## table of contents

- stretch 1:19.2.1+dfsg-2+deb9u2
- testing 1:21.2.6+dfsg-1
- unstable 1:21.2.6+dfsg-1
- experimental 1:22.0.4+dfsg-1

lists(3erl) | Erlang Module Definition | lists(3erl) |

# NAME¶

lists - List processing functions.# DESCRIPTION¶

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 y*F*x, then x = y (*F*is antisymmetric). - *
- If x
*F*y and y*F*z, then x*F*z (*F*is transitive). - *
- x
*F*y or y*F*x (*F*is total).

An example of a typical ordering function is less than or equal
to: *=</2*.

# EXPORTS¶

all(Pred, List) -> boolean()

Types:

List = [T]

T = term()

Returns *true* if *Pred(Elem)* returns *true* for
all elements *Elem* in *List*, otherwise *false*.

any(Pred, List) -> boolean()

Types:

List = [T]

T = term()

Returns *true* if *Pred(Elem)* returns *true* for
at least one element *Elem* in *List*.

append(ListOfLists) -> List1

Types:

List = List1 = [T]

T = term()

Returns a list in which all the sublists of *ListOfLists*
have been appended.

*Example:*

> lists:append([[1, 2, 3], [a, b], [4, 5, 6]]). [1,2,3,a,b,4,5,6]

append(List1, List2) -> List3

Types:

T = term()

Returns a new list *List3*, which is made from the elements
of *List1* followed by the elements of *List2*.

*Example:*

> lists:append("abc", "def"). "abcdef"

*lists:append(A, B)* is equivalent to *A ++ B*.

concat(Things) -> string()

Types:

Thing = atom() | integer() | float() | string()

Concatenates the text representation of the elements of
*Things*. The elements of *Things* can be atoms, integers, floats,
or strings.

*Example:*

> lists:concat([doc, '/', file, '.', 3]). "doc/file.3"

delete(Elem, List1) -> List2

Types:

List1 = List2 = [T]

T = term()

Returns a copy of *List1* where the first element matching
*Elem* is deleted, if there is such an element.

droplast(List) -> InitList

Types:

InitList = [T]

T = term()

Drops the last element of a *List*. The list is to be
non-empty, otherwise the function crashes with a *function_clause*.

dropwhile(Pred, List1) -> List2

Types:

List1 = List2 = [T]

T = term()

Drops elements *Elem* from *List1* while
*Pred(Elem)* returns *true* and returns the remaining list.

duplicate(N, Elem) -> List

Types:

Elem = T

List = [T]

T = term()

Returns a list containing *N* copies of term *Elem*.

*Example:*

> lists:duplicate(5, xx). [xx,xx,xx,xx,xx]

filter(Pred, List1) -> List2

Types:

List1 = List2 = [T]

T = term()

*List2* is a list of all elements *Elem* in *List1*
for which *Pred(Elem)* returns *true*.

filtermap(Fun, List1) -> List2

Types:

List1 = [Elem]

List2 = [Elem | Value]

Elem = Value = term()

Calls *Fun(Elem)* on successive elements *Elem* of
*List1*. *Fun/2* must return either a Boolean or a tuple *{true,
Value}*. The function returns the list of elements for which *Fun*
returns a new value, where a value of *true* is synonymous with
*{true, Elem}*.

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

Types:

Equivalent to *length(flatten(DeepList))*, but more
efficient.

flatmap(Fun, List1) -> List2

Types:

List1 = [A]

List2 = [B]

A = B = term()

Takes a function from *A*s to lists of *B*s, and a list
of *A*s (*List1*) and produces a list of *B*s by applying the
function to every element in *List1* 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

Types:

List = [term()]

Returns a flattened version of *DeepList*.

flatten(DeepList, Tail) -> List

Types:

Tail = List = [term()]

Returns a flattened version of *DeepList* with tail
*Tail* appended.

foldl(Fun, Acc0, List) -> Acc1

Types:

Acc0 = Acc1 = AccIn = AccOut = term()

List = [T]

T = term()

Calls *Fun(Elem, AccIn)* on successive elements *A* of
*List*, starting with *AccIn == Acc0*. *Fun/2* must return a
new accumulator, which is passed to the next call. The function returns the
final value of the accumulator. *Acc0* is returned if the list is
empty.

*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

Types:

Acc0 = Acc1 = AccIn = AccOut = term()

List = [T]

T = term()

Like *foldl/3*, but the list is traversed from right
to left.

