C++ recursion repeating output

Question

I have this code:

#include <iostream>
#include <vector>
#include <ctime>
#include <math.h>
#include <string>

using namespace std;

int main()
{
    srand(time(0));
    string command_one;
    int slot;
    cout<<"One chip or Quit?\n";
    getline(cin, command_one);
    if(command_one=="One chip"){
        cout<<"Pick a slot between 0 and 8 (inclusive)\n";
        cin>>slot;
        if(slot>=0 and slot<=8){
            double position=slot;
        }
        else{
            cout<<"This Option is invalid!\n";
            main();
        }
    }
    else if(command_one=="Quit"){
        cout<<"Have a nice day! :D";
    }
    else{
        cout<<"This Option is invalid!\n";
        main();
    }
}

When it hits the else loop nested in the if(command_one=="One chip") it returns

"This Option is invalid!
One chip, Multi chip, or Quit?
This Option is invalid!
One chip, Multi chip, or Quit?"

But it should be:

"This Option is invalid!
One chip, Multi chip, or Quit?"

How can this be fixed?

Don't recurse into main? Generally you would make a new function for this and call it from main. — crashmstr
– crashmstr, Commented Sep 10, 2014 at 15:29
extract the recursive functionality to other method, don't use main to recurse. By the way this line: double position=slot; don't do anything. — NetVipeC
– NetVipeC, Commented Sep 10, 2014 at 15:29

Mike Seymour · Accepted Answer · 2014-09-10 15:33:44Z

1

cin>>slot;

This leaves the new-line character following the number in the stream's buffer. The next call to getline will find that, giving an empty line.

You can use ignore to ignore the new-line character and any other garbage at the end of the line:

cin.ignore(-1, '\n'); // ignore any number of characters up to and including new-line

Note that calling main is not allowed (although some compilers might allow it); and unbounded recursion like this could eventually cause a stack overflow. Consider using a loop instead.

answered Sep 10, 2014 at 15:33

Mike Seymour

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Comments

MD. Khairul Basar · Accepted Answer · 2017-05-31 14:39:20Z

0

cin doesn't take the \n character.getline() takes it as an empty line input. Put a getchar() to take that \n character.

#include <iostream>
#include <vector>
#include <ctime>
#include <math.h>
#include <string>
#include <stdio.h>

using namespace std;

int main()
{
    string command_one;
    int slot;
    cout<<"One chip or Quit?\n";
    getline(cin, command_one);
    if(command_one=="One chip")
    {
        cout<<"Pick a slot between 0 and 8 (inclusive)\n";
        cin>>slot;

        //putting a getchar() after cin

        getchar();
        if(slot>=0 and slot<=8)
        {
            double position=slot;
        }
        else
        {
            cout<<"This Option is invalid!\n";
            main();
        }
    }
    else if(command_one=="Quit")
    {
        cout<<"Have a nice day! :D";
    }
    else
    {
        cout<<"This Option is invalid!\n";
        main();
    }
}

edited May 31, 2017 at 14:39

answered Sep 10, 2014 at 16:23

MD. Khairul Basar

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1 Comment

MD. Khairul Basar Over a year ago

when you input number from keyboard and press enter the new line character '\n' is also taken that's why your program was generating output twice but placing a getchar() function after cin the getchar() function takes the new line character '\n' and the program generates output only once...

haccks · Accepted Answer · 2014-09-10 15:34:19Z

0

std::getline:

Extracts characters from input and appends them to str until one of the following occurs (checked in the order listed)
a) end-of-file condition on input, in which case, getline sets eofbit.
b) the next available input character is delim, as tested by Traits::eq(c, delim), in which case the delimiter character is extracted from input, but is not appended to str.

Therefore, you need to consume the leftover \n character.

answered Sep 10, 2014 at 15:34

haccks

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