lists(3)

MODULE
lists - List Processing Functions

DESCRIPTION

This module contains functions for list processing. The functions are organized in two groups: those in the first group perform a particular operation on one or more lists, whereas those in the second group use a user-defined function (given as the first argument) to perform an operation on one list.

EXPORTS

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]

    The result need not be a proper list. The last parameter may be of any datatype and will be the tail in the resulting list. An example:
    > lists:append([[a,b],c]).
    [a,b|c]

    The atom c will be the tail of the list and the list is therefore not proper (a proper list ends with []).

    As a parameter of [] is ignored this example is also valid (although probably useless):
    lists:append([[],d]).

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.

    The behaviour regarding inproper lists is identical to the behaviour of lists:append/1

concat(Things) -> string()

    Types
        Things = [Thing]
        Thing = atom() | integer() | float() | string()

    Concatenates the ASCII list 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(Element, List1) -> List2

    Types
        List1 = list2 = [Element]
        Element = term()

    Returns a copy of List1, but the first occurrence of Element, if present, is deleted.

duplicate(N, Element) -> List

    Types
        N = int()
        List = [Element]
        Element = term()

    Returns a list which contains N copies of the term Element.

  Note:

N must be an integer >= 0. For example:
> lists:duplicate(5, xx).
[xx, xx, xx, xx, xx]

flatlength(DeepList) -> int()

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

flatten(DeepList) -> List

    Types
        DeepList = [term() | DeepList]

    Returns a flattened version of DeepList.

flatten(DeepList, Tail) -> List

    Types
        DeepList = [term() | DeepList]
        Tail = [term()]

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

keydelete(Key, N, TupleList1) -> TupleList2

    Types
        TupleList1 = TupleList2 = [tuple()]
        N = int()
        Key = term()

    Returns a copy of TupleList1 where the first occurrence of a tuple whose Nth element is Key is deleted, if present.

keymember(Key, N, TupleList) -> bool()

    Types
        TupleList = [tuple()]
        N = int()
        Key = term()

    Searches the list of tuples TupleList for a tuple whose Nth element is Key.

keymerge(N, List1, List2)

    Types
        N = int()
        List1 = List2 = [tuple()]

    Returns the sorted list formed by merging List1 and List2. The merge is performed on the Nth element of each tuple. Both List1 and List2 must be key-sorted prior to evaluating this function; otherwise the result is undefined (and probably unexpected). When elements in the input lists compare equal, elements from List1 are picked before elements from List2.

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

    Types
        Key = term()
        N = int()
        TupleList1 = TupleList2 = [tuple()]
        NewTuple = tuple()

    Returns a list of tuples. In this list, a tuple is replaced by the tuple NewTuple. This tuple is the first tuple in the list where the element number N is equal to Key.

keysearch(Key, N, TupleList) -> Result

    Types
        TupleList = [tuple()]
        N = int()
        Key = term()
        Result = {value, tuple()} | false

    Searches the list of the tuples TupleList for Tuple whose Nth element is Key. Returns {value, Tuple} if such a tuple is found, or false if no such tuple is found.

keysort(N, List1) -> List2

    Types
        N = int()
        List1 = List2 = [tuple()]

    Returns a list containing the sorted elements of List1. TupleList1 must be a list of tuples, and the sort is performed on the Nth element of the tuple. The sort is stable.

last(List) -> Element

    Types
        List = [Element]
        Element = term()

    Returns the last element in List.

max(List) -> Max

    Types
        List = [Element]
        Element = Max = term()

    Returns the maximum element of List.

member(Element, List) -> bool()

    Types
        List = [Element]
        Element = term()

    Returns true if Element is contained in the list 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.

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.

merge(Fun, List1, List2) -> List

    Types
        List = List1 = List2 = [Element]
        Fun = fun(Element, Element) -> bool()
        Element = 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 comes before B in the ordering, false otherwise.

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.

min(List) -> Min

    Types
        List = [Element]
        Element = Max = term()

    Returns the minimum element of List.

nth(N, List) -> Element

    Types
        N = int()
        List = [Element]
        Element = term()

    Returns the Nth element of the List. For example:
    > lists:nth(3, [a, b, c, d, e]).
    c

nthtail(N, List1) -> List2

    Types
        N = int()
        List1 = List2 = [Alpha]

    Returns the Nth tail of List. For example:
    > lists:nthtail(3, [a, b, c, d, e]).
    [d, e]

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, List2) -> List3

    Types
        List1 = List2 = List3 = [term()]

    Returns a list where List1 has been reversed and appended to the beginning of List2. Equivalent to reverse(List1) ++ List2. For example:
    > lists:reverse([1, 2, 3, 4], [a, b, c]).
    [4, 3, 2, 1, a, b, c]

seq(From, To) -> [int()]
seq(From, To, Incr) -> [int()]

    Types
        From = To = Incr = 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). If To-From has a different sign from Incr, or if Incr = 0 and From is different from To, an error is signalled (this implies that the result is never an empty list - the first element is always From).

    seq(From, To) is equivalent to seq(From, To, 1).

