I have no idea what you are doing bro...doesn't make any sense to me;
Are you looking for something like this ?
#include
#include
using namespace std;

int main(){
const float pi = 3.14159;
float r;
double diameter, perimeter, area;
string mystr;
cout << "Enter string to find" << endl;
getline(cin, mystr);
cout << "Enter radius" << endl;
cin >> r;
if (r > 0){
if (mystr == "diameter"){
diameter = 2 * r;
cout << "The diamter is :" << diameter << endl;

}
else if (mystr == "perimeter"){
perimeter = r*(2 + pi);
cout << "The perimeter is :" << perimeter << endl;

}
else if (mystr == "area"){
area = pow(r, 2)*pi / 2;
cout << "The area is :" << area << endl;
}

}
else{
cout << "idk wth to do";
}
system("pause");
}