# View Source lists (stdlib v6.0)

List processing functions.

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`

.

# Summary

## Functions

Returns `true`

if `Pred(Elem)`

returns `true`

for all elements `Elem`

in `List`

,
otherwise `false`

. The `Pred`

function must return a boolean.

Returns `true`

if `Pred(Elem)`

returns `true`

for at least one element `Elem`

in
`List`

. The `Pred`

function must return a boolean.

Returns a list in which all the sublists of `ListOfLists`

have been appended.

Returns a new list `List3`

, which is made from the elements of `List1`

followed
by the elements of `List2`

.

Concatenates the text representation of the elements of `Things`

. The elements
of `Things`

can be atoms, integers, floats, or strings.

Returns a copy of `List1`

where the first element matching `Elem`

is deleted, if
there is such an element.

Drops the last element of a `List`

. The list is to be non-empty, otherwise the
function crashes with a `function_clause`

.

Drops elements `Elem`

from `List1`

while `Pred(Elem)`

returns `true`

and returns
the remaining list. The `Pred`

function must return a boolean.

Returns a list containing `N`

copies of term `Elem`

.

Equivalent to `enumerate(1, 1, List1)`

.

Equivalent to `enumerate(Index, 1, List1)`

.

Returns `List1`

with each element `H`

replaced by a tuple of form `{I, H}`

where
`I`

is the position of `H`

in `List1`

. The enumeration starts with `Index`

and
increases by `Step`

in each step.

`List2`

is a list of all elements `Elem`

in `List1`

for which `Pred(Elem)`

returns `true`

. The `Pred`

function must return a boolean.

Calls `Fun(Elem)`

on successive elements `Elem`

of `List1`

in order to update or
remove elements from `List1`

.

Equivalent to `length(flatten(DeepList))`

, but more efficient.

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.

Returns a flattened version of `DeepList`

.

Returns a flattened version of `DeepList`

with tail `Tail`

appended.

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.

Like `foldl/3`

, but the list is traversed from right to left.

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.

Inserts `Sep`

between each element in `List1`

. Has no effect on the empty list
and on a singleton list. For example

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.

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`

.

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)`

.

Returns `true`

if there is a tuple in `TupleList`

whose `N`

th element compares
equal to `Key`

, otherwise `false`

.

Returns the sorted list formed by merging `TupleList1`

and `TupleList2`

.

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`

.

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`

.

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.

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.

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.

Returns the last element in `List`

.

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.

Returns the first element of `List`

that compares greater than or equal to all
other elements of `List`

.

Returns `true`

if `Elem`

matches some element of `List`

, otherwise `false`

.

Returns the sorted list formed by merging `List1`

, `List2`

, and `List3`

. All of
`List1`

, `List2`

, and `List3`

must be sorted before evaluating this function.

Returns the sorted list formed by merging all the sublists of `ListOfLists`

. All
sublists must be sorted before evaluating this function.

Returns the sorted list formed by merging `List1`

and `List2`

. Both `List1`

and
`List2`

must be sorted before evaluating this function.

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.

Returns the first element of `List`

that compares less than or equal to all
other elements of `List`

.

Returns the `N`

th element of `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.

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`

.

Returns `true`

if `List1`

is a prefix of `List2`

, otherwise `false`

.

Returns a list with the elements in `List1`

in reverse order.

Returns a list with the elements in `List1`

in reverse order, with tail `Tail`

appended.

If there is a `Value`

in `List`

such that `Pred(Value)`

returns `true`

, returns
`{value, Value}`

for the first such `Value`

, otherwise returns `false`

. The
`Pred`

function must return a boolean.

Equivalent to `seq(From, To, 1)`

.

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.

Returns a list containing the sorted elements of `List1`

.

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`

.

Splits `List1`

into `List2`

and `List3`

. `List2`

contains the first `N`

elements
and `List3`

the remaining elements (the `N`

th tail).

Partitions `List`

into two lists according to `Pred`

.
`splitwith/2`

behaves as if it is defined as follows

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.

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.

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.

Returns `true`

if `List1`

is a suffix of `List2`

, otherwise `false`

.

Returns the sum of the elements in `List`

.

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. The `Pred`

function must return a boolean.

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.

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.

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.

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.

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.

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.

Returns a list containing the elements of `List1`

with duplicated elements
removed (preserving the order of the elements). The first occurrence of each
element is kept.

Returns a list containing the elements of `List1`

without the elements for which
`Fun`

returned duplicate values (preserving the order of the elements). The
first occurrence of each element is kept.

