- Documentation
- Reference manual
- The SWI-Prolog library
- library(aggregate): Aggregation operators on backtrackable predicates
- library(apply): Apply predicates on a list
- library(assoc): Association lists
- library(broadcast): Broadcast and receive event notifications
- library(charsio): I/O on Lists of Character Codes
- library(check): Consistency checking
- library(clpb): CLP(B): Constraint Logic Programming over Boolean Variables
- library(clpfd): CLP(FD): Constraint Logic Programming over Finite Domains
- library(clpqr): Constraint Logic Programming over Rationals and Reals
- library(csv): Process CSV (Comma-Separated Values) data
- library(debug): Print debug messages and test assertions
- library(error): Error generating support
- library(gensym): Generate unique identifiers
- library(iostream): Utilities to deal with streams
- library(lists): List Manipulation
- member/2
- append/3
- append/2
- prefix/2
- select/3
- selectchk/3
- select/4
- selectchk/4
- nextto/3
- delete/3
- nth0/3
- nth1/3
- nth0/4
- nth1/4
- last/2
- proper_length/2
- same_length/2
- reverse/2
- permutation/2
- flatten/2
- max_member/2
- min_member/2
- sum_list/2
- max_list/2
- min_list/2
- numlist/3
- is_set/1
- list_to_set/2
- intersection/3
- union/3
- subset/2
- subtract/3

- library(main): Provide entry point for scripts
- library(nb_set): Non-backtrackable set
- library(www_browser): Activating your Web-browser
- library(option): Option list processing
- library(optparse): command line parsing
- library(ordsets): Ordered set manipulation
- library(pairs): Operations on key-value lists
- library(persistency): Provide persistent dynamic predicates
- library(pio): Pure I/O
- library(predicate_options): Declare option-processing of predicates
- library(prolog_pack): A package manager for Prolog
- library(prolog_xref): Cross-reference data collection library
- library(quasi_quotations): Define Quasi Quotation syntax
- library(random): Random numbers
- library(readutil): Reading lines, streams and files
- library(record): Access named fields in a term
- library(registry): Manipulating the Windows registry
- library(simplex): Solve linear programming problems
- library(solution_sequences): Modify solution sequences
- library(tabling): Tabled execution (SLG)
- library(thread_pool): Resource bounded thread management
- library(ugraphs): Unweighted Graphs
- library(url): Analysing and constructing URL
- library(varnumbers): Utilities for numbered terms
- library(yall): Lambda expressions

- The SWI-Prolog library
- Packages

- Reference manual

## A.15 library(lists): List Manipulation

- Compatibility
- Virtually every Prolog system has
`library(lists)`

, but the set of provided predicates is diverse. There is a fair agreement on the semantics of most of these predicates, although error handling may vary.

This library provides commonly accepted basic predicates for list manipulation in the Prolog community. Some additional list manipulations are built-in. See e.g., memberchk/2, length/2.

The implementation of this library is copied from many places. These include: "The Craft of Prolog", the DEC-10 Prolog library (LISTRO.PL) and the YAP lists library. Some predicates are reimplemented based on their specification by Quintus and SICStus.

**member**(`?Elem, ?List`)- True if
`Elem`is a member of`List`. The SWI-Prolog definition differs from the classical one. Our definition avoids unpacking each list element twice and provides determinism on the last element. E.g. this is deterministic:member(X, [One]).

- author
- Gertjan van Noord

**append**(`?List1, ?List2, ?List1AndList2`)`List1AndList2`is the concatenation of`List1`and`List2`**append**(`+ListOfLists, ?List`)- Concatenate a list of lists. Is true if
`ListOfLists`is a list of lists, and`List`is the concatenation of these lists.`ListOfLists`must be a list of *possibly*partial lists **prefix**(`?Part, ?Whole`)- True iff
`Part`is a leading substring of`Whole`. This is the same as`append(Part, _, Whole)`

. **select**(`?Elem, ?List1, ?List2`)- Is true when
`List1`, with`Elem`removed, results in`List2`. - [semidet]
**selectchk**(`+Elem, +List, -Rest`) - Semi-deterministic removal of first element in
`List`that unifies with`Elem`. - [nondet]
**select**(`?X, ?XList, ?Y, ?YList`) - Select from two lists at the same positon. True if
`XList`is unifiable with`YList`apart a single element at the same position that is unified with`X`in`XList`and with`Y`in`YList`. A typical use for this predicate is to*replace*an element, as shown in the example below. All possible substitutions are performed on backtracking.?- select(b, [a,b,c,b], 2, X). X = [a, 2, c, b] ; X = [a, b, c, 2] ; false.

