lists
List Processing Functions
This module contains functions for list processing.
Unless otherwise stated, all functions assume that position numbering starts at 1. That is, the first element of a list is at position 1.
Two terms T1
and T2
compare equal if
T1 == T2
evaluates to true
. They match
if T1 =:= T2
evaluates to true
.
Whenever an ordering function
F
is expected as argument, it is assumed that the
following properties hold of F
for all x, y and z:
if x
F
y and yF
x then x = y (F
is antisymmetric);if x
F
y and yF
z then xF
z (F
is transitive);x
F
y or yF
x (F
is total).
An example of a typical ordering function is less than or equal
to, =</2
.
Functions
all(Pred, List) -> boolean()
Pred = fun((Elem :: T) -> boolean())
List = [T]
T = term()
Returns true
if
returns
true
for all elements
in
,
otherwise false
.
any(Pred, List) -> boolean()
Pred = fun((Elem :: T) -> boolean())
List = [T]
T = term()
Returns true
if
returns
true
for at least one element
in
.
append(ListOfLists) -> List1
ListOfLists = [List]
List = List1 = [T]
T = term()
Returns a list in which all the sub-lists of
have been appended. For example:
> lists:append([[1, 2, 3], [a, b], [4, 5, 6]]).
[1,2,3,a,b,4,5,6]
append(List1, List2) -> List3
List1 = List2 = List3 = [T]
T = term()
Returns a new list
which is made from
the elements of
followed by the elements of
. For example:
> lists:append("abc", "def").
"abcdef"
lists:append(A, B)
is equivalent to A ++ B
.
concat(Things) -> string()
Things = [Thing]
Thing = atom() | integer() | float() | string()
Concatenates the text representation of the elements
of
. The elements of
can be atoms,
integers, floats or strings.
> lists:concat([doc, '/', file, '.', 3]).
"doc/file.3"
delete(Elem, List1) -> List2
Elem = T
List1 = List2 = [T]
T = term()
Returns a copy of
where the first element
matching
is deleted, if there is such an
element.
dropwhile(Pred, List1) -> List2
Pred = fun((Elem :: T) -> boolean())
List1 = List2 = [T]
T = term()
Drops elements
from
while
returns true
and returns
the remaining list.
duplicate(N, Elem) -> List
N = integer() >= 0
Elem = T
List = [T]
T = term()
Returns a list which contains
copies of the term
. For example:
> lists:duplicate(5, xx).
[xx,xx,xx,xx,xx]
filter(Pred, List1) -> List2
Pred = fun((Elem :: T) -> boolean())
List1 = List2 = [T]
T = term()
is a list of all elements
in
for which
returns
true
.
filtermap(Fun, List1) -> List2
Fun = fun((Elem) -> boolean() | {true, Value})
List1 = [Elem]
List2 = [Elem | Value]
Elem = Value = term()
Calls
on successive elements Elem
of
.
must return either a boolean
or a tuple {true,
. The function returns the list of elements
for which
returns a new value, where a value of true
is synonymous with {true,
.
That is, filtermap
behaves as if it had been defined as follows:
filtermap(Fun, List1) -> lists:foldr(fun(Elem, Acc) -> case Fun(Elem) of false -> Acc; true -> [Elem|Acc]; {true,Value} -> [Value|Acc] end, end, [], List1).
Example:
> lists:filtermap(fun(X) -> case X rem 2 of 0 -> {true, X div 2}; _ -> false end end, [1,2,3,4,5]).
[1,2]
flatlength(DeepList) -> integer() >= 0
DeepList = [term() | DeepList]
Equivalent to length(flatten(
, but more
efficient.
flatmap(Fun, List1) -> List2
Fun = fun((A) -> [B])
List1 = [A]
List2 = [B]
A = B = term()
Takes a function from
s to lists of
s, and a
list of
s (
) and produces a list of
s by applying the function to every element in
and appending the resulting lists.
That is, flatmap
behaves as if it had been defined as
follows:
flatmap(Fun, List1) -> append(map(Fun, List1)).
