Dealing with bad input

[pieroborrelli]

Hello, I'm a C++ beginner and I have a doubt about bad input, why in this program if I enter the value 2.5 and 3.5 the reading of 3.5 fails ? If I write this program using just one variable it doesn't happen, why ? I have the same problem if I write the same program but using doubles (if I enter two ints, the second reading fails) why ?

	int a = 10;
	int b = 100; 
	cin >> a >> b;
	cout << a << " " << b << "\n"; 

[LB]

Don't read directly from standard input if you don't want to deal with its error state.

#include <iostream>
#include <string>
#include <sstream>

int main()
{
    int a, b;
    std::cout << "Enter two integers on separate lines." << std::endl;
    std::string line;
    while(std::getline(std::cin, line) && !(std::istringstream(line) >> a))
    {
        std::cout << "Please re-enter the first integer." << std::endl;
    }
    while(std::getline(std::cin, line) && !(std::istringstream(line) >> b))
    {
        std::cout << "Please re-enter the second integer." << std::endl;
    }
    std::cout << "You entered " << a << " and " << b << "." << std::endl;
}

(untested, and doesn't handle early end of input)

[PCrumley48]

One feature with cin is that it expects your entry to conform to the variable type you have declared. In the case of int a, a decimal point is undefined in type int, so cin will begin a chase, in which it only ends when you interrupt the program.

I'm not sure about the doubles situation. On my system as long as I input a number it does what it's supposed to do. In this:

	int a = 10;
	int b = 100; 
	cin >> a >> b;
	cout << a << " " << b << "\n";

If I enter 3.5, I get:

3   100
3   100
3   100
3   100
3   100
3   100
3   100
3   100
3   100
3   100
3   100
3   100

If I do this:

	double a = 10;
	double b = 100; 
	cin >> a >> b;
	cout << a << " " << b << "\n"; 

[PCrumley48]

You can't enter a non-int value when you've declared an int, or cin >> will go into an infinite loop. I don't know enough about the cin command, so I can't say why.

I get the same result whether I put in one variable int a, or two int a, int b. Also have no problems with declaring them as doubles.

[LB]

PCrumley48 wrote:

You can't enter a non-int value when you've declared an int, or cin >> will go into an infinite loop. I don't know enough about the cin command, so I can't say why.

It does not go into an infinite loop. It is put into an error state. While a stream is in an error state, all read/write operations do nothing. Either clear the error state (which is a nuisance) or don't allow the stream to enter the error state in the first place (as in my example).

[PCrumley48]

Gotcha. It looks infinite on the monitor. . . ! But right usage is right usage, after all. Like I said, I don't know enough about cin, maybe should read up on it more.

[LB]

std::cin is a std::istream that just happens to take input from standard input. You should research how the C++ I/O Streams library works.