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gar1t
Posted: Wed Dec 02, 2009 5:11 pm Reply with quote
User Joined: 11 Aug 2009 Posts: 55
I'm missing something basic here :\

The lists module has keyfind/3 but no find/2, where you want to return
the position of a particular list item. This is surprising.

Is there a commonly used pattern for this?

Garrett

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Guest
Posted: Wed Dec 02, 2009 5:36 pm Reply with quote
Guest
On Wed, Dec 2, 2009 at 5:10 PM, Garrett Smith <g@rre.tt> wrote:

> I'm missing something basic here :\
>
> The lists module has keyfind/3 but no find/2, where you want to return
> the position of a particular list item. This is surprising.
>
> Is there a commonly used pattern for this?
>
>
>
Hmm, I fail to come up with a good usage scenario for wanting this. What are
you needing the position of an item in a list for?

Robby


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Guest
Posted: Wed Dec 02, 2009 5:52 pm Reply with quote
Guest
On Wed, Dec 02, 2009 at 11:10:25AM -0600, Garrett Smith wrote:
> I'm missing something basic here :\
>
> The lists module has keyfind/3 but no find/2, where you want to return
> the position of a particular list item. This is surprising.
>
> Is there a commonly used pattern for this?
>

gen_leader has a compact little function I've 'borrowed' a couple times:

pos(_, []) ->
100000;
pos(N1,[N1|_]) ->
1;
pos(N1,[_|Ns]) ->
1+pos(N1,Ns).

I agree that something like this would be handy to have in the stdlib
(I've also written a version that finds the index of the first element
that a predicate fun returns true for).

Andrew

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Guest
Posted: Wed Dec 02, 2009 5:54 pm Reply with quote
Guest
I happened to need that once and ended up implementing my own, so I
think you won't find it in the standard library.

Later, I realized there was a more elegant way of doing what I wanted,
so I should agree with Robert in that you probably don't need that
functionality (but we could be wrong, of course...).

find_first(Element, List) when is_list(List) ->
find_first(Element, List, 0).

find_first(_Element, [], Inc) when is_integer(Inc) ->
Inc + 1;
find_first(Element, [Element | _Tail], Inc) when is_integer(Inc) ->
Inc + 1;
find_first(Element, [_ | Tail], Inc) when is_integer(Inc) ->
find_first(Element, Tail, Inc + 1).


Igor.

On Wed, Dec 2, 2009 at 3:10 PM, Garrett Smith <g@rre.tt> wrote:
> I'm missing something basic here :\
>
> The lists module has keyfind/3 but no find/2, where you want to return
> the position of a particular list item. This is surprising.
>
> Is there a commonly used pattern for this?
>
> Garrett

--
"The secret of joy in work is contained in one word - excellence. To
know how to do something well is to enjoy it." - Pearl S. Buck.

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gar1t
Posted: Wed Dec 02, 2009 5:57 pm Reply with quote
User Joined: 11 Aug 2009 Posts: 55
On Wed, Dec 2, 2009 at 11:34 AM, Robert Raschke <rtrlists@googlemail.com> wrote:
> On Wed, Dec 2, 2009 at 5:10 PM, Garrett Smith <g@rre.tt> wrote:
>
>> I'm missing something basic here :\
>>
>> The lists module has keyfind/3 but no find/2, where you want to return
>> the position of a particular list item. This is surprising.
>>
>> Is there a commonly used pattern for this?
>>
> Hmm, I fail to come up with a good usage scenario for wanting this. What are
> you needing the position of an item in a list for?

I'm stitching together record field values and want to get an
insertion position using a field token.

So, this goes in hard with an "insert_at(Value, N, List_Or_Tuple)"
type of function.

In general though, there are several functions in the lists module
that accept a positional argument (N) -- having some easy ways of
getting N seems reasonable. Functions like l/find and rfind are pretty
common in the list APIs I'm familiar with.

