Logic error when checking if input is alphanumeric - c++

Question

The purpose of this program is to check if the character entered by the user is alphanumeric. Once the void function confirms correct entry then it is passed to string test to output a message. I know it's not great coding but it has to be done this way.
I keep getting a logic error & I can't figure out why? Can someone please help?
#include <iostream>
#include <string>
#include <cctype>
using namespace std;
void get_option(char& input);
/**
Takes character entered user input and loops until correct answer
#param y character entered by user
#return to main() once valid entry received
*/
string test(char);
/**
Takes checks character entered user input and loops until correct answer
#param y alphanumeric character entered by user
#return to main() once valid entry received
*/
int main()
{
char y;
//call get_option to prompt for input
get_option(y);
//call test after user input is valid
test(y);
return 0;
}
void get_option(char &x)
{
cout << "Please enter an alphanumeric character: ";
cin >> x;
while (!(isdigit(x)||islower(x)||isupper(x)))
{
cout << "Please enter an alphanumeric character: ";
cin >> x;
}
}
string test(char y)
{
if (isupper(y))
{
cout << "An upper case letter is entered!";
} else if (islower(y)) {
cout << "A lower case letter is entered!";
} else if (isdigit(y)) {
cout << "A digit is entered!";
}
return "";
}

Answer 1

I got the program to work perfectly by changing the return type of the test(char) function:
#include <iostream>
#include <string>
#include <cctype>
using namespace std;
void get_option(char& input);
/**
Takes character entered user input and loops until correct answer
#param y character entered by user
#return to main() once valid entry received
*/
int test(char); //Changed from string to int
/**
Takes checks character entered user input and loops until correct answer
#param y alphanumeric character entered by user
#return to main() once valid entry received
*/
int main()
{
char y;
//call get_option to prompt for input
get_option(y);
//call test after user input is valid
test(y);
return 0;
}
void get_option(char &x)
{
cout << "Please enter an alphanumeric character: ";
cin >> x;
while (!(isdigit(x)||islower(x)||isupper(x)))
{
cout << "Please enter an alphanumeric character: ";
cin >> x;
}
}
int test(char y) //Also changed from string to int
{
if (isupper(y))
{
cout << "An upper case letter is entered!";
} else if (islower(y)) {
cout << "A lower case letter is entered!";
} else if (isdigit(y)) {
cout << "A digit is entered!";
}
return 0;
}
(Testing was on JDoodle using the C++14 compiler.)
(Also, tested using Xcode. Still works)

Answer 2

When I tried it in my setup (g++ 6.4, cygwin), I did not get any output. When I added << endl to the output lines, the output showed up.
I suspect you're experiencing the same problem.
string test(char y)
{
if (isupper(y))
{
cout << "An upper case letter is entered!" << endl; // Add endl
}
else if (islower(y))
{
cout << "A lower case letter is entered!" << endl;
}
else if (isdigit(y))
{
cout << "A digit is entered!" << endl;
}
// This does not make sense but it is syntactically valid.
return 0;
}
JiveDadson is right. The problem was the return 0 line. It causes undefined behavior. Changing the that line to
return "";
fixes the output problem, endl or not. Having the endl is nicer but is not necessary. Fixing the return statement is the most important task.