Comparing a string in an If statement raises error

Question

I am new to C++ (Coming from python). When I am comparing a string in an If statement it is always false.

#include <iostream>
#include <string>
using namespace std;
int main(){
    string op;
    int num1;
    int num2;
    cout << "Operator? ('+', '-', '*', '/' )" << endl;
    cin >> op;
    cout << "Number 1?" << endl;
    cin >> num1;
    cout << "Number 2?" << endl;
    cin >> num2;
    int result;
    cout << op << endl;
    if (op == "+"){
        int result = num1 + num2;
    }
    if (op == "-"){
        int result = num1 - num2;
    }
    if (op == "*"){
        int result = num1 * num2;
    }
    if (op == "/"){
        int result = num1 / num2;
    }
    else{
        cout << "Invalid Operator" << endl;
        return 0;
    }
    cout << "Result is " << result << endl;
}

When I feed it the inputs '+', '1', and '1', It prints '+', meaning it read the input. But then I expect it to give me 'The result is 2', but it prints "Invalid operator". What can i do to fix this?

Answer 1

Seen this a few times recently. If you want to choose one of several alternative using if, then you write it like this

if ... else if ... else if ... else if ... else ...

Not like this (what you have written)

if ... if ... if ... if ... else ...

I know python is strange but I expect in this case it's exactly the same as C++.

Then you have the problem that you have five different result variables

int result;                    // first result variable
cout << op << endl;
if (op == +){
   int result = num1 + num2;   // second result variable
}

should be

int result;
cout << op << endl;
if (op == +) {
   result = num1 + num2;
}

etc. etc.

Answer 2

Since no one has offered a suitable answer in my opinion, the answer is basically a typo (I'm assuming, since you do it the right way the other times). The problem is this line right here:

if (op == +){

Honestly, I'm not sure what this does, but I can tell you it doesn't do what you expect. + is an actual C++ operator. Since it has nothing next to it on either side, I'm not exactly sure what this expression actually does. But again, it is not what you intend.

What you want is a quoted + sign:

if (op == "+"){

The difference with "+" is that "+" is a sting containing the character +, not an actual C++ operator. That's what you actually want. It will do what you expect, and won't always end up with "invalid operator".

---

One other thing before I go, it would be a disservice to you to to not mention that you actually have 2 bugs, even if you only are asking about the one. That is, as others have mentioned, you are creating another variable in another scope when you actually perform your operation:

int result;
cout << op << endl;
if (op == "+"){ // decided to use a fixed version of the if statement as an example
   int result = num1 + num2;
}

In C++, that's a gotch ya. The int in front of result in the second line creates a new variable called result. That new variable goes out of scope almost immediately. Thus, your result isn't saved at all, but instead you get uninitialized data.

The solution is relatively simple: just remove the int keyword in your second example:

result = num1 + num2;

Of course, you will need to do this on all of your operations, not just +.

Answer 3

You have to use else if instead of if every time.

Here is the working code.

#include <iostream>
#include <string>
using namespace std;
int main(){
    string op;
    int num1;
    int num2;
    cout << "Operator? ('+', '-', '*', '/' )" << endl;
    cin >> op;
    cout << "Number 1?" << endl;
    cin >> num1;
    cout << "Number 2?" << endl;
    cin >> num2;
    int result;
    cout << op << endl;
    if (op == "+")
    {
         result = num1 + num2;
    }
    else if (op == "-")
    {
         result = num1 - num2;
    }
    else if (op == "*")
    {
         result = num1 * num2;
    }
    else if (op == "/")
    {
         result = num1 / num2;
    }
    else
    {
        cout << "Invalid Operator" << endl;
        return 0;
    }
    cout << "Result is " << result << endl;
}

Answer 4

As you are new to c++. Few suggestion here

1. As you know c++ is statical typed . So you should know all data types of c++. Like when you are using single character you can use char insted of string
2. You should not redifine variable every time you use.

#include <iostream>
#include <string>
using namespace std;
int main(){
    string op;
    int num1;
    int num2;
    cout << "Operator? ('+', '-', '*', '/' )" << endl;
    cin >> op;
    cout << "Number 1?" << endl;
    cin >> num1;
    cout << "Number 2?" << endl;
    cin >> num2;
    int result;
    cout << op << endl;
    if (op == "+"){
         result = num1 + num2;
    }
    else if (op == "-"){
         result = num1 - num2;
    }
    else if (op == "*"){
         result = num1 * num2;
    }
    else if (op == "/"){
         result = num1 / num2;
    }
    else{
        cout << "Invalid Operator" << endl;
        return 0;
    }
    cout << "Result is " << result << endl;
}

Answer 5

Improved your Code. In this case, Comparing std::string was really not necessary. Have fun!

#include <iostream>

//using namespace std;

int main()
{
    char operation; 
    float num1, num2, result;
    std::cout << "Enter Operator (+,-,/,*) : ";
    std::cin >> operation;
    std::cout << "\nNumber 1 : ";
    std::cin >> num1;
    std::cout << "\nNumber 2 : ";
    std::cin >> num2;
    switch(operation)
    {
        case '+':
        result = num1 + num2;
        break;
        case '-':
        result = num1 - num2;
        break;
        case '/':
        result = num1 / num2;
        break;
        case '*':
        result = num1 * num2;
        break;
        default:
        result = num1 + num2; 
    }
    std::cout << "Result : " << result << std::endl;
}

https://im.ezgif.com/tmp/ezgif-1-76ceb4e91c2c.gif