Example:
> lists:flatmap(fun(X)->[X,X] end, [a,b,c]).
[a,a,b,b,c,c]
flatten(DeepList) -> List
DeepList = [term() | DeepList]
List = [term()]
Returns a flattened version of
.
flatten(DeepList, Tail) -> List
DeepList = [term() | DeepList]
Tail = List = [term()]
Returns a flattened version of
with the tail
appended.
foldl(Fun, Acc0, List) -> Acc1
Fun = fun((Elem :: T, AccIn) -> AccOut)
Acc0 = Acc1 = AccIn = AccOut = term()
List = [T]
T = term()
Calls
on successive elements A
of
, starting with
.
must return a new accumulator which is passed to
the next call. The function returns the final value of
the accumulator.
is returned if the list is empty.
For example:
>lists:foldl(fun(X, Sum) -> X + Sum end, 0, [1,2,3,4,5]).
15 >lists:foldl(fun(X, Prod) -> X * Prod end, 1, [1,2,3,4,5]).
120
foldr(Fun, Acc0, List) -> Acc1
Fun = fun((Elem :: T, AccIn) -> AccOut)
Acc0 = Acc1 = AccIn = AccOut = term()
List = [T]
T = term()
Like foldl/3
, but the list is traversed from right to
left. For example:
>P = fun(A, AccIn) -> io:format("~p ", [A]), AccIn end.
#Fun<erl_eval.12.2225172> >lists:foldl(P, void, [1,2,3]).
1 2 3 void >lists:foldr(P, void, [1,2,3]).
3 2 1 void
foldl/3
is tail recursive and would usually be
preferred to foldr/3
.
foreach(Fun, List) -> ok
Fun = fun((Elem :: T) -> term())
List = [T]
T = term()
Calls
for each element
in
. This function is used for its side effects and
the evaluation order is defined to be the same as the order
of the elements in the list.
keydelete(Key, N, TupleList1) -> TupleList2
Key = term()
N = integer() >= 1
TupleList1 = TupleList2 = [Tuple]
Tuple = tuple()
Returns a copy of
where the first
occurrence of a tuple whose
th element compares equal to
is deleted, if there is such a tuple.
keyfind(Key, N, TupleList) -> Tuple | false
Key = term()
N = integer() >= 1
TupleList = [Tuple]
Tuple = tuple()
Searches the list of tuples
for a
tuple whose
th element compares equal to
.
Returns
if such a tuple is found,
otherwise false
.
keymap(Fun, N, TupleList1) -> TupleList2
Fun = fun((Term1 :: term()) -> Term2 :: term())
N = integer() >= 1
TupleList1 = TupleList2 = [Tuple]
Tuple = tuple()
Returns a list of tuples where, for each tuple in
, the
th element
of the tuple
has been replaced with the result of calling
.
Examples:
>Fun = fun(Atom) -> atom_to_list(Atom) end.
#Fun<erl_eval.6.10732646> 2>lists:keymap(Fun, 2, [{name,jane,22},{name,lizzie,20},{name,lydia,15}]).
[{name,"jane",22},{name,"lizzie",20},{name,"lydia",15}]
keymember(Key, N, TupleList) -> boolean()
Key = term()
N = integer() >= 1
TupleList = [Tuple]
Tuple = tuple()
Returns true
if there is a tuple in
whose
th element compares equal to
, otherwise
false
.
keymerge(N, TupleList1, TupleList2) -> TupleList3
N = integer() >= 1
TupleList1 = [T1]
TupleList2 = [T2]
TupleList3 = [T1 | T2]
T1 = T2 = Tuple
Tuple = tuple()
Returns the sorted list formed by merging
and
. The merge is performed on
the
th element of each tuple. Both
and
must be key-sorted prior to evaluating this
function. When two tuples compare equal, the tuple from
is picked before the tuple from
.
keyreplace(Key, N, TupleList1, NewTuple) -> TupleList2
Key = term()
N = integer() >= 1
TupleList1 = TupleList2 = [Tuple]
NewTuple = Tuple
Tuple = tuple()
Returns a copy of
where the first
occurrence of a T
tuple whose
th element
compares equal to
is replaced with
, if there is such a tuple T
.
keysearch(Key, N, TupleList) -> {value, Tuple} | false
Key = term()
N = integer() >= 1
TupleList = [Tuple]
Tuple = tuple()
Searches the list of tuples
for a
tuple whose
th element compares equal to
.
