How to verify that input is a positive integer in Java

Question

I am trying to create an input that will Verify the input is an Integer, and that it is positive. Right now i have this. How should i check if the integer is positive

EDIT: Also i need it to keep asking until you enter a positive integer

        /**
 * pre: none
 * post: returns a positive integer as entered by the user
 */
public static int getInput(){
    int a;
    Scanner scan = new Scanner(System.in);
    System.out.println("Enter Desired Quantity.");

    while (!scan.hasNextInt()){ //Checks if input is Integer
        System.out.println("Enter A Positive Integer");
        scan.next();

    }
    a = scan.nextInt(); //Assigns entered integer to a
    return a; //replace with correct code
}

low quality question.... should have searched `google` first — Siva, Aug 25 '14 at 17:25
possible duplicate of [Using Scanner to validate user input](http://stackoverflow.com/questions/23021130/using-scanner-to-validate-user-input) — Jeff B, Aug 25 '14 at 18:25

Sergey Kalinichenko · Accepted Answer · 2014-08-25T17:39:44.763

4

You can do this in a single loop, similar to the one that you have for skipping invalid input. However, since the loop needs to ensure two things (i.e. a number is entered, and that number is positive) you need to modify its body.

Since the input needs to be done at least once, a do/while loop is a better choice.

Start with a loop that satisfies the condition that you want, i.e. "I got a number, and that number is positive":

int num = -1;
do {
    // We'll provide content of this in a moment
} while (num <= 0);

Once the loop exits, you know that num > 0, so the only task now is to write a body of the loop that takes us closer to that goal.

Inside the loop we need to check that the user entered a number. If he did, we harvest that number; otherwise, we tell the user to try again:

System.out.print("Please enter a positive integer number: ");
if (scan.hasNextInt()) {
    num = scan.nextInt();
} else {
    System.out.println("I need an int, please try again.");
    scan.nextLine();
}

That's it - now you have a loop body that reads the value for you, and a loop condition that ensures that the value is positive on exit. Combine the loop with the loop body, and try it out. It should do the trick.

edited Aug 25 '14 at 17:39

answered Aug 25 '14 at 17:28

Sergey Kalinichenko

675,664
71
998
1,399

When i test this code , if i input a number such as -20, it does not show the "I need an int, please try again." – Tolkingdom Aug 25 '14 at 17:38
1

because -20 is a valid Integer. – specializt Aug 25 '14 at 17:38
@specializt You can add an unconditional prompt. See the edit for one way of doing it. – Sergey Kalinichenko Aug 25 '14 at 17:40
Yes but i specifically need positive integers only – Tolkingdom Aug 25 '14 at 17:43
Thankyou for your assistance, this is what i needed. – Tolkingdom Aug 25 '14 at 17:49

score 1 · Answer 2 · answered Aug 25 '14 at 17:23

1

Simple:

Scanner input = new Scanner(System.in);

System.out.print("Enter a number: ");
int number = input.nextInt();

if( number == 0)
{ System.out.println("Number is zero"); }
else if (number > 0)
{ System.out.println("Number is positive"); }
else 
{ System.out.println("Number is negative"); }

On a side note:

Check Math.signum()

Returns the signum function of the argument; zero if the argument is zero, 1.0 if the argument is greater than zero, -1.0 if the argument is less than zero.

answered Aug 25 '14 at 17:23

Rahul Tripathi

152,732
28
233
299

How can i make this so it will keep asking for input until it is a positive integer – Tolkingdom Aug 25 '14 at 17:27
@Tolkingdom:- Put that in a do..while loop! – Rahul Tripathi Aug 25 '14 at 17:29
If i enter a Letter instead of a number it crashes the program – Tolkingdom Aug 25 '14 at 17:42

score 0 · Answer 3 · answered Aug 25 '14 at 17:50

You can try this:

public static int getInput(){
   Scanner scan = new Scanner(System.in);

   System.out.println("Enter Desired Quantity.");
   int a = scan.nextInt();
   while (a  < 0){ //Checks if input is Integer
       System.out.println("Enter A Positive Integer");
       a = scan.nextInt();

   }

   return a;
}

How to verify that input is a positive integer in Java

3 Answers3