Do-While loop and hasNextInt()

Question

I know this might sound like a really stupid question but I cannot understand where is my mistake.

Why on the second iteration of the loop, it does not print 'Enter a number:'?

import java.util.Scanner;

public class Main{
    public static void main(String[] args){
        Scanner console = new Scanner(System.in);
        int[] v = new int[10];
        int index = 0;
        do {
            System.out.print("Enter a number:\t");
            v[index] = console.nextInt();
            index++;
        } while(console.hasNextInt());

        for (int i = 0; i < index; i++){
            System.out.print(v[i] + "\t");
        }
        System.out.println("\n" + index);
    }
}

And this is the output:

Enter a number: 1
2
Enter a number: 3
Enter a number: 4
Enter a number: 5
Enter a number: ^D
1       2       3       4       5
5

Are you sure that you want to use a do-while loop, instead of a while loop? — Soutzikevich
– Soutzikevich, Commented May 3, 2019 at 12:22
@P.Soutzikevich if you just substituted a while for the do...while they would not see the first "Entere a number" instead. — Federico klez Culloca
– Federico klez Culloca, Commented May 3, 2019 at 12:44
@FedericoklezCulloca oh I thought the OP had that outside of the loop block too. my bad — Soutzikevich
– Soutzikevich, Commented May 3, 2019 at 15:23

Federico klez Culloca · Accepted Answer · 2019-05-03 12:20:26Z

3

Because hasNextInt blocks until there is an int on the console, therefore not entering the next iteration of the loop.

answered May 3, 2019 at 12:20

Federico klez Culloca

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Comments

Karthick Meenakshi Sundaram · Accepted Answer · 2019-05-03 14:10:44Z

1

Welcome. See answer in comments. Hope it is clear :-)

do {
        System.out.print("Enter a number:\t");  // Prints "Enter a number: "
        v[index] = console.nextInt(); // accepts input "1"
        index++; // increments
    } while(console.hasNextInt()); // waits for input at which point you enter "2"

Ok Let's make it clearer.

The do while loop, executes the do block before it evaluates the while condition. And then if the while condition evaluates to true, it executes the do block again and repeats until the while condition evaluates to false. .

Both console.nextInt and console.hasNextInt read input from the console. So as part of the do block, the "Enter a number:\t" has been printed out, The first nextInt() call has accepted the input "1", which is then followed by the increment, followed by evaluation of the while condition - the console.hasNextInt(), which again waits for input and accepts "2". This explains why the "Enter a number:\t" was not printed before the User Input of "2"

Of course because the value 2 has been entered, the while condition evaluates to true and again the do block is executed and goes on.

Perhaps you need the while loop. This, by contrast, executes the code block only if and as long as the while condition evaluates to true

System.out.print("Enter a number:\t");
while(console.hasNextInt()){
        v[index] = console.nextInt(); 
        index++; // increments
        System.out.print("Enter a number:\t");  
    }

edited May 3, 2019 at 14:10

answered May 3, 2019 at 12:25

Karthick Meenakshi Sundaram

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3 Comments

Kevin Cruijssen Over a year ago

This is exactly the same as OP's code, except that you've added comments (unless I'm skipping over a change you've did by accident).. How does it help understanding the incorrect output?

Kevin Cruijssen Over a year ago

That's indeed a lot better, +1 from me :)

Karthick Meenakshi Sundaram Over a year ago

@KevinCruijssen Thanks for your comment. Happy to improve :-)

Prabhu · Accepted Answer · 2019-05-03 12:24:58Z

0

Think of exiting the loop when you reached the enough size of the array. something like

   do {
            System.out.print("Enter a number:\t");
            v[index] = console.nextInt();
            index++;
        } while(index<10); // if you want the user to enter 10 numbers.

answered May 3, 2019 at 12:24

Prabhu

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