C++ comparing the index of a string to another string? - c++

Question

How can I compare a single character from a string, and another string (which may or may not be greater than one character)
This program gives me almost 300 lines of random errors. The errors don't reference a specific line number either, just a lot of stuff about "char* ", "", or "std::to_string".
#include <iostream>
#include <string>
using std::cout;
using std::string;
int main() {
string str = "MDCXIV";
string test = "D";
if (test == str[4]) { // This line causes the problems
cout << test << endl;
}
return 0;
}

Answer 1

str[4] is a char type, which will not compare with a string.
Compare apples with apples.
Use
test[0] == str[4]
instead.

Answer 2

You need to convert str[4] (which is a char) to a string before you can compare it to another string. Here's a simple way to do this
if (test == string(1, str[4])) {

Answer 3

You're comparing a char to a std::string, this is not a valid comparison.
You're looking for std::string::find, as follows:
if( test.find( str[4] ) != std::string::npos ) cout << test << "\n";
Note that this will return true if test contains str[4].

Answer 4

You're mixing types. It doesn't know how to compare a string (test) to a char (str[4]).
If you change test to a char that will work fine. Or reference the specific character within test you want to compare such as if (test[0] == str[4]) it should compile and run.
However, as this is merely an example and not really the true question what you'll want to do is look at the functionality that the std::string class supplies

Answer 5

Also you need "D" to be a char value not a string value if you are comparing it like that.
std::string myString = "Hello World";
const char *myStringChars = myString.c_str();
You have to turn it into a char array before can access it. Unless you do.
str.at(i);
which you can also write as
str[i] <-- what you did.
Essentially, this all boils down to test needs to initialized like char test = 'D';
Final Output..
#include <iostream>
#include <string>
using std::cout;
using std::string;
int main() {
string str = "MDCXIV";
char test = 'D';
if (test == str[4]) { // This line causes NO problems
cout << test << endl;
}
return 0;
}

Answer 6

I think you might be mixing python with c++. In c++ 'g' refers to a single character g not a string of length 1. "g" refers to an array (string) which is 1 character long and looks like ['g']. So as you can see, if you compare a single character to an array of characters no matter if the array is a single character long, this operation is not defined.
This will work if define it yourself by building a class which is able to compare string of one character long to a single character. Or just overload the == operator for doing just that
Example:
#include <iostream>
#include <string>
using std::cout;
using std::string;
using std::endl;
bool operator == ( const string &lh, const char &rh) {
if (lh.length() == 1) return lh[0] == rh;
return 0;
}
int main() {
string str = "MDCXIV";
string test = "D";
if (test == str[4]) {
cout << test << endl;
}
else cout << "Not a match\n";
return 0;
}