diff options
| -rw-r--r-- | lib/stdlib/doc/src/lists.xml | 8 | ||||
| -rw-r--r-- | lib/stdlib/src/lists.erl | 15 | ||||
| -rw-r--r-- | lib/stdlib/test/lists_SUITE.erl | 12 |
3 files changed, 32 insertions, 3 deletions
diff --git a/lib/stdlib/doc/src/lists.xml b/lib/stdlib/doc/src/lists.xml index 251a383cf8..ee3c51c62c 100644 --- a/lib/stdlib/doc/src/lists.xml +++ b/lib/stdlib/doc/src/lists.xml @@ -123,6 +123,14 @@ </desc> </func> <func> + <name name="droplast" arity="1"/> + <fsummary>Drop the last element of a list</fsummary> + <desc> + <p>Drops the last element of a <c><anno>List</anno></c>. The list should + be non-empty, otherwise the function will crash with a <c>function_clause</c></p> + </desc> + </func> + <func> <name name="dropwhile" arity="2"/> <fsummary>Drop elements from a list while a predicate is true</fsummary> <desc> diff --git a/lib/stdlib/src/lists.erl b/lib/stdlib/src/lists.erl index d6a9f4645d..6303465d3d 100644 --- a/lib/stdlib/src/lists.erl +++ b/lib/stdlib/src/lists.erl @@ -22,7 +22,7 @@ -compile({no_auto_import,[min/2]}). -export([append/2, append/1, subtract/2, reverse/1, - nth/2, nthtail/2, prefix/2, suffix/2, last/1, + nth/2, nthtail/2, prefix/2, suffix/2, droplast/1, last/1, seq/2, seq/3, sum/1, duplicate/2, min/1, max/1, sublist/2, sublist/3, delete/2, unzip/1, unzip3/1, zip/2, zip3/3, zipwith/3, zipwith3/4, @@ -203,6 +203,19 @@ suffix(Suffix, List) -> Delta = length(List) - length(Suffix), Delta >= 0 andalso nthtail(Delta, List) =:= Suffix. +%% droplast(List) returns the list dropping its last element + +-spec droplast(List) -> InitList when + List :: [T, ...], + InitList :: [T], + T :: term(). + +%% This is the simple recursive implementation +%% reverse(tl(reverse(L))) is faster on average, +%% but creates more garbage. +droplast([_T]) -> []; +droplast([H|T]) -> [H|droplast(T)]. + %% last(List) returns the last element in a list. -spec last(List) -> Last when diff --git a/lib/stdlib/test/lists_SUITE.erl b/lib/stdlib/test/lists_SUITE.erl index 92253ef5b9..f4589a8e24 100644 --- a/lib/stdlib/test/lists_SUITE.erl +++ b/lib/stdlib/test/lists_SUITE.erl @@ -61,7 +61,7 @@ zip_unzip/1, zip_unzip3/1, zipwith/1, zipwith3/1, filter_partition/1, otp_5939/1, otp_6023/1, otp_6606/1, otp_7230/1, - suffix/1, subtract/1]). + suffix/1, subtract/1, droplast/1]). %% Sort randomized lists until stopped. %% @@ -2641,4 +2641,12 @@ sub_non_matching(A, B) -> sub(A, B) -> Res = A -- B, Res = lists:subtract(A, B). - + +%% Test lists:droplast/1 +droplast(Config) when is_list(Config) -> + ?line [] = lists:droplast([x]), + ?line [x] = lists:droplast([x, y]), + ?line {'EXIT', {function_clause, _}} = (catch lists:droplast([])), + ?line {'EXIT', {function_clause, _}} = (catch lists:droplast(x)), + + ok. |