*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 is usually preferred to
*foldr/3*.

join(Sep, List1) -> List2

Types:

List1 = List2 = [T]

T = term()

Inserts *Sep* between each element in *List1*. Has no
effect on the empty list and on a singleton list. For example:

> lists:join(x, [a,b,c]). [a,x,b,x,c] > lists:join(x, [a]). [a] > lists:join(x, []). []

foreach(Fun, List) -> ok

Types:

List = [T]

T = term()

Calls *Fun(Elem)* for each element *Elem* in
*List*. 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

Types:

N = integer() >= 1

Tuple = tuple()

Returns a copy of *TupleList1* where the first occurrence of
a tuple whose *N*th element compares equal to *Key* is deleted, if
there is such a tuple.

keyfind(Key, N, TupleList) -> Tuple | false

Types:

N = integer() >= 1

Tuple = tuple()

Searches the list of tuples *TupleList* for a tuple whose
*N*th element compares equal to *Key*. Returns *Tuple* if
such a tuple is found, otherwise *false*.

keymap(Fun, N, TupleList1) -> TupleList2

Types:

N = integer() >= 1

Tuple = tuple()

Returns a list of tuples where, for each tuple in
*TupleList1*, the *N*th element *Term1* of the tuple has been
replaced with the result of calling *Fun(Term1)*.

*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()

Types:

N = integer() >= 1

Tuple = tuple()

Returns *true* if there is a tuple in *TupleList* whose
*N*th element compares equal to *Key*, otherwise *false*.

keymerge(N, TupleList1, TupleList2) -> TupleList3

Types:

TupleList2 = [T2]

TupleList3 = [T1 | T2]

T1 = T2 = Tuple

Tuple = tuple()

Returns the sorted list formed by merging *TupleList1* and
*TupleList2*. The merge is performed on the *N*th element of each
tuple. Both *TupleList1* and *TupleList2* must be key-sorted
before evaluating this function. When two tuples compare equal, the tuple
from *TupleList1* is picked before the tuple from
*TupleList2*.

keyreplace(Key, N, TupleList1, NewTuple) -> TupleList2

Types:

N = integer() >= 1

NewTuple = Tuple

Tuple = tuple()

Returns a copy of *TupleList1* where the first occurrence of
a *T* tuple whose *N*th element compares equal to *Key* is
replaced with *NewTuple*, if there is such a tuple *T*.

keysearch(Key, N, TupleList) -> {value, Tuple} | false

Types:

N = integer() >= 1

Tuple = tuple()

Searches the list of tuples *TupleList* for a tuple whose
*N*th element compares equal to *Key*. Returns *{value,
Tuple}* if such a tuple is found, otherwise *false*.

**Note:**

*keyfind/3*is usually more convenient.

keysort(N, TupleList1) -> TupleList2

Types:

Tuple = tuple()

Returns a list containing the sorted elements of list
*TupleList1*. Sorting is performed on the *N*th element of the
tuples. The sort is stable.

keystore(Key, N, TupleList1, NewTuple) -> TupleList2

Types:

N = integer() >= 1

TupleList2 = [Tuple, ...]

NewTuple = Tuple

Tuple = tuple()

Returns a copy of *TupleList1* where the first occurrence of
a tuple *T* whose *N*th element compares equal to *Key* is
replaced with *NewTuple*, if there is such a tuple *T*. If there
is no such tuple *T*, a copy of *TupleList1* where
[*NewTuple*] has been appended to the end is returned.

keytake(Key, N, TupleList1) -> {value, Tuple, TupleList2} | false

Types:

N = integer() >= 1

Tuple = tuple()

Searches the list of tuples *TupleList1* for a tuple whose
*N*th element compares equal to *Key*. Returns *{value, Tuple,
TupleList2}* if such a tuple is found, otherwise *false*.
*TupleList2* is a copy of *TupleList1* where the first occurrence
of *Tuple* has been removed.

last(List) -> Last

Types:

Last = T

T = term()

Returns the last element in *List*.

map(Fun, List1) -> List2

Types:

List1 = [A]

List2 = [B]

A = B = term()

Takes a function from *A*s to *B*s, and a list of
*A*s and produces a list of *B*s by applying the function to every
element in the list. This function is used to obtain the return values. The
evaluation order depends on the implementation.

mapfoldl(Fun, Acc0, List1) -> {List2, Acc1}

Types:

Acc0 = Acc1 = AccIn = AccOut = term()

List1 = [A]

List2 = [B]

A = B = term()

Combines the operations of *map/2* and
*foldl/3* into one pass.

