Using hasNextInt with do-while loop, no error message for negative integer

Question

I am writing a program that includes a test for whether the user input is a positive integer and displays an error messagge if the entry is a non-integer or negative. My code displays the intended error message if a non-integer is entered, but only re-prompts to enter a positive integer if a negative integer is entered. I've tried adding:

if (n <= 0);
System.out.print("You have not entered a positive integer");

before

 n = input.nextInt();

but that makes an error message appear even if a positive integer is entered.

I have also tried:

while (!input.hasNextInt() && n <= 0)

Any help is appreciated.

        Scanner input = new Scanner( System.in );
        int n = 0;

        do {
        System.out.print("Please enter a positive integer: ");
        while (!input.hasNextInt()){
            System.out.print("You have not entered a positive integer. \nPlease enter a positive integer: ");
            input.next();
        }
        n = input.nextInt();

        } while (n <= 0);

score 1 · Accepted Answer · 2014-07-13 02:39:13Z

1

Try this:

int n = -1;//<-- it won't allow user to skip the loop after first pass without proper input
do {
    System.out.print("Please enter a positive integer: ");
    try {
        n = input.nextInt();//<-- in one pass ask input from user only once
        if(0 > n) {
            System.out.print("You have not entered a positive integer. \nPlease enter a positive integer: ");
        }
    } catch (NumberFormatException ex) {
        System.out.println("Invalid number !");
        break;//<-- break loop here, if don't wana prompt user for next input
    } catch(java.util.InputMismatchException ex) {
        System.out.println("Oops number is required !");
        break;//<-- break loop here, if don't wana prompt user for next input
    }
} while (n <= 0);

edited Jul 13, 2014 at 2:39

answered Jul 13, 2014 at 2:26

user2575725

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2 Comments

Juned Ahsan Over a year ago

Exception is too big a exception to be used here, just use NumberFormatException.

Elizabeth VO Over a year ago

I tried this and got an infinite loop when a non-integer was entered.

user207421 · Accepted Answer · 2014-07-13 02:28:07Z

0

Try this:

int n = -1;
while (input.hasNextInt() && (n = input.nextInt()) < 0)
{
    // print error message about negative number
}

When the loop exits, if 'n' is still -1 you got to the end of the input, e.g. the user typed ctrl/d or ctrl/z depending on the platform.

If the user is at the console you might also need to skip to end of line.

answered Jul 13, 2014 at 2:28

user207421

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3 Comments

Elizabeth VO Over a year ago

I tried this and the program didn't run when a positive integer was entered.

user207421 Over a year ago

"Didn't run" meaning what exactly? It will break out of the loop above as soon as a positive number is entered.

Elizabeth VO Over a year ago

It prompted to enter a positive integer, but when a positive integer was entered, it did not break out of the loop and print results.

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