    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, 1, 0).
    [1]

sort(List1) -> List2

    Types
        List1 = List2 = [term()]

    Returns a list which contains the sorted elements of List1.

sort(Fun, List1) -> List2

    Types
        List1 = List2 = [Element]
        Fun = fun(Element, Element) -> bool()
        Element = term()

    Returns a list which contains the sorted elements of List1, according to the ordering function Fun. Fun(A, B) should return true if A comes before B in the ordering, false otherwise.

sublist(List, N) -> List1

    Types
        List1 = List2 = [term()]
        N = int()

    Returns the first N elements of List. It is not an error for N to exceed the length of the list when List is a proper list - in that case the whole list is returned.

sublist(List1, Start, Length) -> List2

    Types
        List1 = List2 = [term()]
        Start = End = int()

    Returns the sub-list of List starting at Start of length Length. Terminates with a runtime failure if Start is not in List, but a sub-list of a length less than Length is accepted. Start is considered to be in List if Start >= 1 and Start <= length(List)+1.

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

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

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.

ukeymerge(N, List1, List2)

    Types
        N = int()
        List1 = List2 = [tuple()]

    Returns the sorted list formed by merging List1 and List2 while removing consecutive duplicates. The merge is performed on the Nth element of each tuple. Both List1 and List2 must be key-sorted prior to evaluating this function; otherwise the order of the elements in the result will be undefined. When elements in the input lists compare equal, elements from List1 are picked before elements from List2.

ukeysort(N, List1) -> List2

    Types
        N = int()
        List1 = List2 = [tuple()]

    Returns a list containing the sorted elements of List1 with consecutive duplicates removed. TupleList1 must be a list of tuples, and the sort is performed on the Nth element of the tuple. The sort is stable.

umerge(ListOfLists) -> List1

    Types
        ListOfLists = [List]
        List = List1 = [term()]

    Returns the sorted list formed by merging all the sub-lists of ListOfLists while removing duplicates. All sub-lists must be sorted and contain no duplicates prior to evaluating this function.

umerge(List1, List2) -> List3

    Types
        List1 = List2 = List3 = [term()]

    Returns the sorted list formed by merging List1 and List2 while removing duplicates. Both List1 and List2 must be sorted and contain no duplicates prior to evaluating this function.

umerge(Fun, List1, List2) -> List

    Types
        List = List1 = List2 = [Element]
        Fun = fun(Element, Element) -> bool()
        Element = term()

    Returns the sorted list formed by merging List1 and List2 while removing consecutive duplicates. 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 comes before B in the ordering, false otherwise.

umerge3(List1, List2, List3) -> List4

    Types
        List1 = List2 = List3 = List4 = [term()]

    Returns the sorted list formed by merging List1, List2 and List3 while removing duplicates. All of List1, List2 and List3 must be sorted and contain no duplicates prior to evaluating this function.

unzip(List1) -> {List2,List3}

    Types
        List1 = [{X, Y}]
        List2 = [X]
        List3 = [Y]
        X = Y = term()

    "Unzip" a list of two-tuples into two lists, where the first list contains all terms in the first element of each tuple, and the second list 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()

    "Unzip" a list of three-tuples into three lists, where the first list contains all terms in the first element of each tuple, the second list the second element of each tuple, and the third list the third element of each tuple.

usort(List1) -> List2

    Types
        List1 = List2 = [term()]

    Returns a list which contains the sorted elements of List1 without duplicates.

usort(Fun, List1) -> List2

    Types
        List1 = List2 = [Element]
        Fun = fun(Element, Element) -> bool()
        Element = term()

    Returns a list which contains the sorted elements of List1 with consecutive duplicates removed, according to the ordering function Fun. Fun(A, B) should return true if A comes before B in the ordering, false otherwise.

zip(List1, List2) -> List3

    Types
        List1 = [X]
        List2 = [Y]
        List3 = [{X, Y}]
        X = Y = term()

    "Zip" 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 = term()

    "Zip" 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 third list.

zipwith(Combine, List1, List2) -> List3

    Types
        Combine = fun(X, Y) -> term()
        List1 = [X]
        List2 = [Y]
        List3 = [Combine(X, Y)]
        X = Y = 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).