- See also
- selectchk/4 provides a semidet version.

- [semidet]
**selectchk**(`?X, ?XList, ?Y, ?YList`) - Semi-deterministic version of select/4.
**nextto**(`?X, ?Y, ?List`)- True if
`Y`directly follows`X`in`List`. - [det]
**delete**(`+List1, @Elem, -List2`) - Delete matching elements from a list. True when
`List2`is a list with all elements from`List1`except for those that unify with`Elem`. Matching`Elem`with elements of`List1`is uses`\+ Elem \= H`

, which implies that`Elem`is not changed.- See also
- select/3, subtract/3.
- deprecated
- There are too many ways in which one might want to delete elements from
a list to justify the name. Think of matching (= vs.
`==`

), delete first/all, be deterministic or not.

**nth0**(`?Index, ?List, ?Elem`)- True when
`Elem`is the`Index`'th element of`List`. Counting starts at 0.- Errors
`type_error(integer, Index)`

if`Index`is not an integer or unbound.- See also
- nth1/3.

**nth1**(`?Index, ?List, ?Elem`)- Is true when
`Elem`is the`Index`'th element of`List`. Counting starts at 1.- See also
- nth0/3.

- [det]
**nth0**(`?N, ?List, ?Elem, ?Rest`) - Select/insert element at index. True when
`Elem`is the`N`'th (0-based) element of`List`and`Rest`is the remainder (as in by select/3) of`List`. For example:?- nth0(I, [a,b,c], E, R). I = 0, E = a, R = [b, c] ; I = 1, E = b, R = [a, c] ; I = 2, E = c, R = [a, b] ; false.

?- nth0(1, L, a1, [a,b]). L = [a, a1, b].

- [det]
**nth1**(`?N, ?List, ?Elem, ?Rest`) - As nth0/4, but counting starts at 1.
**last**(`?List, ?Last`)- Succeeds when
`Last`is the last element of`List`. This predicate is`semidet`

if`List`is a list and`multi`

if`List`is a partial list.- Compatibility
- There is no de-facto standard for the argument order of
last/2. Be careful when
porting code or use
`append(_, [Last], List)`

as a portable alternative.

- [semidet]
**proper_length**(`@List, -Length`) - True when
`Length`is the number of elements in the proper list`List`. This is equivalent toproper_length(List, Length) :- is_list(List), length(List, Length).

**same_length**(`?List1, ?List2`)- Is true when
`List1`and`List2`are lists with the same number of elements. The predicate is deterministic if at least one of the arguments is a proper list. It is non-deterministic if both arguments are partial lists.- See also
- length/2

**reverse**(`?List1, ?List2`)- Is true when the elements of
`List2`are in reverse order compared to`List1`. - [nondet]
**permutation**(`?Xs, ?Ys`) - True when
`Xs`is a permutation of`Ys`. This can solve for`Ys`given`Xs`or`Xs`given`Ys`, or even enumerate`Xs`and`Ys`together. The predicate permutation/2 is primarily intended to generate permutations. Note that a list of length N has N! permutations, and unbounded permutation generation becomes prohibitively expensive, even for rather short lists (10! = 3,628,800).If both

`Xs`and`Ys`are provided and both lists have equal length the order is`|`

`Xs``|`

`^`

2. Simply testing whether`Xs`is a permutation of`Ys`can be achieved in order log(`|`

`Xs``|`

) using msort/2 as illustrated below with the`semidet`

predicate is_permutation/2:is_permutation(Xs, Ys) :- msort(Xs, Sorted), msort(Ys, Sorted).

