How is ordered a Dictionary in Pharo?

Question

Let's say we have a loop that enters Association s to a Dictionary in a clear order:

| d |

d := Dictionary new: 10.

1 to: 10 do: [ :i |
    d add: i -> (i + 9 printStringBase: 20)
].

d

When I evaluate this code I get a Dictionary in a "twisted" order:

9 -> I
5 -> E
1 -> A
10 ->J
6 -> F
2 -> B
7 -> G
3 -> C
8 -> H
4 -> D

Each time a Dictionary with the same entry data is created it have the same order, so I assume it is a feature not a bug..?

I use Pharo v9.0.21.

Answer 1

A Dictionary is not an ordered collection. Instead it is a keyed collection, keeping key-value pairs.

There is an OrderPreservingDictionary for you to use: https://github.com/pharo-contributions/OrderPreservingDictionary

Answer 2

In addition to this other answer is is worth explaining the apparent disorder shown in the question.

Firstly observe that Dictionary new: 10 will create a new instance of Dictionary with capacity for a prime number p of associations greater than 10 . Say 11, 13, 17, whatever.

Secondly, for every association added, the dictionary will compute the hash value of the key and deduce the location from its remainder modulo p .

Since all keys occurring in the example are instances of SmallInteger , their hashes will be themselves(*). And since these are smaller than p , they will equal the modulo and hence be stored in the slots derived from their values in some implementation-dependent way.

Finally, the printing method is free to enumerate the associations in any order.

---

(*) While this is true in some dialects, I've checked in Pharo and this is not true, 3 hash is not 3 etc. which explains the "twisting" in the case of Pharo.

Answer 3

For completeness of the answer there is such thing as ordered Dictnionary.

At Smalltalk/X the #OrderedDictionary is defined as:

Dictionary subclass:#OrderedDictionary
        instanceVariableNames:'order'
        classVariableNames:''
        poolDictionaries:''
        category:'Collections-Sequenceable'

"/     I am a subclass of Dictionary whose elements (associations) are ordered in a
"/     similar fashion to OrderedCollection.
"/     That is, while being filled via #at:put: messages (or similar Dictionary protocol),
"/     the order in which associations are added, is remembered and accessible via the #atIndex:
"/     or #order messages. 
"/     Therefore, this combines fast access via hashing with a defined order when enumerating.
"/     
"/     [instance variables:]
"/         order <OrderedCollection>       Ordered collection of keys reflecting the order of
"/                                         associations in the dictionary.
"/ 
"/     [complexity:]
"/         access by index: O(1)
"/         access by key: O(1)
"/         searching: O(n)
"/         insertion: mostly O(1)
"/         removal: mostly O(N) (because order will have O(n) behavior)
"/         
"/     [author:]
"/         Ifor Wyn Williams 
"/         Changed by: exept