
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) > bool()
Types Pred = fun(Elem) > bool()
Elem = term()
List = [term()]
Returns true if Pred(Elem) returns true for all elements Elem in List , otherwise
false .
any(Pred, List) > bool()
Types Pred = fun(Elem) > bool()
Elem = term()
List = [term()]
Returns true if Pred(Elem) returns true for at least one element Elem in List .
append(ListOfLists) > List1
Types ListOfLists = [List]
List = List1 = [term()]
Returns a list in which all the sublists of ListOfLists 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
Types List1 = List2 = List3 = [term()]
Returns a new list List3 which is made from the elements of List1 followed by the
elements of List2 . For example:
> lists:append("abc", "def").
"abcdef"
lists:append(A, B) is equivalent to A ++ B .
concat(Things) > string()
Types Things = [Thing]
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.
> lists:concat([doc, '/', file, '.', 3]).
"doc/file.3"
delete(Elem, List1) > List2
Types Elem = term()
List1 = List2 = [term()]
Returns a copy of List1 where the first element matching Elem is deleted, if there
is such an element.
dropwhile(Pred, List1) > List2
Types Pred = fun(Elem) > bool()
Elem = term()
List1 = List2 = [term()]
Drops elements Elem from List1 while Pred(Elem) returns true and returns the
remaining list.
duplicate(N, Elem) > List
Types N = int()
Elem = term()
List = [term()]
Returns a list which contains N copies of the term Elem . For example:
> lists:duplicate(5, xx).
[xx,xx,xx,xx,xx]
filter(Pred, List1) > List2
Types Pred = fun(Elem) > bool()
Elem = term()
List1 = List2 = [term()]
List2 is a list of all elements Elem in List1 for which Pred(Elem) returns true .
flatlength(DeepList) > int()
Types DeepList = [term()  DeepList]
Equivalent to length(flatten(DeepList)) , but more efficient.
flatmap(Fun, List1) > List2
Types Fun = fun(A) > [B]
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 DeepList = [term()  DeepList]
List = [term()]
Returns a flattened version of DeepList .
flatten(DeepList, Tail) > List
Types DeepList = [term()  DeepList]
Tail = List = [term()]
Returns a flattened version of DeepList with the tail Tail appended.
foldl(Fun, Acc0, List) > Acc1
Types Fun = fun(Elem, AccIn) > AccOut
Elem = term()
Acc0 = Acc1 = AccIn = AccOut = term()
List = [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. 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
Types Fun = fun(Elem, AccIn) > AccOut
Elem = term()
Acc0 = Acc1 = AccIn = AccOut = term()
List = [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) > void()
Types Fun = fun(Elem) > void()
Elem = term()
List = [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 ele
ments in the list.
keydelete(Key, N, TupleList1) > TupleList2
Types Key = term()
N = 1..tuple_size(Tuple)
TupleList1 = TupleList2 = [Tuple]
Tuple = tuple()
Returns a copy of TupleList1 where the first occurrence of a tuple whose N th ele
ment compares equal to Key is deleted, if there is such a tuple.
keyfind(Key, N, TupleList) > Tuple  false
Types Key = term()
N = 1..tuple_size(Tuple)
TupleList = [Tuple]
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 Fun = fun(Term1) > Term2
Term1 = Term2 = term()
N = 1..tuple_size(Tuple)
TupleList1 = TupleList2 = [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) > bool()
Types Key = term()
N = 1..tuple_size(Tuple)
TupleList = [Tuple]
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 N = 1..tuple_size(Tuple)
TupleList1 = TupleList2 = TupleList3 = [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
keysorted prior to 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 Key = term()
N = 1..tuple_size(Tuple)
TupleList1 = TupleList2 = [Tuple]
NewTuple = 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 Key = term()
N = 1..tuple_size(Tuple)
TupleList = [Tuple]
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:
This function is retained for backward compatibility. The function lists:keyfind/3 (intro
duced in R13A) is in most cases more convenient.
keysort(N, TupleList1) > TupleList2
Types N = 1..tuple_size(Tuple)
TupleList1 = TupleList2 = [Tuple]
Tuple = tuple()
Returns a list containing the sorted elements of the list TupleList1 . Sorting is
performed on the N th element of the tuples. The sort is stable.
