[erlang-questions] array search problem

[Roelof Wobben]

Thanks,

I see now a output but the wrong one.

It seems that [key| Value] is not working because the second argument is 
a tuple.

So back to the book and reading.

Roelof

Garry Hodgson schreef op 30-1-2015 om 19:44:
> [] is a list with 0 elements.
> [ Key, Tail ] is a list with 2 elements.
> [ Key | Tail ] is a list with Key as its first element,
> and Tail as the rest of the list (possibly [] ).
>
>
> On 1/30/15 1:34 PM, Roelof Wobben wrote:
>> Hello,
>>
>> Im stuck again,
>>
>> I have now this :
>>
>> read(Key, []) ->
>>   {error, Instance};
>>
>> read(Key, [Key, _] )->
>>   Key;
>>
>> read(Key, [_, Tail] ) ->
>>   read(Key, Tail).
>>
>> I see this output :
>>
>> 10> Db1 = db:new().
>> []
>> 11> Db2 = db:write(name,roelof,Db1).
>> [{name,roelof}]
>> 12> Db3 = db:read(name, Db2).
>> ** exception error: no function clause matching 
>> db:read(name,[{name,roelof}]) (db.erl, line 14)
>>
>>
>>
>>
>>
>>
>> Fred Hebert schreef op 30-1-2015 om 18:11:
>>> Hi Roelof.
>>>
>>> I recognize these exercises from the Francesco Cesarini and Simon
>>> Thompson book (published by O'Reilly).
>>>
>>> If you cannot work through these exercises, I strongly recommend you go
>>> back to the chapter that precedes them and re-read it again until it
>>> clicks, or possibly go back to a chapter you may have skipped.
>>>
>>> It will be a better use of your time to go through it and understand it
>>> properly rather than come to the mailing list and ask people in here to
>>> do that part on your behalf.
>>>
>>> That being said, the trick for this exercise is to go through 
>>> recursion.
>>>
>>> The database you mention uses *lists* (*arrays* are a different data
>>> type and have a module to that name, too). Lists are a data structure
>>> defined recursively:
>>>
>>>      [3, 2, 1]
>>>      [3 | [2, 1]]
>>>      [3 | [2 | [1]]]
>>>      [3 | [2 | [1|[]]]]
>>>
>>> Those 4 lists are equivalent. So when you traverse a list by going
>>> [Head | Tail], on each of these, you take one element and then are left
>>> with the rest.
>>>
>>> The definition is therefore [FirstElement | 
>>> RestOfListWhichIsAlsoAList].
>>> The last element of a list is necessarily [], the empty list.
>>>
>>> Recursive functions have two main kinds of clauses (I'm going with an
>>> informal definition here): base cases, and the regular case. The base
>>> case is whenever recursion cannot proceed further. The base case is 
>>> when
>>> you can proceed further.
>>>
>>> To search elements in a list, your base case will therefore be '[]', 
>>> the
>>> empty list, where you can't search further.
>>> The base case will be the other [Element | Rest].
>>>
>>> So your function to search in a DB? To avoid giving you the answer, you
>>> know that if you can't look further, you haven't found the element.
>>> Therefore,
>>>
>>>      lookup(Element, []) ->
>>>          {error, not_found};
>>>      lookup(Element, [??? | RestOfList]) ->
>>>          ???.
>>>
>>> Can you fill in the blanks? If not, go back a few chapters. There's no
>>> shame in doing that and making sure you understand things right before
>>> moving on to more difficult topics.
>>>
>>> Regards,
>>> Fred.
>>>
>>> On 01/30, Roelof Wobben wrote:
>>>> Hello,
>>>>
>>>> Im still struggeling to make the database exercise working.
>>>>
>>>> I have to implement a read method which outputs as this :
>>>>
>>>> 5> db:read(francesco, Db2).
>>>> {ok,london}
>>>> 6>  Db3 = db:write(joern, stockholm, Db2).
>>>> [{joern,stockholm},{lelle,stockholm},{francesco,london}]
>>>> 7>  db:read(ola, Db3).
>>>> {error,instance}
>>>>
>>>> To achieve this do I need to use a try catch or can  I achieve this 
>>>> with
>>>> only pattern matching.
>>>>
>>>> Roelof
>>>>
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