Returns {value,
if such a tuple is found,
otherwise false
.
Note!
This function is retained for backward compatibility.
The function lists:keyfind/3
(introduced in R13A)
is in most cases more convenient.
keysort(N, TupleList1) -> TupleList2
N = integer() >= 1
TupleList1 = TupleList2 = [Tuple]
Tuple = tuple()
Returns a list containing the sorted elements of the list
. Sorting is performed on the
th
element of the tuples. The sort is stable.
keystore(Key, N, TupleList1, NewTuple) -> TupleList2
Key = term()
N = integer() >= 1
TupleList1 = [Tuple]
TupleList2 = [Tuple, ...]
NewTuple = Tuple
Tuple = tuple()
Returns a copy of
where the first
occurrence of a tuple T
whose
th element
compares equal to
is replaced with
, if there is such a tuple T
. If there
is no such tuple T
a copy of
where
[
] has been appended to the end is
returned.
keytake(Key, N, TupleList1) -> {value, Tuple, TupleList2} | false
Key = term()
N = integer() >= 1
TupleList1 = TupleList2 = [tuple()]
Tuple = tuple()
Searches the list of tuples
for a tuple
whose
th element compares equal to
.
Returns {value,
if such a tuple is
found, otherwise false
.
is a copy
of
where the first occurrence of
has been removed.
last(List) -> Last
List = [T, ...]
Last = T
T = term()
Returns the last element in
.
map(Fun, List1) -> List2
Fun = fun((A) -> B)
List1 = [A]
List2 = [B]
A = B = term()
Takes a function from
s to
s, and a list of
s and produces a list of
s by applying
the function to every element in the list. This function is
used to obtain the return values. The evaluation order is
implementation dependent.
mapfoldl(Fun, Acc0, List1) -> {List2, Acc1}
Fun = fun((A, AccIn) -> {B, AccOut})
Acc0 = Acc1 = AccIn = AccOut = term()
List1 = [A]
List2 = [B]
A = B = term()
mapfoldl
combines the operations of map/2
and
foldl/3
into one pass. An example, summing
the elements in a list and double them at the same time:
>lists:mapfoldl(fun(X, Sum) -> {2*X, X+Sum} end,
0, [1,2,3,4,5]).
{[2,4,6,8,10],15}
mapfoldr(Fun, Acc0, List1) -> {List2, Acc1}
Fun = fun((A, AccIn) -> {B, AccOut})
Acc0 = Acc1 = AccIn = AccOut = term()
List1 = [A]
List2 = [B]
A = B = term()
mapfoldr
combines the operations of map/2
and
foldr/3
into one pass.
max(List) -> Max
List = [T, ...]
Max = T
T = term()
Returns the first element of
that compares
greater than or equal to all other elements of
.
member(Elem, List) -> boolean()
Elem = T
List = [T]
T = term()
Returns true
if
matches some element of
, otherwise false
.
merge(ListOfLists) -> List1
ListOfLists = [List]
List = List1 = [T]
T = term()
Returns the sorted list formed by merging all the sub-lists
of
. All sub-lists must be sorted prior to
evaluating this function. When two elements compare equal,
the element from the sub-list with the lowest position in
is picked before the other element.
merge(List1, List2) -> List3
List1 = [X]
List2 = [Y]
List3 = [X | Y]
X = Y = term()
Returns the sorted list formed by merging
and
. Both
and
must be
sorted prior to evaluating this function. When two elements
compare equal, the element from
is picked
before the element from
.
merge(Fun, List1, List2) -> List3
Fun = fun((A, B) -> boolean())
List1 = [A]
List2 = [B]
List3 = [A | B]
A = B = term()
Returns the sorted list formed by merging
and
. Both
and
must be
sorted according to the ordering function
prior to evaluating this function.
