Only accept positive integers

[Joshcannon]

I need this to only accept positive integers. so if I enter a negative or something with a decimal it won't take it. This is what I have now. I just need help where it won't take it if it isn't an integer.

#include <iostream>
#include <cstdlib>
#include <cassert> 
using namespace std;

int main ( )
{
	int N;
	
	do 
	{
	
		cout << endl << "Please enter a positive integer : " ;
		cin >> N ;
	if( N < 0 || N % 0 != 0 ) //
		assert ( N % 0 != 0 );
        cout << "Invalid number." << endl;
	}
	while( N < 0 || N % 0 != 0 ); // 

[admkrk]

N will always be an integer. If you try to enter a number with a decimal point, the decimal point and anything after it will be ignored.

[Joshcannon]

Ohhhh! Thank You!

[booradley60]

If you want to avoid negatives and throw out stuff like people typing in strings, try this:

#include <iostream>
#include <limits>

int main()
{
    int input;
    while(std::cout << "Enter a positive integer: " &&
          (!(std::cin >> input) || input < 0))
    {
        std::cout << "Not a positive integer!\n";
        std::cin.clear();
        std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
    }
    //do stuff with input
    return 0;
}

This basically says, "As long as I can print this prompt, and you type something that puts cin in a fail state, or you give me an integer less than 0, I'm throwing it out and asking you again."

[andywestken]

Or you could use cire's approach, for even better robustness:
http://www.cplusplus.com/forum/beginner/108849/#msg592118

The clear/ignore approach booradley60 suggests will accept strings which begin with a valid number but continue on with other chars. e.g.

42nd Street -> 42
99 red balloons -> 99
3.14 -> 3
1 2 3 4 -> 1

cire's approach ensure that the input is just a number, with optional predicate.

Andy

[admkrk]

The clear/ignore approach booradley60 suggests will accept strings which begin with a valid number but continue on with other chars. e.g.

What do you mean? The type int does that without any help.

# include <iostream>
using namespace std;

int main()
{         
    int x; 
	
    cout << "Enter a number: ";
    cin >> x;
	
    cout << "You entered " << x << '\n';

    system("pause");
    
    return 0;
}

This doesn't check for a positive number and gives trash if it doesn't start with a number, but it will give the same results for the input you used.

[booradley60]

@admkrk
That's his point. What if you were using a file stream instead of cin, you would WANT to have 42nd Street be a failure if you were expecting an int field (probably indicates a malformed file), not just accept the integer.

My only problem with cire's code is that I have not done anything to bone up on C++11, so the predicate part of his code is difficult for me to read.

EDIT (clarification): I am supporting andywestken's suggestion over mine, and this post was intended as a response to admkrk. :)

[giblit]

That's what Andy was saying yours allows for junk to be inputted. Where as cires approach does not allow junk.

basically the OP probably doesn't want someone to input 1.23 for 1 he wants 1 for 1. So yeah ints truncate everything but the value entered is still not valid.

[admkrk]

I didn't consider a file stream and cin was used in the example. I also left off error checking because I didn't know how involved Joshcannon wanted to get with it. cire's approach certainly covers most if not all the possibilities, but it is most likely overkill and unusable for someone in a class and having this kind of problem.

While my answer isn't the best solution, I think it was more appropriate.

So yeah ints truncate everything but the value entered is still not valid.

I did expect a follow up asking about that by the way and I should have mentioned it originally, but since it seemed to have solved his problem the truncating might have been all that was needed.