Major array problem

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Yomi Smito - 04 Jun 2006 02:02 GMT

Why does this method alter the array "double[][] A", if the only one
whose elements are being set is "double[][] C" ? Help would be very
appreciated

static double[][] prueba (double[][] A,double[][] B){
double[][] C=new double[A.length][];
for (int i = 0; i < C.length; i++) {
C[i]=A[i];
}
for (int i = 0; i < C.length; i++) {
for (int j = 0; j < C[i].length; j++) {
C[i][j]=0;
}
}
return C;
}

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Lasse Reichstein Nielsen - 04 Jun 2006 02:18 GMT

> Why does this method alter the array "double[][] A", if the only one
> whose elements are being set is "double[][] C" ?

...

> C[i]=A[i];

...

> C[i][j]=0;

Arrays are objects, so when storing them, you store a reference. You
don't have two-dimensional arrays, only arrays of arrays, so when you
do the first assignment above, you make C[i] reference the same
integer array as A[i]. You then change the value of the j'th entry of
that array.

/L

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Lasse Reichstein Nielsen - lrn@hotpop.com
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Yomi Smito - 04 Jun 2006 02:53 GMT

Lasse Reichstein Nielsen a écrit :

> > Why does this method alter the array "double[][] A", if the only one
> > whose elements are being set is "double[][] C" ?
[quoted text clipped - 14 lines]
> DHTML Death Colors: <URL:http://www.infimum.dk/HTML/rasterTriangleDOM.html>
> 'Faith without judgement merely degrades the spirit divine.'

Ok thanks a lot. Clearly I'm new to OOP.

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Yomi Smito - 04 Jun 2006 02:55 GMT

I'm fixing it by using a more generic "C[i]=new double[A[i].length];"
instead. Thanks.

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blmblm@myrealbox.com - 04 Jun 2006 17:21 GMT

>Lasse Reichstein Nielsen a écrit :
>
[quoted text clipped - 12 lines]
>
>Ok thanks a lot. Clearly I'm new to OOP.

I don't think this is an OOP thing so much as it is a Java thing.
In Java, all you have are primitives and references, and assignment
can't be overloaded. In C++, things are different, and they might
be in other OOP languages as well. (Actually, you would have other
problems passing a multidimensional array as a parameter in C++,
but that's further off-topic.)

Just a nitpick, possibly useful if/when you later learn another
OO language.

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B. L. Massingill
ObDisclaimer: I don't speak for my employers; they return the favor.

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Dimitri Maziuk - 05 Jun 2006 17:28 GMT

Yomi Smito sez:

> Lasse Reichstein Nielsen a écrit :
>
>> > Why does this method alter the array "double[][] A", if the only one
>> > whose elements are being set is "double[][] C" ?

...

>> Arrays are objects, so when storing them, you store a reference.

...

> Ok thanks a lot. Clearly I'm new to OOP.

This has nothing to do with OOP and everything with Java's type
system and terminology. All objects are pointers, and that's what
"reference" means. When it comes to objects, the promise to pass
everything "by value" means "pass the value of the pointer".
So the assignment operator doesn't do what you expect.

Dima

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Well, lusers are technically human. -- Red Drag Diva

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Dusan Juretic - 04 Jun 2006 16:47 GMT

> Why does this method alter the array "double[][] A", if the only one
> whose elements are being set is "double[][] C" ? Help would be very
[quoted text clipped - 12 lines]
> return C;
> }

Your problem is this line: C[i]=A[i], now C[i] references A[i] (so A and
C are now dependent).

---------------
Dusan Juretic D.

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Major array problem

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