should return true
if
compares less
than or equal to
in the ordering, false
otherwise. When two elements compare equal, the element from
is picked before the element from
.
merge3(List1, List2, List3) -> List4
List1 = [X]
List2 = [Y]
List3 = [Z]
List4 = [X | Y | Z]
X = Y = Z = term()
Returns the sorted list formed by merging
,
and
. All of
,
and
must be sorted prior to
evaluating this function. When two elements compare equal,
the element from
, if there is such an element,
is picked before the other element, otherwise the element
from
is picked before the element from
.
min(List) -> Min
List = [T, ...]
Min = T
T = term()
Returns the first element of
that compares
less than or equal to all other elements of
.
nth(N, List) -> Elem
N = integer() >= 1
List = [T, ...]
Elem = T
T = term()
Returns the
th element of
. For example:
> lists:nth(3, [a, b, c, d, e]).
c
nthtail(N, List) -> Tail
N = integer() >= 0
List = [T, ...]
Tail = [T]
T = term()
Returns the
th tail of
, that is, the sublist of
starting at
and continuing up to
the end of the list. For example:
>lists:nthtail(3, [a, b, c, d, e]).
[d,e] >tl(tl(tl([a, b, c, d, e]))).
[d,e] >lists:nthtail(0, [a, b, c, d, e]).
[a,b,c,d,e] >lists:nthtail(5, [a, b, c, d, e]).
[]
partition(Pred, List) -> {Satisfying, NotSatisfying}
Pred = fun((Elem :: T) -> boolean())
List = Satisfying = NotSatisfying = [T]
T = term()
Partitions
into two lists, where the first list
contains all elements for which
returns
true
, and the second list contains all elements for
which
returns false
.
Examples:
>lists:partition(fun(A) -> A rem 2 == 1 end, [1,2,3,4,5,6,7]).
{[1,3,5,7],[2,4,6]} >lists:partition(fun(A) -> is_atom(A) end, [a,b,1,c,d,2,3,4,e]).
{[a,b,c,d,e],[1,2,3,4]}
See also splitwith/2
for a different way to partition
a list.
prefix(List1, List2) -> boolean()
List1 = List2 = [T]
T = term()
Returns true
if
is a prefix of
, otherwise false
.
reverse(List1) -> List2
List1 = List2 = [T]
T = term()
Returns a list with the elements in
in reverse order.
reverse(List1, Tail) -> List2
List1 = [T]
Tail = term()
List2 = [T]
T = term()
Returns a list with the elements in
in reverse order, with the tail
appended. For
example:
> lists:reverse([1, 2, 3, 4], [a, b, c]).
[4,3,2,1,a,b,c]
seq(From, To) -> Seq
From = To = integer()
Seq = [integer()]
seq(From, To, Incr) -> Seq
From = To = Incr = integer()
Seq = [integer()]
Returns a sequence of integers which starts with
and contains the successive results of adding
to
the previous element, until
has been reached or
passed (in the latter case,
is not an element of
the sequence).
defaults to 1.
Failure: If
and
is positive, or if
and
is
negative, or if
and
.
The following equalities hold for all sequences:
length(lists:seq(From, To)) == To-From+1 length(lists:seq(From, To, Incr)) == (To-From+Incr) div Incr
Examples:
>lists:seq(1, 10).
[1,2,3,4,5,6,7,8,9,10] >lists:seq(1, 20, 3).
[1,4,7,10,13,16,19] >lists:seq(1, 0, 1).
[] >lists:seq(10, 6, 4).
[] >lists:seq(1, 1, 0).
