Do While loop skipping user input in C++

Question

Having some difficulty figuring out the right syntax I should be using when trying to get the Do/While loop to work. I would like to write a calculator in C++ that allows the user to enter a string and then it does its thing and prints the result, asks the user if they want to go again or not.

I have a simple main function so far just to get the Do/While loop right, but when I enter y or Y, the program just asks if I want to continue again, it doesnt give me the opporunity to run the "calculator" part again. What am I doing wrong?

#include "stdafx.h"
#include <iostream> //cout, cin
#include <string> //string
#include <algorithm> //remove_if(), end(), begin(), erase()
#include <stack> //stack<type>
#include <ctype.h> //isdigit()
#include <vector> //vectors
#include <stdlib.h>

using namespace std;

int main()
{
    string userInput = ""; //declaring and initialising a string called user input
    char ans;

    do {

        cout << "Welcome to the calculator, please enter your calculation and then press enter when you are done." << endl;
        cin >> userInput;

        userInput.erase(remove_if(userInput.begin(), userInput.end(), isspace), userInput.end()); //removes and then erases any spaces in the string
        userInput.erase(remove_if(userInput.begin(), userInput.end(), isalpha), userInput.end()); // removes and then erases any alphabetic charecters
                                                                                                  //this will leave only numbers and operators
        cout << userInput << endl;
        cout << "Would you like to continue?" << endl;
        cout << "Please enter 'y' or 'n'" << endl;
        cin >> ans;

    } while ((ans == 'y')||(ans == 'Y'));

    return 0;
}

Cannot reproduce. Your loop seems to work. However the code breaks on some input (e.g. containing ä). — wkl
– wkl, Commented Apr 4, 2017 at 9:40
It might depend on how you type the input. cin >> userInput; only reads one word and stops at the first whitespace, leaving the rest in the input buffer. — Bo Persson
– Bo Persson, Commented Apr 4, 2017 at 9:42
@wkl I will get an image and add it to my post so you can see what I am getting. — bchards
– bchards, Commented Apr 4, 2017 at 9:44
@BoPersson I see, I was originally using getline but I thought that was the issue as to why it was skipping through. I would prefer top use getline. — bchards
– bchards, Commented Apr 4, 2017 at 9:44

Bathsheba · Accepted Answer · 2017-04-04 09:52:19Z

4

The problem is that cin >> userInput; stops on the first whitespace, and my money is on a typical input string of yours containing plenty of them.

Change to

std::getline(std::cin, userInput);

This will gobble up a whole line of input, and deal with the newline character automatically for you.

For an easy life, do something similar with ans. Redefine it as a std::string. std::string even has == overloaded for a char type!

(Personally I'd also refrain from commenting standard #include files. Any C++ programmer should know what they "bring in", and this can lead to disinformation as your program expands.)

edited Apr 4, 2017 at 9:52

answered Apr 4, 2017 at 9:46

Bathsheba

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12 Comments

bchards Over a year ago

So even with getline being used, the execution is exactly the same, it just skips right past the opportunity for me to enter another string.

Bathsheba Over a year ago

Did you change the type of ans? I would if I were you. I hate reading in a single char.

bchards Over a year ago

I also do not understand why I would need to change the type of ans I have defined it as a char as the user will only be entering one character? @Bathsheba

Bathsheba Over a year ago

Humour me and try it.

bchards Over a year ago

@Bethsheba I changed it, but no difference

|

bchards · Accepted Answer · 2017-04-04 12:09:31Z

0

So I have finally overcome my issue with help from @Bathsheba and @mutantkeyboard

I was using getline but only for the first instance of user input, like @Bathsheba suggests. This was not working for me as I then left the input for the ans as cin. Changing both of these to getline not just one of them and using a blank project without a pre-compiled header has solved the issue for me. The projects now loops correctly as expected.

Thank you for your help everyone!

answered Apr 4, 2017 at 12:09

bchards

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