Garrett

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gar1t
Posted: Wed Dec 02, 2009 6:02 pm Reply with quote
User Joined: 11 Aug 2009 Posts: 55
On Wed, Dec 2, 2009 at 11:53 AM, Igor Ribeiro Sucupira <igorrs@gmail.com> wrote:
> I happened to need that once and ended up implementing my own, so I
> think you won't find it in the standard library.
>
> Later, I realized there was a more elegant way of doing what I wanted,
> so I should agree with Robert in that you probably don't need that
> functionality (but we could be wrong, of course...).
>
> find_first(Element, List) when is_list(List) ->
>
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Guest
Posted: Wed Dec 02, 2009 6:05 pm Reply with quote
Guest
On Wed, Dec 2, 2009 at 5:53 PM, Garrett Smith <g@rre.tt> wrote:

> On Wed, Dec 2, 2009 at 11:34 AM, Robert Raschke <rtrlists@googlemail.com>
> wrote:
> > On Wed, Dec 2, 2009 at 5:10 PM, Garrett Smith <g@rre.tt> wrote:
> >
> >> I'm missing something basic here :\
> >>
> >> The lists module has keyfind/3 but no find/2, where you want to return
> >> the position of a particular list item. This is surprising.
> >>
> >> Is there a commonly used pattern for this?
> >>
> > Hmm, I fail to come up with a good usage scenario for wanting this. What
> are
> > you needing the position of an item in a list for?
>
> I'm stitching together record field values and want to get an
> insertion position using a field token.
>
> So, this goes in hard with an "insert_at(Value, N, List_Or_Tuple)"
> type of function.
>
> In general though, there are several functions in the lists module
> that accept a positional argument (N) -- having some easy ways of
> getting N seems reasonable. Functions like l/find and rfind are pretty
> common in the list APIs I'm familiar with.
>
> Garrett
>

Not 100% sure what it is exactly you're doing, but it sounds like it could
be a bit easier using tuple_to_list/1 and list comprehensions?

Robby


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Guest
Posted: Wed Dec 02, 2009 6:12 pm Reply with quote
Guest
Robert Raschke wrote:
> On Wed, Dec 2, 2009 at 5:10 PM, Garrett Smith <g@rre.tt> wrote:
>
>
>> I'm missing something basic here :\
>>
>> The lists module has keyfind/3 but no find/2, where you want to return
>> the position of a particular list item. This is surprising.
>>
>> Is there a commonly used pattern for this?
>>
>>
>>
>>
> Hmm, I fail to come up with a good usage scenario for wanting this. What are
> you needing the position of an item in a list for?
>
> Robby
>
>
Find a good usage for lists:nth/2 and lists:nthtail/2 functions, and you
will find a good one for this Smile.


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Guest
Posted: Wed Dec 02, 2009 6:31 pm Reply with quote
Guest
2009/12/2, Garrett Smith <g@rre.tt>:
> I'm missing something basic here :\
>
> The lists module has keyfind/3 but no find/2, where you want to return
> the position of a particular list item. This is surprising.
>
> Is there a commonly used pattern for this?

string:str(List, [Element]).

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Guest
Posted: Wed Dec 02, 2009 6:46 pm Reply with quote
Guest
2009/12/2, Garrett Smith <g@rre.tt>:
[...]
> But if a 20 year old language can get by without these, then by
> definition *I'm* wrong Smile

Well, actually I think in this case Erlang is wrong. There was a
thread here a couple of months ago that some (most?) libraries on OTP
were developed more in an "evolutionary" way, not "intelligently
designed". For example the string module has substr and sub_string,
because the module was merged from two previous string modules. Lookup
functions have different interfaces:

ets:lookup(Table, Key) -> [Objects]
dict:find(Key, Dict) -> {ok, Value} | error
gb_tree:lookup(Key, Tree) -> {value, Val} | none
lists:keysearch(Key, N, TupleList) -> {value, Tuple} | false
proplists:lookup(Key, List) -> none | tuple()

Our local proplist:lookup-clone function returns no_value where
proplists would return none. Expect the unexpected, when you work with
Erlang libraries.

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Guest
Posted: Wed Dec 02, 2009 7:45 pm Reply with quote
Guest
> Hmm, I fail to come up with a good usage scenario for wanting this. What are
> you needing the position of an item in a list for?

FWIW, here's something i wrote recently:

base32_decode(Char) ->
list_index(Char, base32_alphabet()).

base32_alphabet() ->
"0123456789bcdefghjkmnpqrstuvwxyz".


The corresponding encode function uses lists:nth/2.

Can't remember ever needing it before though (in erlang).

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Guest
Posted: Thu Dec 03, 2009 1:00 am Reply with quote
Guest
Hum... when you talked about records, I realized I still have one
function that needs to find the position of an item on a list. The
function is used to get the value of a field F on a record, when F is
defined at runtime.

I don't understand exactly what you need, but I guess it's basically
the same thing, isn't it?

