Arrays

[natarajan raman]

Hi all,

I have a doubt with assigning values to the array variables.
In K&B self test page 49 question 12.
It asks for the line of code to be inserted and still allow the code to compile
____________________________________________________________________
class{
main here{
//code
byte [] [] big = new byte [7][7];
byte [][] b= new byte[2][1];
byte b3=5;
byte b2[][][][]=new byte [2][3][1][2];
//here we need to insert any of the line of code below
}
}
_______________________________________________________________________
they ask for the code that can be inserted
a) b2[0][2]=b; b) b[0][0]=b3; c) b2[1][1][0]=b[0][0]; d) b2[1][2][0]=b;
d) b2[0][1][0][0] = b[0][0]; f) b2[0][1] = big;

The answers are a,b,e and f. The explanation states that "If an array is declared as a two dimensional array, you can't assign a one dimensional array to a one-dimesional array reference"

What's that? If i declare an array as two dimesional what does the one dimensional array got to do with that?

When choice a assigns a 2-D array to 4-d it works fine...then why not option c which assigns 2-D to 3-D.?

I am really confused with array assignment. I tried to search earlier posts too. please help me out. I am losing a lot because of not understanding this concept in other areas too.

Can any of you help me please....Else please atleast specify links to earlier posts for reference.

Thanks a lot in advance.
[ August 08, 2004: Message edited by: natarajan raman ]

[Tybon Wu]

You can only assign arrays of the same dimension to another array. In choice a), b2[0][2] is a 2-dimensional array, and b is also a 2-dimensional array, so b2[0][2]=b is legal. In choice c), b2[1][1][0] is a 1-dimensional array, but b[0][0] is a byte, so b2[1][1][0]=b[0][0] is not legal.

Given byte b2[][][][] = new byte [2][3][1][2];
the following are true:

b2 is a 4-dimensional array
b2[0] is a 3-dimensional array
b2[0][0] is a 2-dimensional array
b2[0][0][0] is a 1-dimensional array
b2[0][0][0][0] is a byte

Hopefully that will make it clear for you...

[natarajan raman]

Dear Tybon,

Quote
_______________________________________________
Given byte b2[][][][] = new byte [2][3][1][2];
the following are true:

b2 is a 4-dimensional array
b2[0] is a 3-dimensional array//isn't that a 1-dimensional array?
b2[0][0] is a 2-dimensional array
b2[0][0][0] is a 1-dimensional array//isn't this a 3-D array?
b2[0][0][0][0] is a byte
_________________________________________________________

Please help.....will be grateful if you do....

Thanks.

[Tybon Wu]

Do you agree with the following?

A n dimensional array is composed of series of n-1 dimensional arrays.
A 4 dimensional array is composed of series of 3 dimensional arrays.
A 3 dimensional array is composed of series of 2 dimensional arrays.
...
...
...

If you agree with the above, then you should agree with the following, which is another way of saying the above:

An element of a n dimensional array is a n-1 dimensional array.
An element of a 4 dimensional array is a 3 dimensional array.
An element of a 3 dimensional array is a 2 dimensional array.
...
...
...

OK if you are still in agreement, we can go back to the previous example:

byte b2[][][][] = new byte [2][3][1][2];

b2 is a 4 dimensional array

We just agreed from above that "An element of a 4 dimensional array is a 3 dimensional array"
And since b2[0] is an element of b2
Therefore b2[0] must be 3 dimensional

We just agreed from above that "An element of a 3 dimensional array is a 2 dimensional array"
And since b2[0][0] is an element of b2[0]
Therefore b2[0][0] must be 2 dimensional
...
...
...

[kapil munjal]

Hi Tybon,

you are very right in saying this

Given byte b2[][][][] = new byte [2][3][1][2];
the following are true:

b2 is a 4-dimensional array
b2[0] is a 3-dimensional array//isn't that a 1-dimensional array?
b2[0][0] is a 2-dimensional array
b2[0][0][0] is a 1-dimensional array//isn't this a 3-D array?
b2[0][0][0][0] is a byte

but I think we need to know a complete explanation of this concept.

If you know any link which tells teh assignment of values to multi-dimensional arrays then please provide us.

Thanks
Kaps

[JulianInactive KennedyInactive]

Tony Morris provides a good explanation of how arrays are implemented in Java. The thrust of the article is that Java does NOT implement multi-dimensional arrays; it only provides support for arrays of arrays. What he doesn't explain clearly is what, for example, a two-dimensional array is as opposed to an array of arrays. The crux of this is that in a 2D array each row has a fixed number of columns (no more, no less). With an array of arrays the number of columns can be different for each row.

Consider the following method of initializing an array of arrays:

byte[][] b = { {0}, {0,0}, {0,0,0} };

This does not produce the same set of data as the following, more conventional, method:

byte[][] b = new byte[3][3];

Both arrays have 3 rows but whilst each row in the first example has a different number of columns, each row in the second example has an equal number of columns.

This may help to clear up the source of your confusion.

Originally posted by Nata Raman
a) b2[0][2]=b; b) b[0][0]=b3; c) b2[1][1][0]=b[0][0]; d) b2[1][2][0]=b;
d) b2[0][1][0][0] = b[0][0]; f) b2[0][1] = big;

The answers are a,b,e and f. The explanation states that "If an array is declared as a two dimensional array, you can't assign a one dimensional array to a one-dimesional array reference"

What's that? If i declare an array as two dimesional what does the one dimensional array got to do with that?

The answer to your question is, as I guess you suspect, "nothing". There appears to be a typo in the explanation. It means to say that you can't assign a two dimensional array (array of arrays) to a one dimensional array reference. Think about it this way: the only thing that you can assign to a Thing reference is an instance of Thing. So, answer d) (the first one in your posting ) will fail to compile as b2[1][2][0] is a one dimensional array reference and b is a two dimensional array. Similarly, answer c) will fail to compile as b2[1][1][0] is a one dimensional array reference and b[0][0] is a primitive byte.

Let me know if that clears things up for you.

Jules

[Murtuza Akhtari]

I think Tybon is right..

Natarajan...the best way I made myself understand this concept was..

b2 is a 4 dimensional array...so it will need 4 indices..
b and big are 2-dimensional arrays..so they need 2 indices..
b3 of course is just a byte.

now look at the answer choices.
a) b2[0][2]=b; ..2 indices given for b2 already + 2 of b i.e. putting a 2-dimensional array within a 4-dimensional array..definitely possible !!

b) b[0][0]=b3;..2 indices given for b(doesnt need anymore) b3 is an element which can be stored at tht location

c) b2[1][1][0]=b[0][0]; 3 indices given..needs 1 more..b[0][0] is element...therefore INCORRECT

d) b2[1][2][0]=b;..3 indices given..needs 1 more..b has 2..so makes 5..therefore INCORRECT

e) b2[0][1][0][0] = b[0][0]; b2 has 4 indices..so it is representing the element at tht location....same with b[0][0]...two elements can be equated.

f) b2[0][1] = big;..again...b2 has 2 indices..needs 2 more...big has 2

Hope that helps more than it confuses!!!
Sorry if i m making this more difficult for u !!
[ August 08, 2004: Message edited by: INXS ]

[natarajan raman]

That's really a great explanation from all you people.

Tybon has given me a clear idea how the array is looked at.

Julian Thanx a lot ......Your explanation was also good.

INXS( I just wonder what your real name is!!!) I just can't believe

anyone can explain this as lucid as you did. Great!. you never confused me.

It's just because of you I could grasp what other two explained easily.

Thank you all.

It's just this great response which you people show makes many like me to

prepare well.

Thank you all again.