Comparing element in string with length-1 string in C++ - c++

Question

I am sorry for the simple question, but I cannot understand why this simple program does not work.
What is a[0] supposed to be other than "a"?
#include <iostream>
using namespace std;
int main(){
string a = "abcd";
string b = "a";
if (a[0]==b){//<------problem here
cout << a << endl;
}
return 0;
}
which returns the error
no match for ‘operator==’ (operand types are ‘char’ and ‘std::__cxx11::string {aka std::__cxx11::basic_string<char>}’)
or simply using string c=a[0]; returns the error:
conversion from ‘char’ to non-scalar type ‘std::__cxx11::string {aka std::__cxx11::basic_string<char>}’ requested
PS: after trying a few things, I can get it to work if I compare a[0]==b[0] or assign c[0]=a[0] because those are now definitely the same type, but I still would like to know what the standard and/or fastest way to carry out a comparison of a substring with another string in C++ is.

Answer 1

You should use std::string::find to find substrings. Using the subscript operator on string returns a single character (scalar), not a string (vector, non-scalar); therefore, they are not the same type and there is no defined comparison.
You can can also use std::string::substr to select a substring which you can directly compare against another string.
Example
#include <iostream>
#include <string>
int
main() {
std::string a = "abcd";
std::string b = "a";
if (a.find(b) != std::string::npos) {
std::cout << a << "\n";
}
if (a.substr(0, 1) == b) {
std::cout << a << "\n";
}
return 0;
}
References
http://en.cppreference.com/w/cpp/string/basic_string/find
http://en.cppreference.com/w/cpp/string/basic_string/substr
http://en.cppreference.com/w/cpp/string/basic_string

Answer 2

what is a[0] supposed to be other than "a"?
It is an 'a' not "a" and it is a char. You cannot compare a char to a string because they are different types. But what you can do is extract a substring from your string that is the length of 1 character:
if (a.substr(0,1)==b)...
Then you will be comparing "a" to "a" because .substr returns a string not a char, even if the length is 1.
Also don't forget to #include <string> if you are working with std::string.

Answer 3

a[0] is a character, which has the value 'a', whereas b is a string, which has a different type. This will work if an operator ==(char, string) is defined (or some variant with const's and/or ref's) but there isn't, at least in the C++ standard, so the compilation should fail. The clang compiler even gives the helpful message,
invalid operands to binary expression ('int' and 'string'...
which indicates that you're comparing different types.
Try changing it to compare a[0] and b[0]. The bracket operator is defined for strings, and returns a character, so you can compare them that way.
edit: This may look as if it doesn't answer the question. It does answer the original question; the OP changed the entirety of the question after the fact (and someone added a corresponding answer) but the original question is still there in the first paragraph. I'll delete the answer if moderators want.