*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}

Types:

Acc0 = Acc1 = AccIn = AccOut = term()

List1 = [A]

List2 = [B]

A = B = term()

Combines the operations of *map/2* and
*foldr/3* into one pass.

max(List) -> Max

Types:

Max = T

T = term()

Returns the first element of *List* that compares greater
than or equal to all other elements of *List*.

member(Elem, List) -> boolean()

Types:

List = [T]

T = term()

Returns *true* if *Elem* matches some element of
*List*, otherwise *false*.

merge(ListOfLists) -> List1

Types:

List = List1 = [T]

T = term()

Returns the sorted list formed by merging all the sublists of
*ListOfLists*. All sublists must be sorted before evaluating this
function. When two elements compare equal, the element from the sublist with
the lowest position in *ListOfLists* is picked before the other
element.

merge(List1, List2) -> List3

Types:

List2 = [Y]

List3 = [X | Y]

X = Y = term()

Returns the sorted list formed by merging *List1* and
*List2*. Both *List1* and *List2* must be sorted before
evaluating this function. When two elements compare equal, the element from
*List1* is picked before the element from *List2*.

merge(Fun, List1, List2) -> List3

Types:

List1 = [A]

List2 = [B]

List3 = [A | B]

A = B = term()

Returns the sorted list formed by merging *List1* and
*List2*. Both *List1* and *List2* must be sorted according to
the **ordering function** *Fun* before evaluating this function.
*Fun(A, B)* is to return *true* if *A* compares less than or
equal to *B* in the ordering, otherwise *false*. When two elements
compare equal, the element from *List1* is picked before the element
from *List2*.

merge3(List1, List2, List3) -> List4

Types:

List2 = [Y]

List3 = [Z]

List4 = [X | Y | Z]

X = Y = Z = term()

Returns the sorted list formed by merging *List1*,
*List2*, and *List3*. All of *List1*, *List2*, and
*List3* must be sorted before evaluating this function. When two
elements compare equal, the element from *List1*, if there is such an
element, is picked before the other element, otherwise the element from
*List2* is picked before the element from *List3*.

min(List) -> Min

Types:

Min = T

T = term()

Returns the first element of *List* that compares less than
or equal to all other elements of *List*.

nth(N, List) -> Elem

Types:

Elem = T

T = term()

Returns the *N*th element of *List*.

*Example:*

> lists:nth(3, [a, b, c, d, e]). c

nthtail(N, List) -> Tail

Types:

Tail = [T]

T = term()

Returns the *N*th tail of *List*, that is, the sublist
of *List* starting at *N+1* and continuing up to the end of the
list.

*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}

Types:

List = Satisfying = NotSatisfying = [T]

T = term()

Partitions *List* into two lists, where the first list
contains all elements for which *Pred(Elem)* returns *true*, and
the second list contains all elements for which *Pred(Elem)* 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]}

For a different way to partition a list, see
*splitwith/2*.

prefix(List1, List2) -> boolean()

Types:

T = term()

Returns *true* if *List1* is a prefix of *List2*,
otherwise *false*.

reverse(List1) -> List2

Types:

T = term()

Returns a list with the elements in *List1* in reverse
order.

reverse(List1, Tail) -> List2

Types:

Tail = term()

List2 = [T]

T = term()

Returns a list with the elements in *List1* in reverse order,
with tail *Tail* appended.

*Example:*

> lists:reverse([1, 2, 3, 4], [a, b, c]). [4,3,2,1,a,b,c]

seq(From, To) -> Seq

seq(From, To, Incr) -> Seq

Types:

Seq = [integer()]

Returns a sequence of integers that starts with *From* and
contains the successive results of adding *Incr* to the previous
element, until *To* is reached or passed (in the latter case, *To*
is not an element of the sequence). *Incr* defaults to 1.

Failures:

- *
- If
*To < From - Incr*and*Incr > 0*. - *
- If
*To > From - Incr*and*Incr < 0*. - *
- If
*Incr =:= 0*and*From =/= To*.

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

Types:

T = term()

Returns a list containing the sorted elements of *List1*.

sort(Fun, List1) -> List2

Types:

List1 = List2 = [T]

T = term()

Returns a list containing the sorted elements of *List1*,
according to the **ordering function** *Fun*. *Fun(A, B)* is to
return *true* if *A* compares less than or equal to *B* in
the ordering, otherwise *false*.

split(N, List1) -> {List2, List3}

Types:

T = term()

Splits *List1* into *List2* and *List3*.
*List2* contains the first *N* elements and *List3* the
remaining elements (the *N*th tail).

splitwith(Pred, List) -> {List1, List2}

Types:

List = List1 = List2 = [T]

T = term()

Partitions *List* into two lists according to *Pred*.
*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]}

For a different way to partition a list, see
*partition/2*.

sublist(List1, Len) -> List2

Types:

Len = integer() >= 0

T = term()

Returns the sublist of *List1* starting at position 1 and
with (maximum) *Len* elements. It is not an error for *Len* to
exceed the length of the list, in that case the whole list is returned.

sublist(List1, Start, Len) -> List2

Types:

Start = integer() >= 1

T = term()

Returns the sublist of *List1* starting at *Start* and
with (maximum) *Len* elements. It is not an error for *Start+Len*
to exceed the length of the list.

