# lists

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 <a name="ordering_function"></a>*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**.



# Functions



## all/2

Return **true** if all elements in a list satisfy **Pred**.

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



## any/2

Return **true** if any of the elements in a list satisfies **Pred**.

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



## append/1

Append a list of lists.

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/2

Append two lists.

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/1

Concatenate a list of atoms.

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/2

Delete an element from a list.

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



## droplast/1

Drop the last element of a list.

Drops the last element of a **List**. The list is to
be non-empty, otherwise the function crashes with a
**function_clause**.



## dropwhile/2

Drop elements from a list while a predicate is **true**.

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



## duplicate/2

Make **N** copies of element.

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

*Example:*



```

> lists:duplicate(5, xx).
[xx,xx,xx,xx,xx]
```



## filter/2

Select elements that satisfy a predicate.

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



## filtermap/2

Filter and map elements that satisfy a function.

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/1

Length of flattened deep list.

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



## flatmap/2

Map and flatten in one pass.

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/1

Flatten a deep list.

Returns a flattened version of **DeepList**.



## flatten/2

Flatten a deep list.

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



## foldl/3

Fold a function over a list.

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/3

Fold a function over a list.

Like [foldl/3](#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/2

Insert an element between elements in a list

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/2

Apply a function to each element of a list.

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/3

Delete an element from a list of tuples.

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/3

Search for an element in a list of tuples.

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/3

Map a function over a list of tuples.

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/3

Test for membership of a list of tuples.

Returns **true** if there is a tuple in
**TupleList** whose **N**th element
compares equal to **Key**, otherwise **false**.



## keymerge/3

Merge two key-sorted lists of tuples.

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/4

Replace an element in a list of tuples.

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/3

Search for an element in a list of tuples.

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**.

This function is retained for backward compatibility. Function
[keyfind/3](#keyfind/3)
is usually more convenient.



## keysort/2

Sort a list of tuples.

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/4

Store an element in a list of tuples.

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/3

Extract an element from a list of tuples.

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/1

Return last element in a list.

Returns the last element in **List**.



## map/2

Map a function over a 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.



## mapfoldl/3

Map and fold in one pass.

Combines the operations of
[map/2](#map/2) and 
[foldl/3](#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/3

Map and fold in one pass.

Combines the operations of
[map/2](#map/2) and 
[foldr/3](#foldr/3) into one pass.



## max/1

Return maximum element of a list.

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



## member/2

Test for membership of a list.

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



## merge/1

Merge a list of sorted lists.

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/2

Merge two sorted lists.

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/3

Merge two sorted list.

Returns the sorted list formed by merging **List1**
and **List2**. Both **List1** and
**List2** must be sorted according to the [ordering function](#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**.



## merge3/3

Merge three sorted lists.

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/1

Return minimum element of a list.

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



## nth/2

Return the **N**th element of a list.

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

*Example:*



```

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



## nthtail/2

Return the **N**th tail of a 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/2

Partition a list into two lists based on a predicate.

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](#splitwith/2).



## prefix/2

Test for list prefix.

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



## reverse/1

Reverse a list.

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



## reverse/2

Reverse a list appending a tail.

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/2



## seq/3

Generate a sequence of integers.

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]
```



## sort/1

Sort a list.

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



## sort/2

Sort a list.

Returns a list containing the sorted elements of
**List1**, according to the [ordering function](#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/2

Split a list into two lists.

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



## search/2

Find the first element that satisfies a predicate.

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/2

Split a list into two lists based on a predicate.

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]}
```

For a different way to partition a list, see 
[partition/2](#partition/2).



## sublist/2

Return a sublist of a certain length, starting at the first position.

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/3

Return a sublist starting at a specified position and with a specified number of elements.

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/2

Subtract the element in one list from another 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.

*Example:*



```

> lists:subtract("123212", "212").
"312".
```

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

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](./ordsets#subtract/2).



## suffix/2

Test for list suffix.

Returns **true** if **List1** is a suffix of
**List2**, otherwise **false**.



## sum/1

Return the sum of elements in a list.

Returns the sum of the elements in **List**.



## takewhile/2

Take elements from a list while a predicate is **true**.

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/3

Merge two key-sorted lists of tuples, removing duplicates.

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/2

Sort a list of tuples, removing duplicates.

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/1

Merge a list of sorted lists, removing duplicates.

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/2

Merge two sorted lists, removing duplicates.

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/3

Merge two sorted lists, removing duplicates.

Returns the sorted list formed by merging **List1**
and **List2**. Both **List1** and
**List2** must be sorted according to the [ordering function](#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.



## umerge3/3

Merge three sorted lists, removing duplicates.

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/1

Unzip a list of two-tuples into two lists.

"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.



## unzip3/1

Unzip a list of three-tuples into three lists.

"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/1

Sort a list, removing duplicates.

Returns a list containing the sorted elements of
**List1** where all except the first element of the
elements comparing equal have been deleted.



## usort/2

Sort a list, removing duplicates.

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](#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/2

Zip two lists into a list of two-tuples.

"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.



## zip3/3

Zip three lists into a list of three-tuples.

"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/3

Zip two lists into one list according to a fun.

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]
```



## zipwith3/4

Zip three lists into one list according to a fun.

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]]
```