Validating double values input

Question

I wrote a program that calculates cylinder and rectangle areas with user inputs. But I want improve my code. When user enter a character instead of number, it should output an error message. So I wrote some extra code but it's not working correctly.

#include <iostream>
#include <ctype.h>

using namespace std;

class area_cl
{
public:
    double height, width;
};

class rectangle : public area_cl
{
public:
    double area(double h, double w)
    {
        return h * w;
    }
};

class cylinder : public area_cl
{
private:
    double pi = 3.14;

public:
    double area(double h, double w)
    {
        return 2 * pi * (w / 2) * ((w / 2) + h);
    }
};

int main()
{
    area_cl dimension;
    rectangle obj1;
    cylinder obj2;
    bool flag = true;

    while (flag)
    {

        cout << "What is the dimensions? (Height and Width)" << endl;

        cin >> dimension.height >> dimension.width;

        if (isdigit(dimension.height) && isdigit(dimension.width))
        {
            flag = false;
        }
        else
        {
            cout << "You are not entered number,please try again." << endl;
        }
    }

    cout << "Rectangle's area is : " << obj1.area(dimension.height, dimension.width) << endl;
    cout << "Cylinder's area is : " << obj2.area(dimension.height, dimension.width) << endl;
    return 0;
}

1. I thought of using isdigit() but my input variables must be double type and probably my code crashes for that. Are there any method in C++, like parsing in C#?

2. I also thought about controlling input with using ASCII codes. For example if ( char variable >= 48 && char variable <= 57) but I couldn't make it work.

I would prefer to solve this problem with first option but I am totally opened to other solutions.

Thanks.

Answer 1

1. I thought of using isdigit() but my input variables must be double type and probably my code crashes for that. Are there any method in C++, like parsing in C#?

isdigit just checks for digits, it's not a good fit for wahat you need. And yes there are better parsings ways in C++, not quite like C#, but at least as nice IMO (I'll get back to it ahead).

---

2. I also thought about controlling input with using ASCII codes. For example if ( char variable >= 48 && char variable <= 57) but I couldn't make it work.

This is also not a good option, you need doubles and that would make checking digit by digit needlessly complicated, especially beacuse this can be achieved very easily in other ways.

For future reference, when using characer comparison, you should use character literals, not all encodings have the digit codes in the same interval as ASCII:

if (char variable >= '0' && char variable <= '9')

Would be more appropriate.

---

I would prefer to solve this problem with first option but I am totally opened to other solutions.

The simpler and more idiomatic way I can think of is to use cin return values to check if the input is indeed valid and take measures (clear buffer and reset stream error state) if it isn't, and ask again for valid values, your code refactored would look similar to:

while (flag)
{
    cout << "What are the dimensions? (Height and Width)" << endl;
    if (cin >> dimension.height && cin >> dimension.width)
    {
        flag = false;
    }
    else
    {
        cin.clear(); // clear error flags
        cin.ignore(numeric_limits<streamsize>::max(),'\n'); // clear buffer *
        cout << "You have not entered numbers, please try again" << end;
    }
}

Or, better IMO:

cout << "What are the dimensions? (Height and Width)" << endl << ">"; // **
while (!(cin >> dimension.height && cin >> dimension.width))           
{
    cin.clear(); // clear error flags
    cin.ignore(numeric_limits<streamsize>::max(), '\n'); // clear buffer *
    cout << "You have not entered numbers, please try again" << endl << ">";
}

---

* - cin.ignore(numeric_limits<streamsize>::max(),'\n'); may require the inclusion of <limits> header.

This routine clears the input buffer, which will have the characters that couldn't be parsed, in case a reading error occurs.

You can read it like:

"until a newline character is found, ignore (and remove from buffer) all the characters that are there".

This will remove all the characters until a \n is found, this is the last character in the input buffer, it's added when you press Enter.

---

** - Notice that I changed the messaging in this second option. The input message is emmited only once at the beggining of the loop, and the error message just asks for new input if there is an input failure. The way you have it now, in case of input failure, the message flow would be:

What is the dimensions? (Height and Width)
1.9 a
What is the dimensions? (Height and Width)
You are not entered number,please try again 
_

I would prefer:

What are the dimensions? (Height and Width)
> 1.9 a
You have not entered numbers, please try again 
>_

Answer 2

isdigit() checks if a char is digit. When you pass it a double, it will be converted to an int. Let's say your input was 1.2 which is converted to (int) 1 which is not a digit '0' through '9'.

You can use cin.fail() to determine if it was able obtain input. If not, clear the failed state, and flush stdin with cin.ignore() of the data that could not be read. cin.bad() and cin.eof() covered in similar fashion:

#include <limits>
#include <stdlib.h>

...

retry:
    cout << "What is the dimensions? (Height and Width)" << endl;
    cin >> dimension.height >> dimension.width;
    if(cin.bad()) return EXIT_FAILURE;
    if(cin.eof()) return EXIT_SUCCESS;
    if(cin.fail()) {
        cout << "You have not entered number, please try again" << endl << ">";
        cin.clear();
        cin.ignore(numeric_limits<streamsize>::max(), '\n');
        goto retry;
    }

or the non-goto variant (for discussion with @anastaciu):

#include <limits>
#include <stdlib.h>

...

for(;;) {
    cout << "What is the dimensions? (Height and Width)" << endl;
    cin >> dimension.height >> dimension.width;
    if(cin.bad()) return EXIT_FAILURE;
    else if(cin.eof()) return EXIT_SUCCESS;
    else if(cin.fail()) {
        cout << "You have not entered number, please try again" << endl << ">";
        cin.clear();
        cin.ignore(numeric_limits<streamsize>::max(), '\n');
    } else
        break;
}

Another option is to read a line of text and parse that. It would allow you to provide a better error handling than losing all user input.