[erlang-questions] (Newbie) extracting value for one key

Gleb Peregud <>
Tue Aug 19 14:38:10 CEST 2008


On Tue, Aug 19, 2008 at 2:18 PM, Brian Candler <> wrote:
> In Erlang, it seems a list of {key,value} tuples is often returned, e.g.
>
> 17> P = process_info(self()).
> [{current_function,{erl_eval,do_apply,5}},
>  {initial_call,{erlang,apply,2}},
>  {status,running},
>  {message_queue_len,0},
>  {messages,[]},
>  {links,[<0.25.0>]},
>  {dictionary,[]},
>  {trap_exit,false},
>  {error_handler,error_handler},
>  {priority,normal},
>  {group_leader,<0.24.0>},
>  {heap_size,610},
>  {stack_size,28},
>  {reductions,526},
>  {garbage_collection,[{fullsweep_after,65535}]}]
>
> Question: What's the idiomatic way to extract the value for a particular
> key? For example, to extract "stack_size" from the above I could write
>
> 18> hd([ V || {stack_size,V} <- P ]).
> 28
>
> but is there a better way to do this?
>
> Thanks,
>
> Brian.
>
> P.S. I found the example at
> http://www.erlang.org/doc/programming_examples/list_comprehensions.html#3.6
> which shows how to factor the comprehension into a function.
>
> I also found "element(2, element(2, lists:keysearch(stack_size, 1, P)))."
> but that's rather verbose.
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> 
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>


Hello,

Take a look at module proplists [1], specifically at function
get_value/2,3 or lookup/2 (or other in there)

1: man proplists or http://www.erlang.org/doc/man/proplists.html

BR
-- 
Gleb Peregud
http://gleber.pl/

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