keystore(Key, N, TupleList1, NewTuple) > TupleList2
Types Key = term()
N = 1..tuple_size(Tuple)
TupleList1 = TupleList2 = [Tuple]
NewTuple = 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 Key = term()
N = 1..tuple_size(Tuple)
TupleList1 = TupleList2 = [Tuple]
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, other
wise false . TupleList2 is a copy of TupleList1 where the first occurrence of Tuple
has been removed.
last(List) > Last
Types List = [term()], length(List) > 0
Last = term()
Returns the last element in List .
map(Fun, List1) > List2
Types Fun = fun(A) > B
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 is implementation dependent.
mapfoldl(Fun, Acc0, List1) > {List2, Acc1}
Types 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}
Types 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
Types List = [term()], length(List) > 0
Max = term()
Returns the first element of List that compares greater than or equal to all other
elements of List .
member(Elem, List) > bool()
Types Elem = term()
List = [term()]
Returns true if Elem matches some element of List , otherwise false .
merge(ListOfLists) > List1
Types ListOfLists = [List]
List = List1 = [term()]
Returns the sorted list formed by merging all the sublists of ListOfLists . All
sublists must be sorted prior to evaluating this function. When two elements com
pare equal, the element from the sublist with the lowest position in ListOfLists
is picked before the other element.
merge(List1, List2) > List3
Types List1 = List2 = List3 = [term()]
Returns the sorted list formed by merging List1 and List2 . Both List1 and List2
must be sorted prior to 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 Fun = fun(A, B) > bool()
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 prior to evaluating this
function. Fun(A, B) should return true if A compares less than or equal to B in the
ordering, false otherwise. When two elements compare equal, the element from List1
is picked before the element from List2 .
merge3(List1, List2, List3) > List4
Types List1 = List2 = List3 = List4 = [term()]
Returns the sorted list formed by merging List1 , List2 and List3 . All of List1 ,
List2 and List3 must be sorted prior to 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 List = [term()], length(List) > 0
Min = term()
Returns the first element of List that compares less than or equal to all other
elements of List .
nth(N, List) > Elem
Types N = 1..length(List)
List = [term()]
Elem = term()
Returns the N th element of List . For example:
> lists:nth(3, [a, b, c, d, e]).
c
nthtail(N, List1) > Tail
Types N = 0..length(List1)
List1 = Tail = [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. 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, NonSatisfying}
Types Pred = fun(Elem) > bool()
Elem = term()
List = Satisfying = NonSatisfying = [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]}
See also splitwith/2 for a different way to partition a list.
prefix(List1, List2) > bool()
Types List1 = List2 = [term()]
Returns true if List1 is a prefix of List2 , otherwise false .
reverse(List1) > List2
Types List1 = List2 = [term()]
Returns a list with the top level elements in List1 in reverse order.
reverse(List1, Tail) > List2
Types List1 = Tail = List2 = [term()]
Returns a list with the top level elements in List1 in reverse order, with the tail
Tail appended. For 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 From = To = Incr = int()
Seq = [int()]
Returns a sequence of integers which starts with From and contains the successive
results of adding Incr to the previous element, until To has been reached or passed
(in the latter case, To is not an element of the sequence). Incr defaults to 1.
Failure: If To<FromIncr and Incr is positive, or if To>FromIncr and Incr is nega
tive, or if Incr==0 and From/=To .
The following equalities hold for all sequences:
length(lists:seq(From, To)) == ToFrom+1
length(lists:seq(From, To, Incr)) == (ToFrom+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 List1 = List2 = [term()]
Returns a list containing the sorted elements of List1 .
sort(Fun, List1) > List2
Types Fun = fun(Elem1, Elem2) > bool()
Elem1 = Elem2 = term()
List1 = List2 = [term()]
Returns a list containing the sorted elements of List1 , according to the ordering
function Fun . Fun(A, B) should return true if A compares less than or equal to B
in the ordering, false otherwise.
split(N, List1) > {List2, List3}
Types N = 0..length(List1)
List1 = List2 = List3 = [term()]
Splits List1 into List2 and List3 . List2 contains the first N elements and List3
the rest of the elements (the N th tail).
splitwith(Pred, List) > {List1, List2}
Types Pred = fun(Elem) > bool()
Elem = term()
List = List1 = List2 = [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]}
See also partition/2 for a different way to partition a list.
