How to Make `cin` Accept Only Numerical Input in C ?

Making cin Read Only Numbers

Enforcing numerical input using cin can present challenges, especially when handling mixed input types. This article investigates a solution to ensure that cin accepts only numerical values.

In the provided code, while loops attempt to rectify invalid input by disregarding non-numerical characters. However, this method faces limitations when dealing with multiple inputs.

Solution: Using std::getline and std::string

To overcome this hurdle, we can employ a more comprehensive approach:

1. Read the entire line using std::getline(): This reads the complete line of user input as a string.

2. Convert the string to a double using std::stringstream:

   • Create a stringstream ss from the input line.
   • Attempt to extract a double using the >> operator.

3. Validate the conversion:

   • Check if the conversion is successful and if the stream has reached the end of the string (eof()).
   • If successful, exit the loop. Otherwise, display an error message.

Here's an example implementation:

#include <sstream>
#include <string>

int main()
{
    std::string line;
    double d;
    while (std::getline(std::cin, line))
    {
        std::stringstream ss(line);
        if (ss >> d)
        {
            if (ss.eof())
            {   // Success
                break;
            }
        }
        std::cout << "Error!" << std::endl;
    }
    std::cout << "Finally: " << d << std::endl;
}

This solution ensures that both integers and floating-point numbers are processed correctly, with non-numerical characters being discarded completely.

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