Problem with C++ Language

I am going through a hectic error in c++ language. I am having some critical errors and problems writing a simple program. The only errors I am getting is "undeclared identifier", but the variable is declared and initialized through user input before it is using it

Please help me with this problem

Example

doy.cpp: In function `int main()':
doy.cpp:25: `DayOfYear' undeclared (first use this function)
doy.cpp:25: (Each undeclared identifier is reported only once for each function
it appears in.)
doy.cpp:25: parse error before `;' token
Meaning
You used "DayOfYear" in your code, but the compiler has not seen a definition for "DayOfYear". It doesn't know what "DayOfYear" is.
Usual Causes
You forgot to include the header file that defines the class/struct/function/etc
You misspelled the name of the identifier

You cannot nest functions and subroutines in C. Thus, you cannot put the code for Startstr() within main(). You need to do this:
Code:
char *Startstr(char * s, char * sub); // Function prototype
int main() {
.. code for main ..
}

char *Startstr(char * s, char * sub) {
.. code for Startstr
}

You need to declare s and sub within main(). This is the cause of your error.
Code:
int main() {
char s[50], sub[50];
.. rest of code
}

You're using the strstr() function. You need to #include <cstring> instead of <string>. Also, you need to declare that you're using the standard namespace:
Code:
#include <iostream>
#include <cstring>

using namespace std;

If you don't have that using statement, you will need to qualify all your objects/functions like this:
std::cout << "...";
std::cin >> "....";