Thread: validating user input

He means cin.good( )

cin.good() has drawbacks. As long as the input begins with a digit, cin.good() will return "true", truncating the first invalid character and anything that follows. That is, '25g9j' will be held as '25'. Not too good. (Pardon the pun. )

This has been suggested before (by Prelude, if not others). Place your input into a string and validate the individual characters. On success, convert the string to a numeric value.

Not quite as bad as it seems.

Code:

#include <iostream>
#include <cstdlib>  // system call
#include <ctype>    // isdigit()
#include <cmath>    // atof (ASCII to float (double) )
#include <iomanip>  // setprecision()

inline void pause() { system("PAUSE"); }

int main(void)
{
char* input = " ";
double numVal;
bool valid = true;

std::cout << "Enter a numeric value: -> ";
std::cin >> input;

    for ( unsigned int i = 0; i < strlen(input); i++ ){

        if ( !isdigit(input[i]) && input[i] != '.'){

            std::cerr << "\nError! " << "\'" << input[i] << "\'"
                      << " is an invalid character.\n\n";
            valid = false;
        }
    }

    if ( valid ){
        numVal = atof(input);
        std::cout << setiosflags(std::ios::fixed)<< std::setprecision(2)
                  << "\nnumVal = " << numVal << "\n\n";
    }

pause();
return 0;
}

I don't attest to the elegance of my approach here, but it works. Note that I've written the code using 'atof' vice 'atoi'. It can be easily modified to suit your particular needs.

-Skipper

You might want to allocate some space for the input, too.

>That is, '25g9j' will be held as '25'. Not too good.

Unless that was required. Using good/fail/etc in this way allows simple parsing of strings. Besides, if you want to alter it's behaviour you could do something like -

Code:

#include <iostream>

using std::cin;
using std::cout;

int main()
{
	double numVal;
	
	cin >> numVal;
	while(cin.fail())
	{
		cin.clear();
		cin.ignore();
		cin >> numVal;
	}
	
	//Ensure everthing on line is valid
	if(cin.get()!='\n')
		cout << "Input invalid\n";
	else
		cout << numVal;
	
	return 0;
}

If you needed to include other delimeters (' ') the code would be straightforward.

>I mean is there any differences between them
Yes, fail() tests only the failbit for the stream state, good() tests badbit, eofbit, and failbit and will return true only if none of them are set.

-Prelude

> Yes, fail() tests only the failbit for the stream state, good() tests badbit, eofbit, and failbit and will return true only if none of them are set.
Doesn't fail() also test badbit?

(Sorry, just happened to be revising my code to account for multiple decimal points when I noticed your post. )

-Skipper

>Doesn't fail() also test badbit?
Yes, I should stop posting in a hurry, it hurts my reputation.

-Prelude

Nah, it's still intact.

-Skipper