Checking if an array is null or not

[jlausin]

I created an array of books and named it catalog as follows:

Book[] catalog;
catalog = new Book[20];

Based on the input command, I can either list this catalog or add items to the catalog. When I list this catalog, how can I check if this catalog is null or not?

Thanks for your help

[Shashi Sadasivan]

I created an array of books and named it catalog as follows:

Book[] catalog;
catalog = new Book[20];

Based on the input command, I can either list this catalog or add items to the catalog. When I list this catalog, how can I check if this catalog is null or not?

Thanks for your help

Hi,
once u create catalog array, each elemnt is assigned to be null (as it is your own predefined class)
so if you want to check if an element is null

if(catalog[0] == null)

will do the trick

[Plater]

For the situtation:

Book[] mybooks;
 

If you were to check mybooks[0] you would get an exception.
Use

if (mybooks==null)
 

However, I generally make it a point to assign things like:

Book[] mybooks= new Book[0];
 

Then I can do this:

if (mybooks.Length==0)
 

to determine if it's "empty"

[Shashi Sadasivan]

For the situtation:

Book[] mybooks;
 

If you were to check mybooks[0] you would get an exception.
Use

if (mybooks==null)
 

However, I generally make it a point to assign things like:

Book[] mybooks= new Book[0];
 

Then I can do this:

if (mybooks.Length==0)
 

to determine if it's "empty"

Hi Plater
if you do

Book[] mybooks= new Book[10];

mybooks.Length will result in 10 regardless of wether you have assigned any array items with any objects.
this code creates an aray of 10 null elements
However if it was an int, or frloat array it would have made an array with 10 elements each set to 0
I did try it to confirm it.
However using a List<T> or a dynamic array will give a better functionality

[r035198x]

Hi Plater
if you do

Book[] mybooks= new Book[10];

mybooks.Length will result in 10 regardless of wether you have assigned any array items with any objects.
this code creates an aray of 10 null elements
However if it was an int, or frloat array it would have made an array with 10 elements each set to 0
I did try it to confirm it.
However using a List<T> or a dynamic array will give a better functionality

He used zero for the intial size of his array not 10 (ten).

Arrays are so Fortranesque. Use them only when the number of objects to be stored is known before hand. Then you only need to intialize the array once with the appropriate size. Otherwise ArrayList is the way to go.

[Shashi Sadasivan]

He used zero for the intial size of his array not 10 (ten).

Arrays are so Fortranesque. Use them only when the number of objects to be stored is known before hand. Then you only need to intialize the array once with the appropriate size. Otherwise ArrayList is the way to go.

Hi,
i think i should have had my post more subtle, didnt want to offend, but what i meant was that by using the length you cannot determine the number of elemnts which have been assigned.

apologies for any harh looking reply i had in there...i have no such intensions!! :)

[r035198x]

Hi,
i think i should have had my post more subtle, didnt want to offend, but what i meant was that by using the length you cannot determine the number of elemnts which have been assigned.

apologies for any harh looking reply i had in there...i have no such intensions!! :)

Of course it doesn't look harsh at all. You just overlooked that he had used zero to initialize the array so the length would be zero like he expected.
The approach is not a very good one of course because he creates a useless array, reserving unneccessary memory which has to be gced when he finally initializes the array to the proper sized array.

[Plater]

Hi,
i think i should have had my post more subtle, didnt want to offend, but what i meant was that by using the length you cannot determine the number of elemnts which have been assigned.

apologies for any harh looking reply i had in there...i have no such intensions!! :)

But using myBooks[0] when it was delcared as
"Books[] myBooks;"
or
"Books[] myBooks = new Books[0];"
would throw an exception.
You would want to check for null on the object FIRST, before checking for null on their individual indexes.
And declaring an "empty" array is done with setting the size to [0] since you cann't assign "null" to an array declaration explicitly.