This function is retained for backward compatibility. Function keyfind/3
is usually more convenient.
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 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
.
Returns true
if Pred(Elem)
returns true
for all elements Elem
in List
, otherwise false
.
Returns true
if Pred(Elem)
returns true
for at least one element Elem
in List
.
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]
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
.
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"
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.
Returns a list containing N
copies of term Elem
.
Example:
> lists:duplicate(5, xx). [xx,xx,xx,xx,xx]
List2
is a list of all elements Elem
in List1
for which Pred(Elem)
returns true
.
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]
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.
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
.
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.
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
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
.
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, []). []
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.
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.
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
.
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}]
1..tuple_size(Tuple)
Returns true
if there is a tuple in TupleList
whose N
th element compares equal to Key
, otherwise false
.
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
.
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
.
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
.
This function is retained for backward compatibility. Function keyfind/3
is usually more convenient.
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.
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.
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.
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.
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}
Combines the operations of map/2
and foldr/3
into one pass.
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 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.
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
.
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
.
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
.
Returns the first element of List
that compares less than or equal to all other elements of List
.
1..length(List)
Returns the N
th element of List
.
Example:
> lists:nth(3, [a, b, c, d, e]). c
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]). []
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
.
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]
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]
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
.
0..length(List1)
Splits List1
into List2
and List3
. List2
contains the first N
elements and List3
the remaining elements (the N
th tail).
If there is a Value
in List
such that Pred(Value)
returns true
, returns {value, Value}
for the first such Value
, otherwise returns false
.
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
.
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.
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). []
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
.
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.
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.
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.
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.
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.
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.
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.
"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.
"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.
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
.
"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.
"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.
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]
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]]
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Licensed under the Apache License, Version 2.0.