为什么这个 c++ 程序不是 function 正确？

Question

Right now, I'm learning C++. I have made my first little program, an addition calculator. I write two numbers and it adds them. Can you tell me why my output is 0?

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

int main () {
    int a;
    int b;
    string number1;
    string number2;
    number1 = a;
    number2 = b;
    int output;
    output = a + b;
    getline (cin, number1);
    getline (cin, number2);
    cout << output;
}

Output:

1
1
0

Answer 1

Step through your code in order and you'll see the problem:

int main () {
    int a;
    int b;
    string number1;
    string number2;
    number1 = a;
    number2 = b;
    int output;
    output = a + b;
    getline (cin, number1);
    getline (cin, number2);
    cout << output;
}

You define four variables, fiddle with two of them in a weird way, and then calculate output as a+b.

By this point you haven't actually interacted with the user. You finish doing all that, and THEN you accept your two values (as strings) and output the previously-calculated value.

You never use the values you pull in from input, as you do that after doing anything else.

Maybe try this:

#include <iostream>
#include <string>

int main () {
    std::string number1;
    std::string number2;
    std::getline (std::cin, number1);
    std::getline (std::cin, number2);

    int a = std::stol(number1);
    int b = std::stol(number2);
    int output = a + b;
    std::cout << output << std::endl;
}

That is, define your strings and then get them from the user.

std::stol is "string to long". It converts the strings to numbers.

The rest should make sense.