    Examples:
    > lists:zipwith(fun(X, Y) -> X+Y end, [1,2,3], [4,5,6]).
    [5,7,9]
    > lists:zipwith(fun(X, Y) -> [X|Y] end, [a,b,c], [4,5,6]).
    [[a|4],[b|5],[c|6]]

zipwith3(Combine, List1, List2, List3) -> List4

    Types
        Combine = fun(X, Y) -> term()
        List1 = [X]
        List2 = [Y]
        List3 = [Z]
        List4 = [Combine(X, Y, Z)]
        X = Y = Z = term()

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

    zipwith3(fun(X, Y, Z) -> {X, Y, Z} end, List1, List2, List2) 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]]

all(Pred, List) -> bool()

    Types
        Pred = fun(A) -> bool()
        List = [A]

    Returns true if all elements X in List satisfy Pred(X).

any(Pred, List) -> bool()

    Types
        Pred = fun(Element) -> bool()
        List = [Element]
        Element = term()

    Returns true if any of the elements in List satisfies Pred.

dropwhile(Pred, List1) -> List2

    Types
        Pred = fun(A) -> bool()
        List1 = List2 = [A]

    Drops elements X from List1 while Pred(X) is true and returns the remaining list.

filter(Pred, List1) -> List2

    Types
        Pred = fun(A) -> bool()
        List1 = List2 = [A]

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

flatmap(Function, List1) -> Element

    Types
        Function = fun(A) -> B
        List1 = [A]
        Element = [B]

    flatmap behaves as if it had been defined as follows:
    flatmap(Func, List) ->
        append(map(Func, List))

foldl(Function, Acc0, List) -> Acc1

    Types
        Function = fun(A, AccIn) -> AccOut
        List = [A]
        Acc0 = Acc1 = AccIn = AccOut = term()

    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(Function, Acc0, List) -> Acc1

    Types
        Function = fun(A, AccIn) -> AccOut
        List = [A]
        Acc0 = Acc1 = AccIn = AccOut = term()

    Calls Function on successive elements of List together with an extra argument Acc (short for accumulator). Function must return a new accumulator which is passed to the next call. Acc0 is returned if the list is empty. foldr differs from foldl in that the list is traversed "bottom up" instead of "top down". foldl is tail recursive and would usually be preferred to foldr.

foreach(Function, List) -> void()

    Types
        Function = fun(A) -> void()
        List = [A]

    Applies the function Function to each of the elements 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.

map(Func, List1) -> List2

    Types
        Func = fun(A) -> B
        List1 = [A]
        List2 = [B]

    map takes a function from As to Bs, and a list of As and produces a list of Bs 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(Function, Acc0, List1) -> {List2, Acc}

    Types
        Function = fun(A, AccIn) -> {B, AccOut}
        Acc0 = Acc1 = AccIn = AccOut = term()
        List1 = [A]
        List2 = [B]

    mapfold combines the operations of map and foldl into one pass. For example, we could sum 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(Function, Acc0, List1) -> {List2, Acc}

    Types
        Function = fun(A, AccIn) -> {B, AccOut}
        Acc0 = Acc1 = AccIn = AccOut = term()
        List1 = [A]
        List2 = [B]

    mapfold combines the operations of map and foldr into one pass.

partition(Pred, List) -> {Satisfying, NonSatisfying}

    Types
        Pred = fun(A) -> bool()
        List = [A]
        List = Satisfying = [A] where Pred(A) == true
        List = NonSatisfying = [A] where Pred(A) == false
        A = term()

    Partition List into two lists, where the first list contains all elements for which Pred(A) returns true, and the second list contains all elements for which Pred(A) 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.

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

    Types
        Pred = fun(A) -> bool()
        List = List1 = List2 = [A]

    Partitions List into List1 and List2. List1 contains the first N elements and List2 the rest of the elements.

    Note that List == List1 ++ List2.

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

    Types
        Pred = fun(A) -> bool()
        List = List1 = List2 = [A]

    Partitions List into List1 and List2 according to Pred.

    splitwith behaves as if it had been defined as follows:
    splitwidth(Pred, List) ->
              {takewhile(Pred, List), dropwhile(Pred, List)}.

    Note also that List == List1 ++ List2.

    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.

takewhile(Pred, List1) -> List2

    Types
        Pred = fun(A) -> bool()
        List1 = List2 = [A]

    Returns the longest prefix of List1 for which all elements X in List1 satisfy Pred(X).

Relics

Some of the exported functions in lists.erl are not documented. In particular, this applies to a number of maps and folds which have an extra argument for environment passing. These functions are no longer needed because Erlang 4.4 and later releases have Funs.

  Note:

Any undocumented functions in lists should not be used.

AUTHORS
Joe Armstrong - support@erlang.ericsson.se
Robert Virding - support@erlang.ericsson.se