My simple c++ calculator is not working as it should be [duplicate]

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• C++ multiple strings inside an if statement 5 answers

This is a suppposed to be simple calculator which can plus, minus, mulyiply or divide two numbers that are input into the console (int a, b). The problem is probably with my if / else statemnts. Even though I input "Minus" , "Multiply" or "Divide" into the console, the operator (std::string operator) is always set to "plus", thus adding the two numbers even though that was not my desired operator.

Ive tried removing the [int plus();] function alltogether to see the result, then the default and fixed operator changed to minus.

#include <iostream>
// the problem is probably at [void core();]

std::string operation;
int a;
int b;

class common {
public:
    void print() {
        std::cout << "a: ";
        std::cin >> a;
        std::cout << "b: ";
        std::cin >> b;
    }
};

int plus() {
    common plus1;
    plus1.print();
    int ans = a + b;
    std::cout << "ANS: " << ans << "\n";
    return a + b;
}

int minus() {
    common minus1;
    minus1.print();
    int ans = a - b;
    std::cout << "ANS: " << ans << "\n";
    return a - b;
}

int multiply() {
    common multiply1;
    multiply1.print();
    int ans = a * b;
    std::cout << "ANS: " << ans << "\n";
    return a * b;
}

int divide() {
    common divide1;
    divide1.print();
    int ans = a / b;
    std::cout << "ANS: " << ans << "\n";
    return a / b;
}

void core() {
    std::cout << "\nplus / minus / multiply / divide\n"
        << "operation: ";
    std::cin >> operation;
    if (operation == "plus" or "Plus") {
        plus();
    }
    else if (operation == "minus" or "Minus") {
        minus();
    }
    else if (operation == "multiply" or "Multiply") {
        multiply();
    }
    else if (operation == "divide" or "Divide") {
        divide();
    }
    else {
        std::cout << "Invalid Operation!\n";
    }

}

int main() {
    for (int i = 0; i < 99; i++) {
        core();
    }
}

When i set [std::string operation] as "multiply" through [std::cin >> operation], i expected the [multiply();] function to be called but instead, [plus();] function is called no matter what operator i set.