[erlang-questions] Clueless am I

[Donald Steven]

Stefan and Hugo,

Is there any data structure in Erlang that will accept modification?  
This is quite beyond me as to how to add elements to a data structure.  
I've hit the same dead end with lists.  If one has, for example, a 
database, and one wants to add a customer, do you really need to have a 
new (second) database?

Don

On 04/25/2016 03:29 PM, Stefan Schmiedl wrote:
> Hi Don,
>
> these are not the arrays you're looking for.
>
> http://erlang.org/doc/man/array.html starts with "Functional, extendible
> arrays".
> A bit further down you can find "The representation is not documented and is
> subject to change without notice."
>
> So the array datatype is not the contiguous block of memory you are used to,
> but a functional data structure. This means that an array, is basically
> read-only.
>
> Mathematical functions are reproducible: You pass in the same arguments, you
> get
> the same result. So the array A1 in your non-working example references the
> original
> datastructure and your code is equivalent to
>
>>       A1 = array:set(0, nextevent(), array:new(20)),
>>       A2 = array:set(1, nextevent(), A1),
>>       A3 = array:set(2, nextevent(), A1),
>>       A4 = array:set(3, nextevent(), A1),
>>       A5 = array:set(4, nextevent(), A1),
> s.
>
>> -----Ursprüngliche Nachricht-----
>> Von: erlang-questions-bounces@REDACTED [mailto:erlang-questions-
>> bounces@REDACTED] Im Auftrag von Donald Steven
>> Gesendet: Montag, 25. April 2016 21:16
>> An: erlang-questions@REDACTED
>> Betreff: [erlang-questions] Clueless am I
>>
>> Dear friends,
>>
>> Please forgive me for being clueless.  (I'm sure it's because I come to
> Erlang
>> steeped in C.)
> Yes. SCNR.
>
>   Below are 2 very simple programs.  Both run: the first one
>> works (but requires all these variables which are meaningless for my
>> purposes) but the second one  does not set the array values and I don't
>> understand why not or how to do this.
>>
>> Thank for any help you can provide.
>>
>> Don
>>
>> %% THIS WORKS
>>
>> -module(arraytest).
>> -export([main/0]).
>>
>> main() ->
>>
>>       A1 = array:set(0, nextevent(), array:new(20)),
>>       A2 = array:set(1, nextevent(), A1),
>>       A3 = array:set(2, nextevent(), A2),
>>       A4 = array:set(3, nextevent(), A3),
>>       A5 = array:set(4, nextevent(), A4),
>>
>>       io:format("~nArray: ~p~n", [A5]),
>>       io:format("Array size: ~p~n~n", [array:size(A5)]).
>>
>> nextevent() ->
>>
>>       [rand:uniform()].
>>
>> ============================================
>>
>> %% THIS DOES NOT WORK
>>
>> -module(arraytest).
>> -export([main/0]).
>>
>> main() ->
>>
>>       A1 = array:set(0, nextevent(), array:new(20)),
>>       array:set(1, nextevent(), A1),
>>       array:set(2, nextevent(), A1),
>>       array:set(3, nextevent(), A1),
>>       array:set(4, nextevent(), A1),
>>
>>       io:format("~nArray: ~p~n", [A1]),
>>       io:format("Array size: ~p~n~n", [array:size(A1)]).
>>
>> nextevent() ->
>>
>>       [rand:uniform()].
>>
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