"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.

"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.

Returns a list containing the sorted elements of `List1`

where all except the
first element of the elements comparing equal have been deleted.

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`

.

Equivalent to `zip3(List1, List2, List3, fail)`

.

"Zips" three lists 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.

Equivalent to `zip(List1, List2, fail)`

.

"Zips" two lists 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.

Combines the elements of three lists 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)`

.

Combines the elements of two lists 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)`

.

# Functions

-spec all(Pred, List) -> boolean() when Pred :: fun((Elem :: T) -> boolean()), List :: [T], T :: term().

Returns `true`

if `Pred(Elem)`

returns `true`

for all elements `Elem`

in `List`

,
otherwise `false`

. The `Pred`

function must return a boolean.

-spec any(Pred, List) -> boolean() when Pred :: fun((Elem :: T) -> boolean()), List :: [T], T :: term().

Returns `true`

if `Pred(Elem)`

returns `true`

for at least one element `Elem`

in
`List`

. The `Pred`

function must return a boolean.

-spec append(ListOfLists) -> List1 when ListOfLists :: [List], List :: [T], 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]
```

-spec append(List1, List2) -> List3 when List1 :: [T], List2 :: [T], List3 :: [T], 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`

.

-spec concat(Things) -> string() when 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.

*Example:*

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

-spec delete(Elem, List1) -> List2 when Elem :: T, List1 :: [T], List2 :: [T], T :: term().

Returns a copy of `List1`

where the first element matching `Elem`

is deleted, if
there is such an element.

-spec droplast(List) -> InitList when List :: [T, ...], 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`

.

-spec dropwhile(Pred, List1) -> List2 when Pred :: fun((Elem :: T) -> boolean()), List1 :: [T], List2 :: [T], T :: term().

Drops elements `Elem`

from `List1`

while `Pred(Elem)`

returns `true`

and returns
the remaining list. The `Pred`

function must return a boolean.

-spec duplicate(N, Elem) -> List when N :: non_neg_integer(), 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]
```

-spec enumerate(List1) -> List2 when List1 :: [T], List2 :: [{Index, T}], Index :: integer(), T :: term().

Equivalent to `enumerate(1, 1, List1)`

.

-spec enumerate(Index, List1) -> List2 when List1 :: [T], List2 :: [{Index, T}], Index :: integer(), T :: term().

Equivalent to `enumerate(Index, 1, List1)`

.

-spec enumerate(Index, Step, List1) -> List2 when List1 :: [T], List2 :: [{Index, T}], Index :: integer(), Step :: integer(), T :: term().

Returns `List1`

with each element `H`

replaced by a tuple of form `{I, H}`

where
`I`

is the position of `H`

in `List1`

. The enumeration starts with `Index`

and
increases by `Step`

in each step.

That is, `enumerate/3`

behaves as if it had been defined as
follows:

```
enumerate(I, S, List) ->
{List1, _ } = lists:mapfoldl(fun(T, Acc) -> {{Acc, T}, Acc+S} end, I, List),
List1.
```

The default values for `Index`

and `Step`

are both `1`

.

*Examples:*

```
> lists:enumerate([a,b,c]).
[{1,a},{2,b},{3,c}]
```

```
> lists:enumerate(10, [a,b,c]).
[{10,a},{11,b},{12,c}]
```

```
> lists:enumerate(0, -2, [a,b,c]).
[{0,a},{-2,b},{-4,c}]
```

-spec filter(Pred, List1) -> List2 when Pred :: fun((Elem :: T) -> boolean()), List1 :: [T], List2 :: [T], T :: term().

`List2`

is a list of all elements `Elem`

in `List1`

for which `Pred(Elem)`

returns `true`

. The `Pred`

function must return a boolean.

-spec filtermap(Fun, List1) -> List2 when Fun :: fun((Elem) -> boolean() | {true, Value}), List1 :: [Elem], List2 :: [Elem | Value], Elem :: term(), Value :: term().

Calls `Fun(Elem)`

on successive elements `Elem`

of `List1`

in order to update or
remove elements from `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]
```

-spec flatlength(DeepList) -> non_neg_integer() when DeepList :: [term() | DeepList].

Equivalent to `length(flatten(DeepList))`

, but more efficient.

-spec flatmap(Fun, List1) -> List2 when Fun :: fun((A) -> [B]), List1 :: [A], List2 :: [B], A :: term(), 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]
```

Returns a flattened version of `DeepList`

.

-spec flatten(DeepList, Tail) -> List when DeepList :: [term() | DeepList], Tail :: [term()], List :: [term()].