[1]
sort(List1) -> List2
List1 = List2 = [T]
T = term()
Returns a list containing the sorted elements of
.
sort(Fun, List1) -> List2
Fun = fun((A :: T, B :: T) -> boolean())
List1 = List2 = [T]
T = term()
Returns a list containing the sorted elements of
, according to the ordering function
.
should return true
if
compares less than or equal to
in the
ordering, false
otherwise.
split(N, List1) -> {List2, List3}
N = integer() >= 0
List1 = List2 = List3 = [T]
T = term()
Splits
into
and
.
contains the first
elements and
the rest of the elements (the
th tail).
splitwith(Pred, List) -> {List1, List2}
Pred = fun((T) -> boolean())
List = List1 = List2 = [T]
T = term()
Partitions
into two lists according to
. splitwith/2
behaves as if it is defined
as follows:
splitwith(Pred, List) -> {takewhile(Pred, List), dropwhile(Pred, List)}.
Examples:
>lists:splitwith(fun(A) -> A rem 2 == 1 end, [1,2,3,4,5,6,7]).
{[1],[2,3,4,5,6,7]} >lists:splitwith(fun(A) -> is_atom(A) end, [a,b,1,c,d,2,3,4,e]).
{[a,b],[1,c,d,2,3,4,e]}
See also partition/2
for a different way to partition
a list.
sublist(List1, Len) -> List2
List1 = List2 = [T]
Len = integer() >= 0
T = term()
Returns the sub-list of
starting at position 1
and with (max)
elements. It is not an error for
to exceed the length of the list, in that case
the whole list is returned.
sublist(List1, Start, Len) -> List2
List1 = List2 = [T]
Start = integer() >= 1
Len = integer() >= 0
T = term()
Returns the sub-list of
starting at
and with (max)
elements. It is not an error for
to exceed the length of the list.
>lists:sublist([1,2,3,4], 2, 2).
[2,3] >lists:sublist([1,2,3,4], 2, 5).
[2,3,4] >lists:sublist([1,2,3,4], 5, 2).
[]
subtract(List1, List2) -> List3
List1 = List2 = List3 = [T]
T = term()
Returns a new list
which is a copy of
, subjected to the following procedure: for each
element in
, its first occurrence in
is deleted. For example:
> lists:subtract("123212", "212").
"312".
lists:subtract(A, B)
is equivalent to A -- B
.
Warning!
The complexity of lists:subtract(A, B)
is proportional
to length(A)*length(B)
, meaning that it will be very slow if
both A
and B
are long lists.
(Using ordered lists and
ordsets:subtract/2
is a much better choice if both lists are long.)
suffix(List1, List2) -> boolean()
List1 = List2 = [T]
T = term()
Returns true
if
is a suffix of
, otherwise false
.
sum(List) -> number()
List = [number()]
Returns the sum of the elements in
.
takewhile(Pred, List1) -> List2
Pred = fun((Elem :: T) -> boolean())
List1 = List2 = [T]
T = term()
Takes elements
from
while
returns true
, that is,
the function returns the longest prefix of the list for which
all elements satisfy the predicate.
ukeymerge(N, TupleList1, TupleList2) -> TupleList3
N = integer() >= 1
TupleList1 = [T1]
TupleList2 = [T2]
TupleList3 = [T1 | T2]
T1 = T2 = Tuple
Tuple = tuple()
Returns the sorted list formed by merging
and
. The merge is performed on the
th element of each tuple. Both
and
must be key-sorted without duplicates
prior to evaluating this function. When two tuples compare
equal, the tuple from
is picked and the
one from
deleted.
ukeysort(N, TupleList1) -> TupleList2
N = integer() >= 1
TupleList1 = TupleList2 = [Tuple]
Tuple = tuple()
Returns a list containing the sorted elements of the list
where all but the first tuple of the
tuples comparing equal have been deleted. Sorting is
performed on the
th element of the tuples.
umerge(ListOfLists) -> List1
ListOfLists = [List]
List = List1 = [T]
T = term()
Returns the sorted list formed by merging all the sub-lists
of
. All sub-lists must be sorted and
contain no duplicates prior to evaluating this function.