sublist(List1, Len) > List2
Types List1 = List2 = [term()]
Len = int()
Returns the sublist of List1 starting at position 1 and with (max) 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 List1 = List2 = [term()]
Start = 1..(length(List1)+1)
Len = int()
Returns the sublist of List1 starting at Start and with (max) Len elements. It is
not an error for Start+Len 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
Types List1 = List2 = List3 = [term()]
Returns a new list List3 which is a copy of List1 , subjected to the following pro
cedure: for each element in List2 , its first occurrence in List1 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 ord
sets:subtract/2 is a much better choice if both lists are long.)
suffix(List1, List2) > bool()
Returns true if List1 is a suffix of List2 , otherwise false .
sum(List) > number()
Types List = [number()]
Returns the sum of the elements in List .
takewhile(Pred, List1) > List2
Types Pred = fun(Elem) > bool()
Elem = term()
List1 = List2 = [term()]
Takes elements Elem from List1 while Pred(Elem) returns true , that is, the func
tion returns the longest prefix of the list for which all elements satisfy the
predicate.
ukeymerge(N, TupleList1, TupleList2) > TupleList3
Types N = 1..tuple_size(Tuple)
TupleList1 = TupleList2 = TupleList3 = [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
keysorted without duplicates prior to evaluating this function. When two tuples
compare equal, the tuple from TupleList1 is picked and the one from TupleList2
deleted.
ukeysort(N, TupleList1) > TupleList2
Types N = 1..tuple_size(Tuple)
TupleList1 = TupleList2 = [Tuple]
Tuple = tuple()
Returns a list containing the sorted elements of the list TupleList1 where all but
the first tuple of the tuples comparing equal have been deleted. Sorting is per
formed on the N th element of the tuples.
umerge(ListOfLists) > List1
Types ListOfLists = [List]
List = List1 = [term()]
Returns the sorted list formed by merging all the sublists of ListOfLists . All
sublists must be sorted and contain no duplicates prior to evaluating this func
tion. When two elements compare equal, the element from the sublist with the low
est position in ListOfLists is picked and the other one deleted.
umerge(List1, List2) > List3
Types List1 = List2 = List3 = [term()]
Returns the sorted list formed by merging List1 and List2 . Both List1 and List2
must be sorted and contain no duplicates prior to evaluating this function. When
two elements compare equal, the element from List1 is picked and the one from List2
deleted.
umerge(Fun, List1, List2) > List3
Types Fun = fun(A, B) > bool()
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
prior to evaluating this function. Fun(A, B) should return true if A compares less
than or equal to B in the ordering, false otherwise. When two elements compare
equal, the element from List1 is picked and the one from List2 deleted.
umerge3(List1, List2, List3) > List4
Types List1 = List2 = List3 = List4 = [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 prior to 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
one deleted.
unzip(List1) > {List2, List3}
Types List1 = [{X, Y}]
List2 = [X]
List3 = [Y]
X = Y = term()
"Unzips" a list of twotuples 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 List1 = [{X, Y, Z}]
List2 = [X]
List3 = [Y]
List4 = [Z]
X = Y = Z = term()
"Unzips" a list of threetuples 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 List1 = List2 = [term()]
Returns a list containing the sorted elements of List1 where all but the first ele
ment of the elements comparing equal have been deleted.
usort(Fun, List1) > List2
Types Fun = fun(Elem1, Elem2) > bool()
Elem1 = Elem2 = term()
List1 = List2 = [term()]
Returns a list which contains the sorted elements of List1 where all but the first
element of the elements comparing equal according to the ordering function Fun have
been deleted. Fun(A, B) should return true if A compares less than or equal to B in
the ordering, false otherwise.
zip(List1, List2) > List3
Types List1 = [X]
List2 = [Y]
List3 = [{X, Y}]
X = Y = term()
"Zips" two lists of equal length into one list of twotuples, where the first ele
ment 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
Types List1 = [X]
List2 = [Y]
List3 = [Z]
List3 = [{X, Y, Z}]
X = Y = Z = term()
"Zips" three lists of equal length into one list of threetuples, 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
Types 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 X, Y
of list elements from the two lists, the element in the result list will be Com
bine(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 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
X, Y, Z of list elements from the three lists, the element in the result list will
be 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]]
Ericsson AB stdlib 1.17.3 lists(3erl) 
