CODEWITHSUNDEEP
java
Erik
Erik

Reputation: 503

Scanner for array's and checking for negatives

I'm trying to write a program that scans in values from the console and stores them in the array. I'm trying to allow the user to specify the length of the array using the scanner. An array length as you know, can only be positive, so I'm trying to check, here's what I have so far that in my mind should be working.

    int[] array; 
int index = 0;

max = new Scanner(System.in);

System.out.println("How long is your array?");

while (!max.hasNextInt() || max.nextInt() <=0 ) 
{
  System.out.println("Arrays aren't negative, unless your's is " +
                "smaller then physically posible, try again");
  max.next(); 
}
maxSize = max.nextInt();


array = new int[maxSize];

System.out.println("Enter up to " + maxSize + " integers");

input = new Scanner(System.in);


while(input.hasNextInt())
{
  array[index] = input.nextInt();
  index++;
}

return array;

It's working somewhat, here's the output How long is your array?

-2

Arrays aren't negative, unless your's is smaller then physically posible, try again

-3

-4

Arrays aren't negative, unless your's is smaller then physically posible, try again

2

3

4

Enter up to 4 integers

Any help is appreciated, and also it does not seem to be storing the array at all so any help with that is appreciated however I just while testing the max size so I'm going to try and figure it out. I'm thinking it may be from using two scanners now that I'm looking at my code, this is my first experience with scanners. Thanks in advance!

Upvotes: 0

Views: 1322

Answers (2)

galchen
galchen

Reputation: 5290

max.nextInt() <=0 reads from the scanner, but if you do so, you miss storing the value.

you will want something like this:

int[] array; 
Scanner sc = new Scanner(System.in);
System.out.println("How long is your array?");
int len = sc.nextInt();
while (len <= 0) {
    System.out.println("Arrays aren't negative, unless your's is " +
                "smaller then physically posible, try again");
    len = sc.nextInt(); 
}
array = new int[len];

also. it is pointless to use 2 scanners on system.in. use the same scanner object.

use scanner.hasNextInt() if you want to enter multiple values in the same line

Upvotes: 1

Jon Skeet
Jon Skeet

Reputation: 1500335

The trouble is that you're reading once to check the size, then you're reading the size separately.

I would extract it into a separate method, which should make life easier in terms of the readability of your bigger method.

public static int readNonNegativeInteger(Scanner scanner)
{
    // You may want some way of letting the user quit...
    while (true)
    {
        while (!scanner.hasNextInt())
        {
            System.out.println("Please enter an integer.");
            scanner.next();
        }
        int candidate = scanner.nextInt();
        if (candidate >= 0)
        {
            // Success!
            return candidate;
        }
        System.out.println("The value must be non-negative. Please try again.");
    }
}

Note that while (true) loops are relatively rare and some commenters may object. However, in this case it feels like the simplest approach to me - we want to keep going until we've successfully read a value, and the way we break isn't by satisfying a condition - it's by returning. I suspect it would be hard to find a clearer way of expressing the same flow without repeating the condition or having a pointless "done" variable.

Upvotes: 1

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