Erlang ets remove / filter element

Question

I use elang ets table as a simple cache. I want to use a process to scan the table and remove expired elements (multiple).

with ets:foldl

expire_table_example() ->
Tab = ets:new(ets_tab, [named_table, set]),
ets:insert(Tab, [{a, 1}, {b, 2}, {c, 3}, {d, 4}, {e, 5},{f,7}]),
Tab1 = ets:foldl(fun({Key, Val}, Acc) ->
if
   (Val > 3) -> [{Key, Val} | Acc];
   true -> Acc
 end
end, Tab, Tab),
io:format("end ~p ~n", [Tab1]).

I got

[{f,7},{e,5},{d,4}|ets_tab] %% the ets_tab is NOT expected.

How Can I fix This?

Any other API's would do this better?

The reason you're getting the atom ets_tab in your list is that you're putting it there by using Tab as the second argument to ets:foldl/3, which is the initial value of the fold accumulator. You should be using [] for the initial value instead. — Steve Vinoski
Actually, I want to return an empty ets table there. if I use [], it could be an empty list. right? — user3644708
If you want to empty the whole table, just use ets:delete_all_objects/1. — Steve Vinoski

P_A P_A · Accepted Answer · 2015-12-22T14:57:50

You can't use ets table as accumulator.

For your purpose you can use ets:select_delete/2:

1> Tab = ets:new(ets_tab, [named_table, set]).
ets_tab
2> ets:insert(Tab, [{a, 1}, {b, 2}, {c, 3}, {d, 4}, {e, 5},{f,7}]).
true
3> ets:select_delete(Tab, [{{'$1','$2'},[{'=<','$2',3}],[true]}]).
3
4> ets:tab2list(Tab).
[{f,7},{e,5},{d,4}]

Or you can use ets:tab2list/1 to get list of all values, filter them and after that re-insert to table:

1> Tab = ets:new(ets_tab, [named_table, set]).
ets_tab
2> ets:insert(Tab, [{a, 1}, {b, 2}, {c, 3}, {d, 4}, {e, 5},{f,7}]).
true
3> L = ets:tab2list(Tab).
[{f,7},{e,5},{d,4},{c,3},{b,2},{a,1}]
4> L2 = lists:filter(fun({Key,Val}) -> Val > 3 end, L).
[{f,7},{e,5},{d,4}]
5> ets:delete_all_objects(Tab).
true
6> ets:insert(Tab, L2).
true
7> ets:tab2list(Tab).
[{f,7},{e,5},{d,4}]

Erlang ets remove / filter element

3 Answers