Number of even substrings in a string of digits

Given a string of digits 0 – 9. The task is to count number of substrings which when convert into integer form an even number.


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, substring must end wit 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 index of that even digit plus one.

// C++ program to count number of substring
// which are even integer in a string of digits.
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 Program
int main()
    char str[] = "1234";
    cout << evenNumSubstring(str) << endl;
    return 0;



Time Complexity: O(length of string).

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