if statement followed by return 0

Question

I have some code like:

#include <iostream>
#include <string>

int main() {
    std::string question;

    std::getline(std::cin, question);
    
    if (question == "yes") {
        std::cout << "Let's rock and roll!" << std::endl;
        return 0; // This line
    } if (question == "no") {
        std::cout << "Too bad then..." << std::endl;   
    } else {
        std::cout << "What do you mean by that?" << std::endl;
    }
    return 0;
}

If I don't write the commented return 0 line and input yes, the output is Let's rock and roll! followed by What do you mean by that?. It should only output Let's rock and roll!.

But I don't need to put return 0 in the if (question=="no"){...} block. If I input no, the output is just Too bad then....

Why do I need the return 0 in the first case, but not the second?

Answer 1

Control flow is your issue here:

     if(question == "yes"){
        std::cout<<"Lets rock and roll!"<<std::endl;
        return 0;
     }if (question == "no"){
        std::cout<<"Too bad then..."<<std::endl;   
     } else{
        std::cout<<"What do you mean by that?"<<std::endl;
     }

Let's format this a bit better by surrounding if/else statements/blocks with newlines and adding some whitespace around operators. Your compiler doesn't care (much) about whitespace when compiling code, but it conveys a lot of meaning to humans reading code, and poor formatting can convey a meaning that isn't reflected by the actual code.

     if (question == "yes") {
        std::cout << "Lets rock and roll!" << std::endl;
        return 0;
     }

     if (question == "no") {
        std::cout << "Too bad then..." << std::endl;   
     } 
     else {
        std::cout << "What do you mean by that?" << std::endl;
     }

These are two different conditionals. The first one being triggered does not stop the second if/else from being evaluated. In fact, if question equals "yes" then it cannot equal "no" so the else clause in the second if/else must be executed.

By including return 0; in the first conditional block, the function exits immediately, thus skipping everything after it. The second if/else is not evaluated and "What do you mean by that?" is never printed.

You likely wanted this to be a single if/else. Now only one of these blocks will be executed. Because an else is included as a catch-all in the event none of the previous conditions were met, it is guaranteed one branch will be executed.

     if (question == "yes") {
        std::cout << "Lets rock and roll!" << std::endl;
     }
     else if (question == "no") {
        std::cout << "Too bad then..." << std::endl;   
     } 
     else {
        std::cout << "What do you mean by that?" << std::endl;
     }

An alternative approach entirely is to use a mapping of a string input to a response. This scales better to a larger assortment of inputs/responses.

#include <iostream>
#include <string>
#include <unordered_map>

int main() {
    const std::unordered_map<std::string, std::string> map = {
        {"yes", "Lets rock and roll!"},
        {"no", "Too bad then..."}
    };
    std::string input;

    std::getline(std::cin, input);
    
    try {
        std::cout << map.at(input) << '\n';
    }
    catch (std::out_of_range) {
        std::cout << "What do you mean by that?\n";
    }
}