CODEWITHSUNDEEP
listerlang
Mathias Fernström
Mathias Fernström

Reputation: 173

How to specify return value based on if element is found in list?

I have a list of tuples looking e.g. like this:

[{apple, fruit},{pear, fruit},{tomato, vegetable}]

One of my functions is supposed to go through the list and return an element by key like this:

1> db:find(pear, Db).
{ok,fruit}

My function does this and looks like this:

find(Key, DbRef) -> [{ok, Element} || {Key1, Element} <- DbRef, Key =:= Key1].

But I also want it to return something like this:

2> db:find(cucumber, Db).
{error,unknown}

when the Key is not found in the list.

Is this possible??

Upvotes: 0

Views: 334

Answers (3)

Muzaaya Joshua
Muzaaya Joshua

Reputation: 7836

This sounds simple: 

db:find(Key,Proplist)->
    case proplists:get_value(Key,Proplist) of
        undefined -> {error,unknown};
        Value -> {ok,Value}
    end.

Upvotes: 0

Petr Kozorezov
Petr Kozorezov

Reputation: 426

Use lists:keyfind/3 or proplists:get_value/2,3.

lists:keyfind/3 is the very best option for this task. It is a BIF and, as such, is written in C. It is far and away faster than the proplists module or any other pure-erlang implementation. It's also capable of working with more complex tuples with keys in different positions (as would be the case if you had a list of records).

Examples

Using the lists module:

{pear, fruit} = lists:keyfind(pear, 1, Db),
false = lists:keyfind(cucumber, 1, Db).

Using the proplists module:

fruit = proplists:get_value(pear, Db),
undefined = proplists:get_value(cucumber, Db),
{error, instance} = proplists:get_value(cucumber, Db, {error, instance}).

Or even mix the two, with the proplists-style keyfind:

get_value(Key, List) -> get_value(Key, List, undefined).
get_value(Key, List, Default) ->
    case lists:keyfind(Key, 1, List) of
        false -> Default;
        {Key, Value} -> Value
    end.

Usage:

fruit = get_value(pear, Db),
undefined = get_value(cucumber, Db),
{error, instance} = get_value(cucumber, Db, {error, instance}).

Upvotes: 6

rvirding
rvirding

Reputation: 20916

You have to recursively step down the list looking at each element. If it has the right key then you return the value otherwise you call yourself recursively to look at the rest of the list. If you reach the end then you know that the element is not there and you return the {error,instance}.

Upvotes: 1

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