[erlang-questions] lists:mapfind?
Richard A. O'Keefe
<>
Mon Aug 21 01:22:10 CEST 2017
On 19/08/17 1:04 AM, zxq9 wrote:
> Good point.
>
> What are the cases not covered by the lists module?
> filter/2, partition/2, matching/guarded list comprehensions, etc?
We have all(Pred, List)
any(Pred, List)
but one(Pred, List) is missing.
% one(Pred, List) is true when List has exactly one element X
% such that Pred(X) is true. Yes, this is useful.
one(Pred, [X|Xs]) ->
case Pred(X)
of true -> not(any(Pred, Xs))
; false -> one(Pred, Xs)
end;
one(_, []) ->
false.
% first(Pred, List) returns {ok,Value} when Value is the
% first element of List such that Pred(Value), or error
% if there is no such value.
first(Pred, [X|Xs]) ->
case Pred(X)
of true -> {ok,X}
; false -> first(Pred, Xs)
end;
first(_, []) ->
error.
% last(Pred, List) returns {ok,Value} when Value is the
% last element of List such that Pred(Value), or error
% if there is no such value.
last(Pred, [X|Xs]) ->
case Pred(X)
of true -> last_aux(Pred, Xs, X)
; false -> last(Pred, Xs)
end;
last(_, []) ->
error.
last_aux(Pred, [X|Xs], Y) ->
case Pred(X)
of true -> last_aux(Pred, Xs, X)
; false -> last_aux(Pred, Xs, Y)
end;
last_aux(_, _, Y) ->
{ok, Y}.
% foldl1(Fun, List) is a relative of foldl/3 used when
% the function has no (representable) left identity,
% such as max and min. It is an error if List is empty.
foldl1(Fun, [X|Xs]) ->
foldl(Fun, X, Xs).
foldr1 is also missing.
The whole scan{l,r}{,1} family are missing.
As mentioned several years ago, unfold/2 is missing.
unfold(Fun, State) ->
case Fun(State)
of {Head,State1} -> [Head | unfold(Fun, State1)]
; [] -> []
end.
takewhile and dropwhile are there, and drop is there
under the name nthtail, but take is not there.
uniq(List) -> List'
uniq(Eql, List) -> List'
returns a subsequence of List with *adjacent*
duplicates dropped, Whenever there is a run
...x1,x2,...,xn such that Eql(xk_1, xk) for
2<=k<=n, x1 is included in the result and
x2...xn are not. The default equality function
is exact equality. O(length(List)).
nub(List) -> List'
nub(Eql, List) -> List'
returns a subsequence of List with duplicates
dropped. Whenever there is a pair ...xi...xj...
such that Eql(xi, xj), xj is excluded from the
result. The default equality function is
exact equality. O(length(List)**2).
group(List) -> List_Of_Lists
group(Eql, List) -> List_Of_Lists
returns a list of lists whose concatenation
is the original List. If [...xi...xj...] is
an element of the result, Eql(xi, xj). The
Eql function should be transitive but need not
be an equivalence; the default is exact equality.
O(length(List)).
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