Number of even substrings in a string of digits

08 Jul, 2022
Given a string of digits 0 – 9. The task is to count a number of substrings which when converting into integer form an even number.
Examples : 
Input : str = "1234".
Output : 6
"2", "4", "12", "34", "234", "1234" 
are 6 substring which are even.

Input : str = "154".
Output : 3

Input : str = "15".
Output : 0
For a number to be even, the substring must end with an even digit. We find all the even digits in the string and for each even digit, count the number of substrings ending with it. Now, observe that the number of substrings will be an index of that even digit plus one. 
Implementation:
C++
Java
Python3
C#
PHP
Javascript
// C++ program to count number of substring// which are even integer in a string of digits.#include<bits/stdc++.h>using namespace std; // Return the even number substrings.int evenNumSubstring(char str[]){    int len = strlen(str);    int count = 0;     for (int i = 0; i < len; i++)    {        int temp = str[i] - '0';         // If current digit is even, add        // count of substrings ending with        // it. The count is (i+1)        if (temp % 2 == 0)            count += (i + 1);    }     return count;} // Driven Programint main(){    char str[] = "1234";    cout << evenNumSubstring(str) << endl;    return 0;}
// Java program to count number of// substring which are even integer// in a string of digits.public class GFG {         // Return the even number substrings.    static int evenNumSubstring(String str)    {        int len = str.length();        int count = 0;             for (int i = 0; i < len; i++)        {            int temp = str.charAt(i) - '0';                 // If current digit is even, add            // count of substrings ending with            // it. The count is (i+1)            if (temp % 2 == 0)                count += (i + 1);        }             return count;    }         public static void main(String args[])    {                 String str= "1234";                 System.out.println(evenNumSubstring(str));    }} // This code is contributed by Sam007.
# Python 3 program to count number of substring# which are even integer in a string of digits.  # Return the even number substrings.def evenNumSubstring(str):    length = len(str)    count = 0     for i in range(0,length,1):        temp = ord(str[i]) - ord('0')         # If current digit is even, add        # count of substrings ending with        # it. The count is (i+1)        if (temp % 2 == 0):            count += (i + 1)     return count # Driven Programif __name__ == '__main__':    str = ['1','2','3','4']    print(evenNumSubstring(str)) # This code is contributed by# Surendra_Gangwar
// C# program to count number of// substring which are even integer// in a string of digits.using System; public class GFG {         // Return the even number substrings.    static int evenNumSubstring(string str)    {        int len = str.Length;        int count = 0;             for (int i = 0; i < len; i++)        {            int temp = str[i] - '0';                 // If current digit is even,            // add count of substrings            // ending with it. The count            // is (i+1)            if (temp % 2 == 0)                count += (i + 1);        }             return count;    }         // Driver code    public static void Main()    {        string str= "1234";             Console.Write(            evenNumSubstring(str));    }} // This code is contributed by Sam007.
<?php// PHP program to count number// of substring which are even// integer in a string of digits. // Return the even number substrings.function evenNumSubstring($str){    $len = strlen($str);    $count = 0;     for ($i = 0; $i < $len; $i++)    {        $temp = $str[$i] - '0';         // If current digit is even, add        // count of substrings ending with        // it. The count is (i+1)        if ($temp % 2 == 0)            $count += ($i + 1);    }     return $count;} // Driver Code$str = "1234";echo evenNumSubstring($str),"\n" ; // This code is contributed by jit_t   ?>
<script>     // Javascript program to count number of    // substring which are even integer    // in a string of digits.         // Return the even number substrings.    function evenNumSubstring(str)    {        let len = str.length;        let count = 0;               for (let i = 0; i < len; i++)        {            let temp = str[i] - '0';                   // If current digit is even,            // add count of substrings            // ending with it. The count            // is (i+1)            if (temp % 2 == 0)                count += (i + 1);        }               return count;    }         let str= "1234";           document.write(evenNumSubstring(str));         </script>
6
Time Complexity: O(length of string). 
This article is contributed by Anuj Chauhan. 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. 
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