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# Number of even substrings in a string of digits

• Difficulty Level : Easy
• Last Updated : 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```
Recommended Practice

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++

 `// C++ program to count number of substring``// which are even integer in a string of digits.``#include``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;``}`

## Java

 `// 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.`

## Python3

 `# 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 Program``if` `__name__ ``=``=` `'__main__'``:``    ``str` `=` `[``'1'``,``'2'``,``'3'``,``'4'``]``    ``print``(evenNumSubstring(``str``))` `# This code is contributed by``# Surendra_Gangwar`

## C#

 `// 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

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## Javascript

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Output

`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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