C++ 错误：在此 scope 中未声明“变量”

Question

I have this simple C++ program and when I try to compile it I get two errors: 'klasa' was not declared in this scope 'oznaka' was not declared in this scope

Does anybody know how can I fix it?

Note: I am still beginner in C++. :]

#include <iostream>
#include <string>
using namespace std;

int main(){
    int T;
    cin >> T;

    if(T>=73){
        string klasa = "A";
        string oznaka = "XL";
    }
    else if(63<=T && T<73){
        string klasa = "A";
        string oznaka = "L";
    }
    else if(53<=T && T<63){
        string klasa = "A";
        string oznaka = "M";
    }
    else if(43<=T && T<53){
        string klasa = "A";
        string oznaka = "S";
    }
    else if(T<43){
        string klasa = "B";
        string oznaka = "XS";
    }

    cout << klasa << " " << oznaka;
}

Answer 1

The reason for the error is that you're declaring each of klasa and oznaka inside the "if statement", as such you can only reach these variables within their scope (ie inside each of their respective "if statements")

#include <iostream>
#include <string>
using namespace std;

int main(){
    int T;
    cin >> T;
    string klasa = "";
    string oznaka = "";

    if(T>=73){
        klasa = "A";
        oznaka = "XL";
    }
    else if(63<=T && T<73){
        klasa = "A";
        oznaka = "L";
    }
    else if(53<=T && T<63){
        klasa = "A";
        oznaka = "M";
    }
    else if(43<=T && T<53){
        klasa = "A";
        oznaka = "S";
    }
    else if(T<43){
        klasa = "B";
        oznaka = "XS";
    }

    cout << klasa << " " << oznaka;
}

The code above places the declaration of the variables in a more global scope, so you can access them inside the "if statements" and outside of them too.

Answer 2

Along with declaring the variables outside of the scope of the conditionals, I'd also reverse your order of the comparisons to make it read better and less chance of making a mistake:

string klasa, oznaka;

if (T < 43) {
    klasa = "B";
    oznaka = "XS";
}
else if (T < 53) {
    klasa = "A";
    oznaka = "S";
}
else if (T < 63) {
    klasa = "A";
    oznaka = "M";
}
else if (T < 73) {
    klasa = "A";
    oznaka = "L";
}
else {
    klasa = "A";
    oznaka = "XL";
}

Answer 3

Variables are declared in a scope. { opens a scope and } closes a scope. Within a scope you can access variables from outside scopes but not from inside scopes.

You can think of it like storeys of a building. On each floor, you can only look down, not up.

int main() { // "ground-level"
    int x = 0;

    { // "1st floor"
        x = 1;     // we can "look down" 

        { // "2nd floor" 
            int y = 0;
            x = 2;
        }
     
        y = 1;     // error ! we cannot "look up"

        { // another "2nd floor", but not connected to above
             x = 3;
             y = 2; // error ! (there are no "windows to look horizontally")
        }
    }

}

For your case, that means you need to declare the variables outside of the if if you want to access them outside of the if :

#include <iostream>
#include <string>

int main(){

    std::string klasa;
    std::string oznaka;
    if(...){
        klasa = "A";
        oznaka = "XL";
    }
    std::cout << klasa << " " << oznaka;
}

Answer 4

Read the code to get the explanation:

#include <iostream>
#include <string>
using namespace std;

int main(){
    int T;      // T is defined here and exists until the closing bracket
    cin >> T;

    if(T>=73){
        string klasa = "A"; // klasa is defined here
        string oznaka = "XL"; // oznaka is defined from here on.
    } // here klasa, and oznaka don't exist anymore.
    else if(63<=T && T<73){
        string klasa = "A"; // a different klasa is defined here
        string oznaka = "L"; // a different oznaka is defined here.
        // the second set of klasa & oznaka cease to exist after this point.
    } // here klasa, and oznaka don't exist anymore.
    else if(53<=T && T<63){
        string klasa = "A"; // a third klasa is defined here.
        string oznaka = "M"; // a third oznaka is defined here.
        // the third group cease to exist after this point.
    } // here klasa, and oznaka don't exist anymore.
    else if(43<=T && T<53){
        string klasa = "A"; // Idem....
        string oznaka = "S";
    } // here klasa, and oznaka don't exist anymore.
    else if(T<43){
        string klasa = "B";
        string oznaka = "XS";
    } // here klasa, and oznaka don't exist anymore.

    // neither klasa nor oznaka are defined at this point, only T
    // (and cin with is defined in the global scope, outside of main())

    cout << klasa << " " << oznaka; // error.
} // here, T doesn't exist anymore.

this code will do the work:

#include <iostream>
#include <string>
using namespace std;

int main(){
    int T;
    cin >> T;

    string klasa = "?";  // put some initializer here.
    string oznaka = "?"; // and here.

    if(T>=73){
        klasa = "A";
        oznaka = "XL";
    }
    else if(63<=T && T<73){
        klasa = "A";
        oznaka = "L";
    }
    else if(53<=T && T<63){
        klasa = "A";
        oznaka = "M";
    }
    else if(43<=T && T<53){
        klasa = "A";
        oznaka = "S";
    }
    else if(T<43){
        klasa = "B";
        oznaka = "XS";
    }

    // so if you get here with klasa or oznaka having "?" you can test
    // that you have not covered all the possible values in the tests
    // above.

    cout << klasa << " " << oznaka;
}