Returns a flattened version of `DeepList`

with tail `Tail`

appended.

-spec foldl(Fun, Acc0, List) -> Acc1 when Fun :: fun((Elem :: T, AccIn) -> AccOut), Acc0 :: term(), Acc1 :: term(), AccIn :: term(), 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
```

-spec foldr(Fun, Acc0, List) -> Acc1 when Fun :: fun((Elem :: T, AccIn) -> AccOut), Acc0 :: term(), Acc1 :: term(), AccIn :: term(), 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`

.

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.

-spec join(Sep, List1) -> List2 when Sep :: T, List1 :: [T], 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, []).
[]
```

-spec keydelete(Key, N, TupleList1) -> TupleList2 when Key :: term(), N :: pos_integer(), TupleList1 :: [Tuple], 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.

-spec keyfind(Key, N, TupleList) -> Tuple | false when Key :: term(), N :: pos_integer(), 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`

.

-spec keymap(Fun, N, TupleList1) -> TupleList2 when Fun :: fun((Term1 :: term()) -> Term2 :: term()), N :: pos_integer(), TupleList1 :: [Tuple], TupleList2 :: [Tuple], 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}]
```

-spec keymember(Key, N, TupleList) -> boolean() when Key :: term(), N :: pos_integer(), TupleList :: [Tuple], Tuple :: tuple().

Returns `true`

if there is a tuple in `TupleList`

whose `N`

th element compares
equal to `Key`

, otherwise `false`

.

-spec keymerge(N, TupleList1, TupleList2) -> TupleList3 when N :: pos_integer(), TupleList1 :: [T1], TupleList2 :: [T2], TupleList3 :: [T1 | T2], T1 :: Tuple, 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`

.

-spec keyreplace(Key, N, TupleList1, NewTuple) -> TupleList2 when Key :: term(), N :: pos_integer(), TupleList1 :: [Tuple], TupleList2 :: [Tuple], 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`

.

-spec keysearch(Key, N, TupleList) -> {value, Tuple} | false when Key :: term(), N :: pos_integer(), 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. Function

`keyfind/3`

is usually more convenient.

-spec keysort(N, TupleList1) -> TupleList2 when N :: pos_integer(), TupleList1 :: [Tuple], TupleList2 :: [Tuple], 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.

-spec keystore(Key, N, TupleList1, NewTuple) -> TupleList2 when Key :: term(), N :: pos_integer(), TupleList1 :: [Tuple], 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.

-spec keytake(Key, N, TupleList1) -> {value, Tuple, TupleList2} | false when Key :: term(), N :: pos_integer(), TupleList1 :: [tuple()], 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.

-spec last(List) -> Last when List :: [T, ...], Last :: T, T :: term().

Returns the last element in `List`

.

-spec map(Fun, List1) -> List2 when Fun :: fun((A) -> B), List1 :: [A], List2 :: [B], A :: term(), 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.

-spec mapfoldl(Fun, Acc0, List1) -> {List2, Acc1} when Fun :: fun((A, AccIn) -> {B, AccOut}), Acc0 :: term(), Acc1 :: term(), AccIn :: term(), AccOut :: term(), List1 :: [A], List2 :: [B], A :: term(), 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}
```

-spec max(List) -> Max when List :: [T, ...], Max :: T, T :: term().

Returns the first element of `List`

that compares greater than or equal to all
other elements of `List`

.

Returns `true`

if `Elem`

matches some element of `List`

, otherwise `false`

.

-spec merge3(List1, List2, List3) -> List4 when List1 :: [X], List2 :: [Y], List3 :: [Z], List4 :: [X | Y | Z], X :: term(), Y :: term(), 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`

.

-spec merge(ListOfLists) -> List1 when ListOfLists :: [List], List :: [T], 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.

-spec merge(List1, List2) -> List3 when List1 :: [X], List2 :: [Y], List3 :: [X | Y], X :: term(), 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`

.

-spec merge(Fun, List1, List2) -> List3 when Fun :: fun((A, B) -> boolean()), List1 :: [A], List2 :: [B], List3 :: [A | B], A :: term(), 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`

.

-spec min(List) -> Min when List :: [T, ...], Min :: T, T :: term().

Returns the first element of `List`

that compares less than or equal to all
other elements of `List`

.

-spec nth(N, List) -> Elem when N :: pos_integer(), List :: [T, ...], Elem :: T, T :: term().

Returns the `N`

th element of `List`

.

