c++ Making sure program doesn't divide by 0

Question

I'm currently writing a program to check if a user inputs 0.0 or any variation of 0. However, if I were to write the following code with the user input as 0.0 I don't get either right or wrong as an output the program just ends.

int main(){
    double num;
    cin >> num;
    if (num==0){
        cout << "wrong";
    }
    else {
        cout <<"right";
    }
}

Can someone explain the reason I don't get anything and possibly how to do the comparison I need?

Update: The code I was running is correct, it didn't output because I wasn't calling the function in main. The only reason my post has the code in main was to simplify my question.

@ruakh The return from main is optional according c++ standards. — πάντα ῥεῖ, Nov 05 '16 at 23:31
@bolov `main` in C++ can have a return statement, but it's not required. — Eli Sadoff, Nov 05 '16 at 23:33
How are you running your program? (Is this on a unixish system? Did you call it `test`?) — melpomene, Nov 05 '16 at 23:33
I'm guessing that you ran this on Windows, and the terminal window closed before you could see the result. Add an input operation at the end to prevent this. — interjay, Nov 05 '16 at 23:38
@πάντα ῥεῖ First of all, if the user input 0 then the value will be exactly zero and it's fine to use `==`. Second, this is not correct use of machine epsilon. — interjay, Nov 05 '16 at 23:42
@interjay So finally we [cannot reproduce](http://ideone.com/Rfskup) the situation from the OP's example, yes. — πάντα ῥεῖ, Nov 05 '16 at 23:47
@πάνταῥεῖ Then vote to close as not reproducible. Since OP doesn't answer the comments we won't know what the problem was. — interjay, Nov 05 '16 at 23:48
@interjay: [That's not the best way to prevent this.](http://stackoverflow.com/a/36374595/560648) — Lightness Races in Orbit, Nov 05 '16 at 23:51

score 3 · Answer 1 · answered Nov 05 '16 at 23:33

3

It's impossible for the program you have given to output nothing. You must be running different code. You may need to recompile it. Or, you might not be seeing the output for some reason. Try adding \n at the end of the strings you print, in case the next prompt is overwriting it somehow.

answered Nov 05 '16 at 23:33

Dan

12,409
3
50
87

That's exactly what I was thinking but when I ran it both visual studio and cpp.sh gave me the same no output when I originally asked this question. However it works now oddly enough. I wish I screen grabbed it because it blew my mind. – John McNair Nov 06 '16 at 00:25

score 1 · Answer 2 · answered Nov 06 '16 at 00:13

1

You may need << std::endl or << std::flush at the end of your cout statements.

answered Nov 06 '16 at 00:13

interputed

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5

"If the main function returns to its original caller, or the exit(3) function is called, all open files are closed (hence all output streams are flushed) before program termination." – Dan Nov 06 '16 at 10:18

score 0 · Answer 3 · answered Nov 06 '16 at 00:45

This program is guarenteed to print out right or wrong, you must be running different code or maybe your ide/settings are somehow disabling output to the console, like a gui application

As a side note, do some research on the ternary operator, and your code could be shortened to:

double num;
cin >> num;
std::cout << (num == 0) ? "wrong":"right" << std::endl;

c++ Making sure program doesn't divide by 0

3 Answers3