Accessing an element in an associativity list

Accessing an element in an associativity list

[Nordlöw]

I have the following list of lists structure

(defvar c++-stl-containers
  '(
    ("string" "<string>" "String of char.")
    ("wstring" "<wstring>" "String of wide char (wchar_t).")
    ("vector" "<vector>" "Vectors contain contiguous elements stored
as an array.")
   )
 )

and I want to do something like this:
  (index-first 'c++-stl-containers "string" 2)
should evaluate to
  "<string>"

Is there any emacs lisp convenience function for accessing a row given
its, say, first (key) element? I know this is slow but speed is of no
importance here. Of course if a hash table is as easy to use then of
course I would like to use that.

Thanks in advance,
Nordlöw

Re: Accessing an element in an associativity list

[Joost Kremers]

Nordlöw wrote:
> I have the following list of lists structure
>
> (defvar c++-stl-containers
>   '(
>     ("string" "<string>" "String of char.")
>     ("wstring" "<wstring>" "String of wide char (wchar_t).")
>     ("vector" "<vector>" "Vectors contain contiguous elements stored
> as an array.")
>    )
>  )

note, lisp is not C, so don't depend on the parens to show grouping, use
indentation for visual cues. let emacs handle the parentheses. just format
the code like so:

(defvar c++-stl-containers
  '(("string" "<string>" "String of char.")
    ("wstring" "<wstring>" "String of wide char (wchar_t).")
    ("vector" "<vector>" "Vectors contain contiguous elements stored as an array.")))

> and I want to do something like this:
>   (index-first 'c++-stl-containers "string" 2)
> should evaluate to
>   "<string>"
> 
> Is there any emacs lisp convenience function for accessing a row given
> its, say, first (key) element?

you can use ASSOC:

(assoc "string" c++-stl-containers)

  ===> ("string" "<string>" "String of char.")

ASSOC uses EQUAL as comparison predicate, so you can use it with strings as
keys.

(info "(elisp) Association Lists") for more details.

> I know this is slow but speed is of no
> importance here. Of course if a hash table is as easy to use then of
> course I would like to use that.

(info "(elisp) Hash Tables") should tell you all you need to know. note
that hash tables by default uses eql for key lookup. if you want to use
strings as keys, pass a :test equal argument.

HTH


-- 
Joost Kremers                                      joostkremers@yahoo.com
Selbst in die Unterwelt dringt durch Spalten Licht
EN:SiS(9)

Re: Accessing an element in an associativity list

[Tassilo Horn]

Joost Kremers <joostkremers@yahoo.com> writes:

Hi!

> (defvar c++-stl-containers
>   '(("string" "<string>" "String of char.")
>     ("wstring" "<wstring>" "String of wide char (wchar_t).")
>     ("vector" "<vector>" "Vectors contain contiguous elements stored as an array.")))
>
>> and I want to do something like this:
>>   (index-first 'c++-stl-containers "string" 2)
>> should evaluate to
>>   "<string>"
>> 
>> Is there any emacs lisp convenience function for accessing a row
>> given its, say, first (key) element?
>
> you can use ASSOC:
>
> (assoc "string" c++-stl-containers)
>
>   ===> ("string" "<string>" "String of char.")

For the sake of completeness:  To get to "<string>" from the key
"string" you'd use

      (cadr (assoc "string" c++-stl-containers))

here, which is a shortcut for

      (car (cdr (assoc "string" c++-stl-containers))).

Or you could use one of

      (nth 1 (assoc "string" c++-stl-containers))
      (second (assoc "string" c++-stl-containers)).

Bye,
Tassilo