Function that doesn't allow user inputted negative numbers

[myka316]

I'm trying to work a function into my program that won't allow the user to input a negative number. The function I have so far is:

int negatives ()
{
     int input;

     cin >> input;

     if (input < 0)
     {
          cout << "Input cannot be less than 0." << endl;
     }
     return input;
}

However, when I insert it into another function:

void userinput(double &feet, double &inches)
{

	cout << "Enter number of feet: ";
	cin >> negatives ();

	cout << "Enter number of inches: ";
	cin >> negatives ();

}

the program fails to compile. I'm probably making yet another horribly amateur mistake but I'm completely stumped. Any suggestions?

[3qX21hU5]

Your writing cin >> negatives (); // Trying to write into a function call Delete the cin and the >> operator.

Also I would suggest you reread about functions and how they work because your not stoping the user from entering a negative number with your function and its not even storing the number it is giving it basically just tells the user if the number is negative then erases everything.

[myka316]

Thank you for the reply. How would I go about not only informing the user that they entered a negative number but prevent my program from continuing to the next input option?

[JLBorges]

#include <iostream>

int get_non_negative()
{
    int value ;
    std::cout << "enter a non-negative integer: " ;

    if( std::cin >> value && value >= 0 ) return value ;
    else
    {
        std::cin.clear() ; // clear possible error state
        std::cin.ignore( 1000, '\n' ) ; // empty the input buffer
        return get_non_negative() ; // and try again
    }
}

int main()
{
    int value = get_non_negative() ;
    // use value
}

[myka316]

JLBorges,

Thank you for the example. I tried to implement it into my program but once the user enters positive numbers (after entering negative numbers and being told to enter non-negatives) my equation function will use the initial negative inputs instead of the corrected inputs.

I placed the "get_non_negative()" function after the cin >> in my input coding

void userinput(double &feet, double &inches)
{
	cout << "Enter number of feet: ";
	cin >> feet;
        get_non_negative();

	cout << "Enter number of inches: ";
	cin >> inches;
        get_non_negative();
}

I tried inserting the get_non_negatives() three different positions (before cout, after, and after cin) to no avail.

[JLBorges]

First, modify get_non_negative() to accept and return a non-negative real number:

double get_non_negative()
{
    double value ;
    std::cout << "enter a non-negative number: " ;

    if( std::cin >> value && value >= 0 ) return value ;
    else
    {
        std::cin.clear() ; // clear possible error state
        std::cin.ignore( 1000, '\n' ) ; // empty the input buffer
        return get_non_negative() ; // and try again
    }
}

Then, modify userinput() to use the return value of get_non_negative():

void userinput( double &feet, double &inches )
{
	std::cout << " number of feet? ";
	// cin >> feet; 
        feet = get_non_negative(); // ***

	std::cout << " number of inches? ";
	// cin >> inches;
        inches = get_non_negative(); // ***
}