*Examples:*

> 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

Types:

T = term()

Returns a new list *List3* that is a copy of *List1*,
subjected to the following procedure: for each element in *List2*, its
first occurrence in *List1* is deleted.

*Example:*

> lists:subtract("123212", "212"). "312".

*lists:subtract(A, B)* is equivalent to *A -- B*.

**Warning:**

*lists:subtract(A, B)*is proportional to

*length(A)*length(B)*, meaning that it is very slow if both

*A*and

*B*are long lists. (If both lists are long, it is a much better choice to use ordered lists and

*ordsets:subtract/2*.

suffix(List1, List2) -> boolean()

Types:

T = term()

Returns *true* if *List1* is a suffix of *List2*,
otherwise *false*.

sum(List) -> number()

Types:

Returns the sum of the elements in *List*.

takewhile(Pred, List1) -> List2

Types:

List1 = List2 = [T]

T = term()

Takes elements *Elem* from *List1* while
*Pred(Elem)* 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

Types:

TupleList2 = [T2]

TupleList3 = [T1 | T2]

T1 = T2 = Tuple

Tuple = tuple()

Returns the sorted list formed by merging *TupleList1* and
*TupleList2*. The merge is performed on the *N*th element of each
tuple. Both *TupleList1* and *TupleList2* must be key-sorted
without duplicates before evaluating this function. When two tuples compare
equal, the tuple from *TupleList1* is picked and the one from
*TupleList2* is deleted.

ukeysort(N, TupleList1) -> TupleList2

Types:

Tuple = tuple()

Returns a list containing the sorted elements of list
*TupleList1* where all except the first tuple of the tuples comparing
equal have been deleted. Sorting is performed on the *N*th element of
the tuples.

umerge(ListOfLists) -> List1

Types:

List = List1 = [T]

T = term()

Returns the sorted list formed by merging all the sublists of
*ListOfLists*. All sublists must be sorted and contain no duplicates
before evaluating this function. When two elements compare equal, the
element from the sublist with the lowest position in *ListOfLists* is
picked and the other is deleted.

umerge(List1, List2) -> List3

Types:

List2 = [Y]

List3 = [X | Y]

X = Y = term()

Returns the sorted list formed by merging *List1* and
*List2*. Both *List1* and *List2* must be sorted and contain
no duplicates before evaluating this function. When two elements compare
equal, the element from *List1* is picked and the one from *List2*
is deleted.

umerge(Fun, List1, List2) -> List3

Types:

List1 = [A]

List2 = [B]

List3 = [A | B]

A = B = term()

Returns the sorted list formed by merging *List1* and
*List2*. Both *List1* and *List2* must be sorted according to
the **ordering function** *Fun* and contain no duplicates before
evaluating this function. *Fun(A, B)* is to return *true* if
*A* compares less than or equal to *B* in the ordering, otherwise
*false*. When two elements compare equal, the element from *List1*
is picked and the one from *List2* is deleted.

umerge3(List1, List2, List3) -> List4

Types:

List2 = [Y]

List3 = [Z]

List4 = [X | Y | Z]

X = Y = Z = term()

Returns the sorted list formed by merging *List1*,
*List2*, and *List3*. All of *List1*, *List2*, and
*List3* must be sorted and contain no duplicates before evaluating this
function. When two elements compare equal, the element from *List1* is
picked if there is such an element, otherwise the element from *List2*
is picked, and the other is deleted.

unzip(List1) -> {List2, List3}

Types:

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}

Types:

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

Types:

T = term()

Returns a list containing the sorted elements of *List1*
where all except the first element of the elements comparing equal have been
deleted.

usort(Fun, List1) -> List2

Types:

List1 = List2 = [T]

T = term()

Returns a list containing the sorted elements of *List1*
where all except the first element of the elements comparing equal according
to the **ordering function** *Fun* have been deleted. *Fun(A,
B)* is to return *true* if *A* compares less than or equal to
*B* in the ordering, otherwise *false*.

zip(List1, List2) -> List3

Types:

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 the corresponding element in the second list.

zip3(List1, List2, List3) -> List4

Types:

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 the 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

Types:

List1 = [X]

List2 = [Y]

List3 = [T]

X = Y = T = term()

Combines the elements of two lists of equal length into one list.
For each pair *X, Y* of list elements from the two lists, the element
in the result list is *Combine(X, Y)*.

*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

Types:

List1 = [X]

List2 = [Y]

List3 = [Z]

List4 = [T]

X = Y = Z = T = term()

Combines the elements of three lists of equal length into one
list. For each triple *X, Y, Z* of list elements from the three lists,
the element in the result list is *Combine(X, Y, Z)*.

*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]]

stdlib 3.2 | Ericsson AB |