Unintialized local variable 'e' used - c++

Question

I have this code. It should work perfectly. It's a circle calculator; I'm doing is as an exercise. I want the user to have the option to return the the 'main menu.' I made a yes/no prompt using char* e; but it's uninitialized. How can I initialize
#include <iostream>
using namespace std;
class Circlecalc {
public:
double const pi = 3.1415962543;
double diameter;
double radius;
double circumference;
};
int _welcome() {
Circlecalc calc;
cout << endl;
int i = 0;
char* e;
cin >> i;
while (i != 5)
{
switch (i) {
case(1) :
cout << "Enter your radius." << endl;
cin >> calc.radius;
cout << endl;
cout << (calc.radius * 2) * calc.pi << endl;
cout << "Exit? [Y/N]" << endl;
cin >> e;
if (e == "Y") {
_welcome();
}
else if (e == "N") {
}
else {
cerr << "Unsupported function" << endl;
}
case(2) :
cout << "Enter your diameter" << endl;
cin >> calc.diameter;
cout << endl;
cout << (calc.diameter * 2) * calc.pi << endl;
cout << "Exit? [Y/N]" << endl;
cin >> e;
if (e == "Y") {
_welcome();
}
else if (e == "N") {
}
else {
cerr << "Unsupported function" << endl;
}
break;
case(3) :
cout << "Enter the circumference" << endl;
cin >> calc.circumference;
cout << endl;
cout << (calc.circumference / 2) / calc.pi;
cout << "Exit? [Y/N]" << endl;
cin >> e;
if (e == "Y") {
_welcome();
}
else if (e == "N") {
}
else {
cerr << "Unsupported function" << endl;
}
break;
case(4) :
cout << "Enter the circumference" << endl;
cin >> calc.circumference;
cout << endl;
cout << calc.circumference / calc.pi;
cout << "Exit? [Y/N]" << endl;
cin >> e;
if (e == "Y") {
_welcome();
}
else if (e == "N") {
}
else {
cerr << "Unsupported function" << endl;
}
break;
case(5) :
return(0);
break;
default:
cerr << "Unsupported function." << endl;
}
}
}

Answer 1

Instead of:
char* e;
use:
std::string e;
The reason you get:
Unintialized local variable 'e' used
is that e is not set when passed to operator>> that is used by cin, to initialize it assign an array to it, ie:
char arr[128] = {0};
char* e = arr;
operator>> for cin stream expect that you have provided some memory buffer where read string is located, char* e; is not bound to any such buffer, and that would end in (possibly) crash (Undefined Behaviour).

Answer 2

In this case you do not need to. If you only want a single letter input from the user just use a char like
char response;
Then you would compare it against a character literal instead of a string literal like
if (response == 'N' || response == 'n')
If you want to compare against a string like "no" or "No" then I suggest you use a std::string and not worry about having to allocate memory for the string.