Hi florent

Read K. Beck idiom on redefining = normally you MUST also redefine hash

the pattern Kent suggested was

Book>>= anBook (self author = anBook author ) and: [self name = anBook name]

Book>>hash self author hash bitXor: self name hash

Now most of the time I try to use sortedCollection to avoid to have to fix the equality of an object, SortedCollection sortBlock: [:a :b | a name > b name and.....]

On 5 août 06, at 12:56, florent trolat wrote:

Mathieu a écrit :

...

florent trolat a écrit :

...

Hi,

I've define an '=' message for an oject and I want to use this object in a Collection.

I've seen that 'incudes:' message use '=' or not '==' and it's not fun for me!

I think that '=' message is to compare two objects and '==' to know if the two object are one!

So for me 'includes:' must use '==' or not '='...

May be I have not understand something...

Thanks to help me....

Hi florent

You can do

---

= anObject

^anObject == self

---

But it is not pretty so the best way is to remove = and the default behavior of = is == (unless inherit from someone else...).

Ok I understand ... it was my job to see this, sorry... _______________________________________________ Beginners mailing list Beginners@lists.squeakfoundation.org http://lists.squeakfoundation.org/mailman/listinfo/beginners

cdrick wrote:

2006/8/7, stéphane ducasse <ducasse@iam.unibe.ch mailto:ducasse@iam.unibe.ch>:

Hi florent

Read K. Beck idiom on redefining = normally you MUST also redefine hash

the pattern Kent suggested was


Book>>= anBook
        (self author = anBook author ) and: [self name = anBook
name]     (a)

Book>>hash
        self author hash bitXor: self  name hash     (b)

I'm aware of that... but I just don't understand what's: 'hash bitXor:'

why (a) is not sufficient ?

hash tables doesn't talk to me too much ;). What I imagine is that bitXOr gives a logical (byte) representation ? and these are used in a table (hash) to access objects (pointers)?

Set and Dictionaries have a special ways of storing objects. They compute a position in an array from a number calculated on the object. This number is returned from the #hash method sent on the object and manipulated to have an index in the array (using a modulo operator for example).

http://en.wikipedia.org/wiki/Hash_table

As an example, you want to store aBook in an empty hash table.

aHashTable add: aBook

image the hash table has an array of size 10 filled with nil because the table is empty.

So, when adding, the hash table will compute the index in which the object will be stored:

HashTable>>computeIndexFor: anObject ^ anObject hash \ self array size

\ is the implementation of the modulo function which basically returns a number between 0 and the parameter based on the receiver.

Then, the add: method is:

HashTable>>add: anObject self array at: (self computeIndexFor: anObject) put: anObject.

This is very very simplified but the idea is there in my opinion.

Now, you should find how the hash table can store more than 10 objects. Hint: the array does not have to be resized (but sometimes it will for efficiency).

Is it clearer ? Feel free to ask more