*Example:*

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

-spec nthtail(N, List) -> Tail when N :: non_neg_integer(), List :: [T, ...], 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]).
[]
```

-spec partition(Pred, List) -> {Satisfying, NotSatisfying} when Pred :: fun((Elem :: T) -> boolean()), List :: [T], Satisfying :: [T], 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`

.

Returns `true`

if `List1`

is a prefix of `List2`

, otherwise `false`

.

-spec reverse(List1) -> List2 when List1 :: [T], List2 :: [T], T :: term().

Returns a list with the elements in `List1`

in reverse order.

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]
```

-spec search(Pred, List) -> {value, Value} | false when Pred :: fun((T) -> boolean()), List :: [T], Value :: T.

If there is a `Value`

in `List`

such that `Pred(Value)`

returns `true`

, returns
`{value, Value}`

for the first such `Value`

, otherwise returns `false`

. The
`Pred`

function must return a boolean.

Equivalent to `seq(From, To, 1)`

.

-spec seq(From, To, Incr) -> Seq when From :: integer(), To :: integer(), Incr :: integer(), 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]
```

-spec sort(List1) -> List2 when List1 :: [T], List2 :: [T], T :: term().

Returns a list containing the sorted elements of `List1`

.

-spec sort(Fun, List1) -> List2 when Fun :: fun((A :: T, B :: T) -> boolean()), List1 :: [T], 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`

.

-spec split(N, List1) -> {List2, List3} when N :: non_neg_integer(), List1 :: [T], List2 :: [T], List3 :: [T], T :: term().

Splits `List1`

into `List2`

and `List3`

. `List2`

contains the first `N`

elements
and `List3`

the remaining elements (the `N`

th tail).

-spec splitwith(Pred, List) -> {List1, List2} when Pred :: fun((T) -> boolean()), List :: [T], List1 :: [T], 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]}
```

The `Pred`

function must return a boolean. For a different way to partition a
list, see `partition/2`

.

-spec sublist(List1, Len) -> List2 when List1 :: [T], List2 :: [T], Len :: non_neg_integer(), 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.

-spec sublist(List1, Start, Len) -> List2 when List1 :: [T], List2 :: [T], Start :: pos_integer(), Len :: non_neg_integer(), 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).
[]
```

-spec subtract(List1, List2) -> List3 when List1 :: [T], List2 :: [T], List3 :: [T], 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`

.

Returns `true`

if `List1`

is a suffix of `List2`

, otherwise `false`

.

Returns the sum of the elements in `List`

.

-spec takewhile(Pred, List1) -> List2 when Pred :: fun((Elem :: T) -> boolean()), List1 :: [T], 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. The `Pred`

function must return a boolean.

-spec ukeymerge(N, TupleList1, TupleList2) -> TupleList3 when N :: pos_integer(), TupleList1 :: [T1], TupleList2 :: [T2], TupleList3 :: [T1 | T2], T1 :: Tuple, 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.

-spec ukeysort(N, TupleList1) -> TupleList2 when N :: pos_integer(), TupleList1 :: [Tuple], TupleList2 :: [Tuple], 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.

-spec umerge3(List1, List2, List3) -> List4 when List1 :: [X], List2 :: [Y], List3 :: [Z], List4 :: [X | Y | Z], X :: term(), Y :: term(), 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.

-spec umerge(ListOfLists) -> List1 when ListOfLists :: [List], List :: [T], 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.

-spec umerge(List1, List2) -> List3 when List1 :: [X], List2 :: [Y], List3 :: [X | Y], X :: term(), 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.

-spec umerge(Fun, List1, List2) -> List3 when Fun :: fun((A, B) -> boolean()), List1 :: [A], List2 :: [B], List3 :: [A | B], A :: term(), 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.

-spec uniq(List1) -> List2 when List1 :: [T], List2 :: [T], T :: term().

Returns a list containing the elements of `List1`

with duplicated elements
removed (preserving the order of the elements). The first occurrence of each
element is kept.

*Examples:*

```
> lists:uniq([3,3,1,2,1,2,3]).
[3,1,2]
> lists:uniq([a, a, 1, b, 2, a, 3]).
[a, 1, b, 2, 3]
```

-spec uniq(Fun, List1) -> List2 when Fun :: fun((T) -> any()), List1 :: [T], List2 :: [T], T :: term().

Returns a list containing the elements of `List1`

without the elements for which
`Fun`

returned duplicate values (preserving the order of the elements). The
first occurrence of each element is kept.

*Examples:*