On Wed, Dec 2, 2009 at 3:53 PM, Garrett Smith <g@rre.tt> wrote:
> On Wed, Dec 2, 2009 at 11:34 AM, Robert Raschke <rtrlists@googlemail.com> wrote:
>> On Wed, Dec 2, 2009 at 5:10 PM, Garrett Smith <g@rre.tt> wrote:
>>
>>> I'm missing something basic here :\
>>>
>>> The lists module has keyfind/3 but no find/2, where you want to return
>>> the position of a particular list item. This is surprising.
>>>
>>> Is there a commonly used pattern for this?
>>>
>> Hmm, I fail to come up with a good usage scenario for wanting this. What are
>> you needing the position of an item in a list for?
>
> I'm stitching together record field values and want to get an
> insertion position using a field token.
>
> So, this goes in hard with an "insert_at(Value, N, List_Or_Tuple)"
> type of function.
>
> In general though, there are several functions in the lists module
> that accept a positional argument (N) -- having some easy ways of
> getting N seems reasonable. Functions like l/find and rfind are pretty
> common in the list APIs I'm familiar with.
>
> Garrett
>
> ________________________________________________________________
> erlang-questions mailing list. See http://www.erlang.org/faq.html
> erlang-questions (at) erlang.org
>
>



--
"The secret of joy in work is contained in one word - excellence. To
know how to do something well is to enjoy it." - Pearl S. Buck.

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Guest
Posted: Thu Dec 03, 2009 1:18 am Reply with quote
Guest
By the way, this is the function (with some details like module names
removed/changed):

field_value(R, F) when is_record(R, test), is_atom(F) ->
P = find_first(F, record_info(fields, test)),
lists:nth(P + 1, tuple_to_list(R)).

I don't think there would be something easier without the function to
get the position of the list item (find_first, in my example).

Igor.

On Wed, Dec 2, 2009 at 10:58 PM, Igor Ribeiro Sucupira <igorrs@gmail.com> wrote:
> Hum... when you talked about records, I realized I still have one
> function that needs to find the position of an item on a list. The
> function is used to get the value of a field F on a record, when F is
> defined at runtime.
>
> I don't understand exactly what you need, but I guess it's basically
> the same thing, isn't it?
>
> On Wed, Dec 2, 2009 at 3:53 PM, Garrett Smith <g@rre.tt> wrote:
>> On Wed, Dec 2, 2009 at 11:34 AM, Robert Raschke <rtrlists@googlemail.com> wrote:
>>> On Wed, Dec 2, 2009 at 5:10 PM, Garrett Smith <g@rre.tt> wrote:
>>>
>>>> I'm missing something basic here :\
>>>>
>>>> The lists module has keyfind/3 but no find/2, where you want to return
>>>> the position of a particular list item. This is surprising.
>>>>
>>>> Is there a commonly used pattern for this?
>>>>
>>> Hmm, I fail to come up with a good usage scenario for wanting this. What are
>>> you needing the position of an item in a list for?
>>
>> I'm stitching together record field values and want to get an
>> insertion position using a field token.
>>
>> So, this goes in hard with an "insert_at(Value, N, List_Or_Tuple)"
>> type of function.
>>
>> In general though, there are several functions in the lists module
>> that accept a positional argument (N) -- having some easy ways of
>> getting N seems reasonable. Functions like l/find and rfind are pretty
>> common in the list APIs I'm familiar with.
>>
>> Garrett
>>
>> ________________________________________________________________
>> erlang-questions mailing list. See http://www.erlang.org/faq.html
>> erlang-questions (at) erlang.org
>>
>>
>
>
>
> --
> "The secret of joy in work is contained in one word - excellence. To
> know how to do something well is to enjoy it." - Pearl S. Buck.
>



--
"The secret of joy in work is contained in one word - excellence. To
know how to do something well is to enjoy it." - Pearl S. Buck.

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ngocdaothanh
Posted: Thu Dec 03, 2009 1:24 am Reply with quote
User Joined: 06 Dec 2008 Posts: 43
> string:str(List, [Element]).

What a surprise! Thank you.

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gar1t
Posted: Thu Dec 03, 2009 1:52 am Reply with quote
User Joined: 11 Aug 2009 Posts: 55
On Wed, Dec 2, 2009 at 7:23 PM, Ngoc Dao <ngocdaothanh@gmail.com> wrote:
>> string:str(List, [Element]).
>
> What a surprise! Thank you.

A good example of "most astonishment" Smile

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