[erlang-questions] Spurious '100' in array:new

Fred Youhanaie <>
Tue Jan 29 01:52:57 CET 2019


Don

If you mean using a single ~p to print the whole map with the array inside it, then you'll end up with the extra stuff too. io:format cannot extract the data part of the array automatically.

However, you can use the array:to_list/1 function to explicitly extract the data part of the array, and then create a "printable" map using maps:update/3, i.e.

Map_with_array = ...

% extract the array data as a list
Array_list = array:to_list( maps:get(arrayKey, Map_with_array) ).

% create the printable map, by replacing the array structure with the list equivalent
Map_printable = maps:update( arrayKey, Array_list, Map_with_array ).

% print the whole map with a single ~p
io:format("The map: ~p.~n", [Map_printable]).

Do bear in mind that array:to_list/1 will give you a long list with 19 x {0,unspecified,0} elements for your initial array!

Cheers,
Fred

On 28/01/2019 20:26, Donald Steven wrote:
> Thanks Jesper, that's most helpful to know.  Is there a particular way to print it in io:format that will *not* show the 'extra stuff'. (I used ~p)
> 
> Best,
> 
> Don
> 
> On 1/28/2019 11.01 AM, Jesper Louis Andersen wrote:
>> On Sat, Jan 26, 2019 at 6:29 PM Fred Youhanaie < <mailto:>> wrote:
>>
>>     That's part of the internal structure of the array module, the data structure is opaque and should not be relied upon.
>>
>>
>> For full reference, you can glean on the internal structure:
>>
>> Erlang/OTP 21 [erts-10.2.1] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [hipe] [dtrace]
>>
>> Eshell V10.2.1  (abort with ^G)
>> 1> rr(array).
>> [array]
>> 2> array:new(19, {default, {0, unspecified, 0}}).
>> #array{size = 19,max = 0,
>>        default = {0,unspecified,0},
>>        elements = 100}
>> 3> array:new(9, {default, {0, unspecified, 0}}).
>> #array{size = 9,max = 0,
>>        default = {0,unspecified,0},
>>        elements = 10}
>>
>> Here is what is happening: In Erlang, an Array is a very flat tree with a branching factor of 10. It is stored in `elements`. But instead of storing a tree, you may store an integer encoding how 
>> much more space is allowed in this part of the tree when it is expanded. You have created a tree of size 19, which means it needs two expansions of size 10, or 100---though not all of those 
>> expansions will be used, as it is extra padding. If you had created an array of size 101, it would have been 1000, and so on. It is always the next log10 ceiling value. In the example with size 9, 
>> only one expansion is needed in which case we have a `10` in the elements.
>>
>> You might ask why it isn't represented as a traditional array. This is because of persistence in data structures: older versions of the tree can be kept alive, so you cannot overwrite an older 
>> array. It is useful in a number of situations, the most important one being that you can know the state of a crashing process before it crashed, and what event led to said crash.
>>
>>
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