```
> lists:uniq(fun({X, _}) -> X end, [{b, 2}, {a, 1}, {c, 3}, {a, 2}]).
[{b, 2}, {a, 1}, {c, 3}]
```

-spec unzip3(List1) -> {List2, List3, List4} when List1 :: [{A, B, C}], List2 :: [A], List3 :: [B], List4 :: [C], A :: term(), B :: term(), 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.

-spec unzip(List1) -> {List2, List3} when List1 :: [{A, B}], List2 :: [A], List3 :: [B], A :: term(), 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.

-spec usort(List1) -> List2 when List1 :: [T], List2 :: [T], 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.

-spec usort(Fun, List1) -> List2 when Fun :: fun((T, T) -> boolean()), List1 :: [T], 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`

.

-spec zip3(List1, List2, List3) -> List4 when List1 :: [A], List2 :: [B], List3 :: [C], List4 :: [{A, B, C}], A :: term(), B :: term(), C :: term().

Equivalent to `zip3(List1, List2, List3, fail)`

.

-spec zip3(List1, List2, List3, How) -> List4 when List1 :: [A], List2 :: [B], List3 :: [C], List4 :: [{A | DefaultA, B | DefaultB, C | DefaultC}], A :: term(), B :: term(), C :: term(), How :: fail | trim | {pad, {DefaultA, DefaultB, DefaultC}}, DefaultA :: term(), DefaultB :: term(), DefaultC :: term().

"Zips" three lists 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.

For a description of the `How`

parameter, see `zip/3`

.

-spec zip(List1, List2) -> List3 when List1 :: [A], List2 :: [B], List3 :: [{A, B}], A :: term(), B :: term().

Equivalent to `zip(List1, List2, fail)`

.

-spec zip(List1, List2, How) -> List3 when List1 :: [A], List2 :: [B], List3 :: [{A | DefaultA, B | DefaultB}], A :: term(), B :: term(), How :: fail | trim | {pad, {DefaultA, DefaultB}}, DefaultA :: term(), DefaultB :: term().

"Zips" two lists 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.

The `How`

parameter specifies the behavior if the given lists are of different
lengths.

- The call will fail if the given lists are not of equal length. This is the default.`fail`

- Surplus elements from the longer list will be ignored.`trim`

*Examples:*`> lists:zip([a, b], [1, 2, 3], trim). [{a,1},{b,2}] > lists:zip([a, b, c], [1, 2], trim). [{a,1},{b,2}]`

- The shorter list will be padded to the length of the longer list, using the respective elements from the given`{pad, Defaults}`

`Defaults`

tuple.*Examples:*`> lists:zip([a, b], [1, 2, 3], {pad, {x, 0}}). [{a,1},{b,2},{x,3}] > lists:zip([a, b, c], [1, 2], {pad, {x, 0}}). [{a,1},{b,2},{c,0}]`

-spec zipwith3(Combine, List1, List2, List3) -> List4 when Combine :: fun((X, Y, Z) -> T), List1 :: [X], List2 :: [Y], List3 :: [Z], List4 :: [T], X :: term(), Y :: term(), Z :: term(), T :: term().

Equivalent to `zipwith3(Combine, List1, List2, List3, fail)`

.

-spec zipwith3(Combine, List1, List2, List3, How) -> List4 when Combine :: fun((X | DefaultX, Y | DefaultY, Z | DefaultZ) -> T), List1 :: [X], List2 :: [Y], List3 :: [Z], List4 :: [T], X :: term(), Y :: term(), Z :: term(), How :: fail | trim | {pad, {DefaultX, DefaultY, DefaultZ}}, DefaultX :: term(), DefaultY :: term(), DefaultZ :: term(), T :: term().

Combines the elements of three lists 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)`

.

For a description of the `How`

parameter, see `zip/3`

.

`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]]
```

-spec zipwith(Combine, List1, List2) -> List3 when Combine :: fun((X, Y) -> T), List1 :: [X], List2 :: [Y], List3 :: [T], X :: term(), Y :: term(), T :: term().

Equivalent to `zipwith(Combine, List1, List2, fail)`

.

-spec zipwith(Combine, List1, List2, How) -> List3 when Combine :: fun((X | DefaultX, Y | DefaultY) -> T), List1 :: [X], List2 :: [Y], List3 :: [T], X :: term(), Y :: term(), How :: fail | trim | {pad, {DefaultX, DefaultY}}, DefaultX :: term(), DefaultY :: term(), T :: term().

Combines the elements of two lists 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)`

.

For a description of the `How`

parameter, see `zip/3`

.

`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]
```