fatal error in vector use - string

Question

For a school c++ lab (using microsoft visual studio, hence the system("pause")) I am making a program that will let a user input an email address and the program will spit out the username (before '#') and the site type, either the type based on the last three letters of the address (com is commercial ventures) or the last two letter country code (us is united states).
#include <iostream>
#include <string>
using namespace std;
void getemail(string &email);
void finduser(string email);
void findsitetype(string email);
int main()
{
string email;
getemail(email);
finduser(email);
findsitetype(email);
system("pause");
return 0;
}
void getemail(string &email)
{
cout << "Please enter your email address: ";
cin >> email;
cout << endl;
}
void finduser(string email)
{
int index = email.find('#');
cout << "Username: ";
for (int i = 0; i < index; i++)
cout << email[i];
cout << endl << endl;
}
void findsitetype(string email)
{
int truesize = size(email);
string lastthree;
for (int i = 0; i < 3; i++)
{
lastthree[i] = email[truesize - i];
}
cout << "Site type: ";
if (lastthree == "edu")
cout << "Educational institutions";
if (lastthree == "org")
cout << "Not-for-profit organizations";
if (lastthree == "gov")
cout << "Government entities";
if (lastthree == "mil")
cout << "Military installations";
if (lastthree == "net")
cout << "Network service providers";
if (lastthree == "com")
cout << "Commercial ventures";
if (email[truesize - 2] == '.')
cout << "Country Code " << email[truesize - 1] << email[truesize];
}
When I run the code, it spits out the username but there seems to be a fatal error when finding the site type. I think it has something to do with my incorrect string use? Any help appreciated.
Debug Assertion Failed!
Program: C:\windows\SYSTEM32\MSVCP140D.dll File: c:\program files
(x86)\microsoft visual studio 14.0\vc\include\xstring Line: 1681
Expression: string subscript out of range
For more information on how your program can cause an assertion
failure, see the Visual C++ documentation on asserts.
(Press Retry to debug the application)

Answer 1

There are a couple things wrong with your code, (1) to get the length of a string, use the length() function, so:
int truesize = size(email);
should be
int truesize = email.length();
I changed your if statements to else ifs because if one of the condition statements evaluates to true, we shouldn't need to check the rest of them.
(2) your for loop is grabbing the email extension in the reverse direction, change:
for (int i = 0; i < 3; i++)
{
lastthree[i] = email[truesize - i];
}
to
lastthree = email.substr(truesize-3, truesize);