The example below illustrates that

`Xs`and`Ys`being proper lists is not a sufficient condition to use the above replacement.?- permutation([1,2], [X,Y]). X = 1, Y = 2 ; X = 2, Y = 1 ; false.

- Errors
`type_error(list, Arg)`

if either argument is not a proper or partial list.

- [det]
**flatten**(`+NestedList, -FlatList`) - Is true if
`FlatList`is a non-nested version of`NestedList`. Note that empty lists are removed. In standard Prolog, this implies that the atom '`[]`

' is removed too. In SWI7,`[]`

is distinct from '`[]`

'.Ending up needing flatten/2 often indicates, like append/3 for appending two lists, a bad design. Efficient code that generates lists from generated small lists must use difference lists, often possible through grammar rules for optimal readability.

- See also
- append/2

- [semidet]
**max_member**(`-Max, +List`) - True when
`Max`is the largest member in the standard order of terms. Fails if`List`is empty.- See also
- - compare/3

- max_list/2 for the maximum of a list of numbers.

- [semidet]
**min_member**(`-Min, +List`) - True when
`Min`is the smallest member in the standard order of terms. Fails if`List`is empty.- See also
- - compare/3

- min_list/2 for the minimum of a list of numbers.

- [det]
**sum_list**(`+List, -Sum`) `Sum`is the result of adding all numbers in`List`.- [semidet]
**max_list**(`+List:list(number), -Max:number`) - True if
`Max`is the largest number in`List`. Fails if`List`is empty.- See also
- max_member/2.

- [semidet]
**min_list**(`+List:list(number), -Min:number`) - True if
`Min`is the smallest number in`List`. Fails if`List`is empty.- See also
- min_member/2.

- [semidet]
**numlist**(`+Low, +High, -List`) `List`is a list [`Low`,`Low`+1, ...`High`]. Fails if`High``<``Low`.- Errors
- -
`type_error(integer, Low)`

-`type_error(integer, High)`

- [semidet]
**is_set**(`@Set`) - True if
`Set`is a proper list without duplicates. Equivalence is based on ==/2. The implementation uses sort/2, which implies that the complexity is N*`log(N)`

and the predicate may cause a resource-error. There are no other error conditions. - [det]
**list_to_set**(`+List, ?Set`) - True when
`Set`has the same elements as`List`in the same order. The left-most copy of duplicate elements is retained.`List`may contain variables. Elements*E1*and*E2*are considered duplicates iff*E1*`==`

*E2*holds. The complexity of the implementation is N*`log(N)`

.- Errors
`List`is type-checked.- See also
- sort/2 can be used to
create an ordered set. Many set operations on ordered sets are order N
rather than order N
`**`

2. The list_to_set/2 predicate is more expensive than sort/2 because it involves, two sorts and a linear scan. - Compatibility
- Up to version 6.3.11, list_to_set/2
had complexity N
`**`

2 and equality was tested using =/2.

- [det]
**intersection**(`+Set1, +Set2, -Set3`) - True if
`Set3`unifies with the intersection of`Set1`and`Set2`. The complexity of this predicate is`|`

`Set1``|`

*`|`

`Set2``|`

- See also
- ord_intersection/3.

- [det]
**union**(`+Set1, +Set2, -Set3`) - True if
`Set3`unifies with the union of`Set1`and`Set2`. The complexity of this predicate is`|`

`Set1``|`

*`|`

`Set2``|`

- See also
- ord_union/3.

- [semidet]
**subset**(`+SubSet, +Set`) - True if all elements of
`SubSet`belong to`Set`as well. Membership test is based on memberchk/2. The complexity is`|`

`SubSet``|`

*`|`

`Set``|`

.- See also
- ord_subset/2.

- [det]
**subtract**(`+Set, +Delete, -Result`) `Delete`all elements in`Delete`from`Set`. Deletion is based on unification using memberchk/2. The complexity is`|`

`Delete``|`

*`|`

`Set``|`

.- See also
- ord_subtract/3.