Java

indienick · 08-22-2006, 06:17 PM

Is there a way in Java to assign values en masse to an array?

To give an idea of what I'm trying to achieve, in Perl, you can do:

Code:

@listofvalues = (1..$anynumber);

to add (in this case) all digits between 1 and $anynumber to the array @listofvalues.

Is there a Java equivalent, or another way of doing this in Java (with that kind of ease)?

So far, I have:

Code:

int limit = Integer.parseInt(args[0]);
double[] list = new double[limit];
int j = 0;
for (int i = 2; i < limit; i++) {
     list[j] = i;
     j++;
}

Is there a way to make this a little more eloquent?

graemef · 08-22-2006, 07:58 PM

As far as I am aware you need to use a loop in Java, but I think that your code will miss the last two elements of the array, you could try the following

Code:

int limit = Integer.parseInt(args[0]);
int start = Integer.parseInt(args[1]);
double[] list = new double[limit];

for (int i = 0; i < limit; i++) {
     list[i] = i+start;
}

indienick · 08-23-2006, 09:46 AM

Thanks for the reply graemef, however I don't need the user to specify a starting point - the starting value is fixed at 2 (which is assigned to list[0]).

How will it miss the last two array elements?

xhi · 08-23-2006, 10:11 AM

> How will it miss the last two array elements?

in your original code j starts out 2 behind i. when i reaches limit (which is also the size of the array) j is still 2 behind i. loop stops and the last 2 in array are left unset.

indienick · 08-23-2006, 10:25 AM

OOOoooooooooooo alright.

thanks xhi.

indienick · 08-23-2006, 10:33 AM

Here's the new code (with line numbers):

Code:

15 int limit = Integer.parseInt(args[0]);
16 int start = 2;
17 
18 int numbers[] = new int[limit];
19
20 for (int i = 0; i < limit; i++) numbers[i] = i + start;

The code compiles without a complaint, however I get an error when I try to run it:

Code:

Exception in thread "main" java.lang.NullPointerException
        at Eratosthenes.main(Eratosthenes.java:20)

What gives?

EDIT: Also, is there a way to create arrays with unspecified (because they're unknown) lengths? (ie. Perl reference again - lists/arrays just exist with variable lengths).

xhi · 08-23-2006, 11:01 AM

> Also, is there a way to create arrays with unspecified (because they're unknown) lengths? (ie. Perl reference again - lists/arrays just exist with variable lengths).

yes, use an ArrayList or other container.

Code:

18 int numbers[] = new int[limit];
19
20 for (int i = 0; i < limit; i++) list[i] = i + start;

you are creating an array named numbers then using array called list.. is that what you intended?

indienick · 08-23-2006, 11:18 AM

Oops, sorry about that - I'm creating and using a list called numbers.

xhi · 08-23-2006, 11:22 AM

is that what was causing the exception?

indienick · 08-23-2006, 11:25 AM

no no...I just overlapped an old code with new code.

And thanks for the ArrayList hint.

xhi · 08-23-2006, 11:30 AM

im not understanding you completely. are you still getting the exception? if you are the only thing that i can see that would be causing it is the args that the program is receiving.

if you are not getting the exception, well good

indienick · 08-23-2006, 12:28 PM

I'm not getting the exception any more, xhi.

Thanks for all your help (you too, graemef).

All I have to do now is fix up the math that calculates which of the numbers from start to limit are prime.