Switch Quantity problem: switch quantity not an integer and could not convert basic_string’ to ‘bool’

I am trying to create a calculator but this happens.. Here's my code. Please help me fix it and give some explanation:

#include <iostream>

using namespace std;

int main()
{
    int a;
    int b;
    string c;
    string d;
    cout<<"Enter No. 1: ";
    cin>>a;
    cout<<"Enter Operation: ";
    cin>>c;
    cout<<"Enter No. 2: ";
    cin>>b;
    cout<<"So you want me to solve this: ";
    cout<<a<<c<<b;
    cout<<"Type Yes or No";
    cin>>d;
    if(d="yes"){
        switch(c)
    {
        case '+':
            cout << a+b;
            break;

        case '-':
            cout << a-b;
            break;

        case '*':
            cout << a*b;
            break;

        case '/':
            cout << a/b;
            break;
    }
    }
        else{
                return 0;
        }
    
    

}

This is the eroor of the code when compiled pleasee fix this code ima noob:

main.cpp: In function ‘int main()’:
main.cpp:21:9: error: could not convert ‘d.std::basic_string<_CharT, _Traits, _Alloc>::operator=, std::allocator >(((const char*)"yes"))’ from ‘std::basic_string’ to ‘bool’
     if(d="yes"){
        ~^~~~~~
main.cpp:22:17: error: switch quantity not an integer
         switch(c)