;                  // if true, then 'mid' is the index of first '1'            if (arr[mid] == 1 && (mid == 0 || arr[mid - 1]                                                    == 0))                return mid;                  // first '1' lies to the left of 'mid'            else if (arr[mid] == 1)                high = mid - 1;                  // first '1' lies to the right of 'mid'            else                low = mid + 1;        }              // 1's are not present in the array        return -1;    }         /* Driver program to test above function */    public static void main(String[] args)    {        int arr[] = { 0, 0, 0, 0, 0, 0, 1, 1, 1, 1 };        int n = arr.length;        System.out.println(indexOfFirstOne(arr, 0,                                              n - 1));             }  }// This code is contributed by Arnav Kr. Mandal.
# Python3 implementation to find# the index of first '1' in a# sorted array of 0's and 1's # function to find the index of first '1'# binary search technique is applieddef indexOfFirstOne( arr, low, high):     while (low <= high):                 mid = int((low + high) / 2)         # if true, then 'mid' is the index of first '1'        if (arr[mid] == 1 and (mid == 0 or arr[mid - 1] == 0)):            return mid         # first '1' lies to the left of 'mid'        elif (arr[mid] == 1):            high = mid - 1         # first '1' lies to the right of 'mid'        else:            low = mid + 1          # 1's are not present in the array    return -1; # Driver program to test abovearr = [0, 0, 0, 0, 0, 0, 1, 1, 1, 1 ]n = len(arr)ans = indexOfFirstOne(arr, 0, n - 1)print (ans) # This code is contributed by saloni1297
// C# program to find the index of// first '1' in a sorted array of 0's and 1'susing System; class GFG{    // function to find the index of first '1'    // binary search technique is applied    public static int indexOfFirstOne(int []arr, int low,                                                int high)    {        while (low <= high) {            int mid = (low + high) / 2;                 // if true, then 'mid' is the index of first '1'            if (arr[mid] == 1 && (mid == 0 || arr[mid - 1]                                                    == 0))                return mid;                 // first '1' lies to the left of 'mid'            else if (arr[mid] == 1)                high = mid - 1;                 // first '1' lies to the right of 'mid'            else                low = mid + 1;        }             // 1's are not present in the array        return -1;    }         // Driver program    public static void Main()    {        int []arr = { 0, 0, 0, 0, 0, 0, 1, 1, 1, 1 };        int n = arr.Length;        Console.Write(indexOfFirstOne(arr, 0, n - 1));             }} // This code is contributed by Sam007
<?php// PHP implementation to find// the index of first '1' in// a sorted array of 0's and 1's // function to find the index// of first '1' binary search// technique is appliedfunction indexOfFirstOne($arr,                         $low,                         $high){    while ($low <= $high)    {        $mid = ceil($low +                    $high) / 2;         // if true, then 'mid' is        // the index of first '1'        if ($arr[$mid] == 1 and            ($mid == 0 or              $arr[$mid - 1] == 0))            return $mid;         // first '1' lies to the        // left of 'mid'        else if ($arr[$mid] == 1)            $high = $mid - 1;         // first '1' lies to        // the right of 'mid'        else            $low = $mid + 1;    }     // 1's are not present    // in the array    return -1;} // Driver Code$arr = array(0, 0, 0, 0, 0,             0, 1, 1, 1, 1);$n = count($arr);echo indexOfFirstOne($arr, 0, $n - 1); // This code is contributed by anuj_67.?>
<script> // Javascript implementation to find the index of first// '1' in a sorted array of 0's and 1's // function to find the index of first '1'// binary search technique is appliedfunction indexOfFirstOne(arr, low, high){    while (low <= high) {        var mid = parseInt((low + high) / 2);         // if true, then 'mid' is the index of first '1'        if (arr[mid] == 1 && (mid == 0 || arr[mid - 1] == 0))            return mid;         // first '1' lies to the left of 'mid'        else if (arr[mid] == 1)            high = mid - 1;         // first '1' lies to the right of 'mid'        else            low = mid + 1;    }     // 1's are not present in the array    return -1;} // Driver program to test abovevar arr = [0, 0, 0, 0, 0, 0, 1, 1, 1, 1];var n = arr.length;document.write( indexOfFirstOne(arr, 0, n - 1)); // This code is contributed by rrrtnx.</script>
Output :  
6
Time Complexity: O(Logn)References: https://www.careercup.com/question?id=17316686This article is contributed by Ayush Jauhari. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above. 
nitin mittal
vt_m
mayanktyagi1709
rrrtnx
Amazon
Binary Search
binary-string
MakeMyTrip
Arrays
Amazon
MakeMyTrip
Arrays
Binary Search
Writing code in comment?
Please use ide.geeksforgeeks.org,
generate link and share the link here.
Maximum and minimum of an array using minimum number of comparisons
Top 50 Array Coding Problems for Interviews
Stack Data Structure (Introduction and Program)
Introduction to Arrays
Multidimensional Arrays in Java
Linear Search
Linked List vs Array
Given an array A[] and a number x, check for pair in A[] with sum as x (aka Two Sum)
Python | Using 2D arrays/lists the right way
Search an element in a sorted and rotated array