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 sub-lists 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 exam-
ple:
> 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 elements 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 element 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 key-sorted 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 New-
Tuple , 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 (introduced 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 New-
Tuple , 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, otherwise 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 sub-lists of ListOfLists . 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 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 evaluat-
ing 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<From-Incr and Incr is positive, or if To>From-Incr and Incr is negative, or 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 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 sub-list 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 sub-list 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 procedure: 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 ordsets: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 function 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 key-sorted 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 performed 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 sub-lists of ListOfLists . 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
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 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 List1 = [{X, Y, Z}]
List2 = [X]
List3 = [Y]
List4 = [Z]
X = Y = Z = 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 List1 = List2 = [term()]
Returns a list containing the sorted elements of List1 where all but the first element 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 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
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 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 correspond-
ing 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 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 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)