/Erlang 21

# lists

lists

## Module Summary

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

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

any(Pred, List) -> boolean()

## Types

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

append(ListOfLists) -> List1

## Types

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

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

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

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

droplast(List) -> InitList

## Types

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

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

duplicate(N, Elem) -> List

## Types

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

Example:

```> lists:duplicate(5, xx).
[xx,xx,xx,xx,xx]```
filter(Pred, List1) -> List2

## Types

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

filtermap(Fun, List1) -> List2

## Types

Calls `Fun(Elem)` on successive elements `Elem` of `List1`. `Fun/1` 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

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

Returns a flattened version of `DeepList`.

flatten(DeepList, Tail) -> List

## Types

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

foldl(Fun, Acc0, List) -> Acc1

## Types

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

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

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

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

1..tuple_size(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

1..tuple_size(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

1..tuple_size(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

1..tuple_size(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

1..tuple_size(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

1..tuple_size(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

1..tuple_size(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. Function `keyfind/3` is usually more convenient.

keysort(N, TupleList1) -> TupleList2

## Types

1..tuple_size(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

1..tuple_size(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

1..tuple_size(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

Returns the last element in `List`.

map(Fun, List1) -> List2

## Types

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

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

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

max(List) -> Max

## Types

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

member(Elem, List) -> boolean()

## Types

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

merge(ListOfLists) -> List1

## Types

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

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

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

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

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

nth(N, List) -> Elem

## Types

1..length(List)

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

Example:

```> lists:nth(3, [a, b, c, d, e]).
c```
nthtail(N, List) -> Tail

## Types

0..length(List)

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

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

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

reverse(List1) -> List2

## Types

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

reverse(List1, Tail) -> List2

## Types

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

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).
```
sort(List1) -> List2

## Types

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

sort(Fun, List1) -> List2

## Types

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

0..length(List1)

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

search(Pred, List) -> {value, Value} | false

## Types

If there is a `Value` in `List` such that `Pred(Value)` returns `true`, returns `{value, Value}` for the first such `Value`, otherwise returns `false`.

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

## Types

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]).
{,[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

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

1..(length(List1)+1)

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

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

The complexity of `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

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

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

1..tuple_size(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

1..tuple_size(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

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

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

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

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

"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

"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

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

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

"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

"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

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

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]]```