Separating array in other arrays

Question

I have an array that I need to split in different arrays. I have an array of Strings and need to split it in different pages (different arrays).

At first, I get the length of the array, using

int size = array.length;

And then, I get the number of pages I need, knowing that each page should only have 10 Strings

int numberOfPages = (int) Math.floor(size/10);

The user then select which page he wants to see

int pageSelected = 2;

After that, I tried to split the array, but got some exceptions. I tried:

Arrays.copyOfRange(array,(0+10*(pageSelected-1),10*10+(pageSelected-1)));

I get an exception when I try to print the values of the new array.

Is there anyway to split an array in 'pages', and display these 'pages' as requestes?

@Edit1 I get a Nullpointer Exception

The exception is a Nullpointer The stacktrace is useless, as it was generated by Bukkit, and has no information regarding the code — LeoColman
– LeoColman, Commented Dec 30, 2014 at 23:49
@j_v_wow_d: it should be Math.ceil((double) size/10) because an integer division already floors. — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Dec 31, 2014 at 0:43

willeM_ Van Onsem · Accepted Answer · 2014-12-31 00:44:26Z

3

The error probably occurs in this line:

Arrays.copyOfRange(array,(0+10*(pageSelected-1),10*10+(pageSelected-1)));

Where there is an error with the brackets (the method requires three arguments, but from the method's perspective, you only provide two: the last two are grouped by brackets). You can use:

Arrays.copyOfRange(array,10*(pageSelected-1),10*10+(pageSelected-1));

(removed 0+ since this has no use).

Furthermore you made a semantical error: 10*10+(pageSelected-1) should be replaced by: 10+10*(pageSelected-1

So the full line reads:

Arrays.copyOfRange(array,10*(pageSelected-1),10+10*(pageSelected-1));

Although a better guideline would be to use small steps:

int i = pageSelected-1;
int g = 10*i;
Arrays.copyOfRange(array,g,g+10);//do something with the result

And to do it perfect, you better use variables for constants such that - if you change your mind - you can easily modify the number of items per page:

int i = pageSelected-1;
int perpage = 10;
int g = perpage*i;
Arrays.copyOfRange(array,g,g+perpage);//do something with the result

Finally a small remark: as @j_v_wow_d says, you should ceil the division, otherwise you will generate one page for 11 items. The correct code for numberOfPages is thus:

int numberOfPages = (int) Math.ceil((double) size/perpage);

edited Dec 31, 2014 at 0:44

answered Dec 30, 2014 at 23:45

willeM_ Van Onsem

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5 Comments

Kick Buttowski Over a year ago

for my learning goal, can you explain what 10*(pageSelected-1) means how did the op come up with it?

willeM_ Van Onsem Over a year ago

10 is the number of items per page. The OP uses an offset of 1 of pageSelected. Thus you decrement it to get a Java array-index. To get the real page (pageSelected-1). Then you multiply it by 10 because every page has 10 items. To get the offset item.

Kick Buttowski Over a year ago

and each page is in each index of an array or from zero to nine counts as a page?

willeM_ Van Onsem Over a year ago

I guess the OP processes the subarray by printing a webpage or something showing these items. Now of course one can also do the math by using indices with the calculated offset. By copying it however, things can be done a bit simpler.

Kick Buttowski Over a year ago

great explanation I think you should add this to your post to be more perfect thank you

wassgren · Accepted Answer · 2014-12-30 23:51:02Z

2

If the pageSelected is indexed so that 0 is the first:

String[] array = {"a", "b", "c", "d", "e", "f"};
int pageSize = 2;
int pageSelected = 2;
final String[] pageData =
        Arrays.copyOfRange(
                array,
                (pageSelected * pageSize),
                (pageSelected * pageSize) + pageSize);

The outcome of this is that pageData contains ["e", "f"].

answered Dec 30, 2014 at 23:51

wassgren

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Comments

Vivin Paliath · Accepted Answer · 2014-12-30 23:48:05Z

1

You could just do size / 10; no need to floor or cast it.

Also, the from expression 10*10+(pageSelected-1) should be 10 + (10 * (pageSelected - 1)).

answered Dec 30, 2014 at 23:48

Vivin Paliath

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1 Comment

willeM_ Van Onsem Over a year ago

And a small brackets error... But that's indeed not the important part.

Philippe Aubertin · Accepted Answer · 2016-03-26 01:57:05Z

1

Arrays.copyOfRange(array,(0+10*(pageSelected-1),10*10+(pageSelected-1)));

I think you mean this :)

Arrays.copyOfRange(array,0+10*(pageSelected-1),10+10*(pageSelected-1));

Look at the position of the + and * after the second comma.

But, it would be easier to read if it was not a one-liner

int offset = 10 * (pageSelected - 1);
Arrays.copyOfRange(array, offset, offset + 10);

Or even better

public static final int PER_PAGE = 10; 

/* (...) */

int offset = PER_PAGE * (pageSelected - 1);
Arrays.copyOfRange(array, offset, offset + PER_PAGE);

edited Mar 26, 2016 at 1:57

answered Dec 30, 2014 at 23:50

Philippe Aubertin

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