When two elements compare equal, the element from the
sub-list with the lowest position in
is
picked and the other one deleted.
umerge(List1, List2) -> List3
List1 = [X]
List2 = [Y]
List3 = [X | Y]
X = Y = term()
Returns the sorted list formed by merging
and
. Both
and
must be
sorted and contain no duplicates prior to evaluating this
function. When two elements compare equal, the element from
is picked and the one from
deleted.
umerge(Fun, List1, List2) -> List3
Fun = fun((A, B) -> boolean())
List1 = [A]
List2 = [B]
List3 = [A | B]
A = B = term()
Returns the sorted list formed by merging
and
. Both
and
must be
sorted according to the ordering function
Fun
and contain no duplicates prior to evaluating
this function.
should return true
if
compares less than or equal to
in the
ordering, false
otherwise. When two elements compare
equal, the element from
is picked and the one from
deleted.
umerge3(List1, List2, List3) -> List4
List1 = [X]
List2 = [Y]
List3 = [Z]
List4 = [X | Y | Z]
X = Y = Z = term()
Returns the sorted list formed by merging
,
and
. All of
,
and
must be sorted and contain no
duplicates prior to evaluating this function. When two
elements compare equal, the element from
is
picked if there is such an element, otherwise the element
from
is picked, and the other one deleted.
unzip(List1) -> {List2, List3}
List1 = [{A, B}]
List2 = [A]
List3 = [B]
A = B = term()
"Unzips" a list of two-tuples into two lists, where the first list contains the first element of each tuple, and the second list contains the second element of each tuple.
unzip3(List1) -> {List2, List3, List4}
List1 = [{A, B, C}]
List2 = [A]
List3 = [B]
List4 = [C]
A = B = C = term()
"Unzips" a list of three-tuples into three lists, where the first list contains the first element of each tuple, the second list contains the second element of each tuple, and the third list contains the third element of each tuple.
usort(List1) -> List2
List1 = List2 = [T]
T = term()
Returns a list containing the sorted elements of
where all but the first element of the elements
comparing equal have been deleted.
usort(Fun, List1) -> List2
Fun = fun((T, T) -> boolean())
List1 = List2 = [T]
T = term()
Returns a list which contains the sorted elements of
where all but the first element of the elements
comparing equal according to the ordering function
have been deleted.
should return
true
if A
compares less than or equal to
B
in the ordering, false
otherwise.
zip(List1, List2) -> List3
List1 = [A]
List2 = [B]
List3 = [{A, B}]
A = B = term()
"Zips" two lists of equal length into one list of two-tuples, where the first element of each tuple is taken from the first list and the second element is taken from corresponding element in the second list.
zip3(List1, List2, List3) -> List4
List1 = [A]
List2 = [B]
List3 = [C]
List4 = [{A, B, C}]
A = B = C = term()
"Zips" three lists of equal length into one list of three-tuples, where the first element of each tuple is taken from the first list, the second element is taken from corresponding element in the second list, and the third element is taken from the corresponding element in the third list.
zipwith(Combine, List1, List2) -> List3
Combine = fun((X, Y) -> T)
List1 = [X]
List2 = [Y]
List3 = [T]
X = Y = T = term()
Combine the elements of two lists of equal length into one
list. For each pair
of list elements from the two
lists, the element in the result list will be
.
zipwith(fun(X, Y) -> {X,Y} end, List1, List2)
is
equivalent to zip(List1, List2)
.
Example:
> lists:zipwith(fun(X, Y) -> X+Y end, [1,2,3], [4,5,6]).
[5,7,9]
zipwith3(Combine, List1, List2, List3) -> List4
Combine = fun((X, Y, Z) -> T)
List1 = [X]
List2 = [Y]
List3 = [Z]
List4 = [T]
X = Y = Z = T = term()
Combine the elements of three lists of equal length into one
list. For each triple
of list elements from
the three lists, the element in the result list will be
.
zipwith3(fun(X, Y, Z) -> {X,Y,Z} end, List1, List2, List3)
is equivalent to zip3(List1, List2, List3)
.
Examples:
>lists:zipwith3(fun(X, Y, Z) -> X+Y+Z end, [1,2,3], [4,5,6], [7,8,9]).
[12,15,18] >lists:zipwith3(fun(X, Y, Z) -> [X,Y,Z] end, [a,b,c], [x,y,z], [1,2,3]).
[[a,x,1],[b,y,2],[c,z,3]]