## Unit data-structures

This unit contains a collection of procedures related to data structures. This unit is used by default, unless the program is compiled with the `-explicit-use` option.

### Lists

#### alist-ref

`alist-ref``KEY``ALIST``#!optional``TEST``DEFAULT`procedureLooks up

`KEY`in`ALIST`using`TEST`as the comparison function (or`eqv?`if no test was given) and returns the cdr of the found pair, or`DEFAULT`(which defaults to`#f`).

#### alist-update

`alist-update``KEY``VALUE``ALIST``#!optional``TEST`procedure`alist-update!``KEY``VALUE``ALIST``#!optional``TEST`procedureIf the list

`ALIST`contains a pair of the form`(KEY . X)`, then this procedure replaces`X`with`VALUE`and returns`ALIST`. If`ALIST`contains no such item, then`alist-update`returns`((KEY . VALUE) . ALIST)`. The optional argument`TEST`specifies the comparison procedure to search a matching pair in`ALIST`and defaults to`eqv?`.`alist-update!`is the destructive version of`alist-update`.

#### atom?

`atom?``X`procedureReturns

`#t`if`X`is not a pair. This is identical to`not-pair?`from Unit srfi-1 but kept for historical reasons.

#### rassoc

`rassoc``KEY``LIST``#!optional``TEST`procedureSimilar to

`assoc`, but compares`KEY`with the`cdr`of each pair in`LIST`using`TEST`as the comparison procedures (which defaults to`eqv?`.

#### butlast

`butlast``LIST`procedureReturns a fresh list with all elements but the last of

`LIST`.

#### chop

`chop``LIST``N`procedureReturns a new list of sublists, where each sublist contains

`N`elements of`LIST`. If`LIST`has a length that is not a multiple of`N`, then the last sublist contains the remaining elements.(chop '(1 2 3 4 5 6) 2) ==> ((1 2) (3 4) (5 6)) (chop '(a b c d) 3) ==> ((a b c) (d))

#### compress

`compress``BLIST``LIST`procedureReturns a new list with elements taken from

`LIST`with corresponding true values in the list`BLIST`.(define nums '(99 100 110 401 1234)) (compress (map odd? nums) nums) ==> (99 401)

#### flatten

`flatten``LIST1``...`procedureReturns

`LIST1 ...`concatenated together, with nested lists removed (flattened).

#### intersperse

`intersperse``LIST``X`procedureReturns a new list with

`X`placed between each element.

#### join

`join``LISTOFLISTS``#!optional``LIST`procedureConcatenates the lists in

`LISTOFLISTS`with`LIST`placed between each sublist.`LIST`defaults to the empty list.(join '((a b) (c d) (e)) '(x y)) ==> (a b x y c d x y e) (join '((p q) () (r (s) t)) '(-)) ==> (p q - - r (s) t)

`join`could be implemented as follows:(define (join lstoflsts #!optional (lst '())) (apply append (intersperse lstoflists lst)) )

#### tail?

`tail?``X``LIST`procedureReturns true if

`X`is one of the tails (cdr's) of`LIST`.

### Queues

#### list->queue

`list->queue``LIST`procedureReturns

`LIST`converted into a queue, where the first element of the list is the same as the first element of the queue. The resulting queue may share memory with the list and the list should not be modified after this operation.

#### make-queue

`make-queue`procedureReturns a newly created queue.

#### queue?

`queue?``X`procedureReturns

`#t`if`X`is a queue, or`#f`otherwise.

#### queue-length

`queue-length``QUEUE`procedureReturns the current number of items stored in

`QUEUE`.

#### queue->list

`queue->list``QUEUE`procedureReturns

`QUEUE`converted into a list, where the first element of the list is the same as the first element of the queue. The resulting list is freshly allocated and does not share memory with the queue object.

#### queue-add!

`queue-add!``QUEUE``X`procedureAdds

`X`to the rear of`QUEUE`.

#### queue-empty?

`queue-empty?``QUEUE`procedureReturns

`#t`if`QUEUE`is empty, or`#f`otherwise.

#### queue-first

`queue-first``QUEUE`procedureReturns the first element of

`QUEUE`. If`QUEUE`is empty an error is signaled

#### queue-last

`queue-last``QUEUE`procedureReturns the last element of

`QUEUE`. If`QUEUE`is empty an error is signaled

#### queue-remove!

`queue-remove!``QUEUE`procedureRemoves and returns the first element of

`QUEUE`. If`QUEUE`is empty an error is signaled

#### queue-push-back!

`queue-push-back!``QUEUE``ITEM`procedurePushes an item into the first position of a queue, i.e. the next

`queue-remove!`will return`ITEM`.

#### queue-push-back-list!

`queue-push-back-list!``QUEUE``LIST`procedurePushes the items in item-list back onto the queue, so that

`(car LIST)`becomes the next removable item.

### Sorting

#### merge

`merge``LIST1``LIST2``LESS?`procedure`merge!``LIST1``LIST2``LESS?`procedureJoins two lists in sorted order.

`merge!`is the destructive version of merge.`LESS?`should be a procedure of two arguments, that returns true if the first argument is to be ordered before the second argument.

#### sort

`sort``SEQUENCE``LESS?`procedure`sort!``SEQUENCE``LESS?`procedureSort

`SEQUENCE`, which should be a list or a vector.`sort!`is the destructive version of sort.

#### sorted?

`sorted?``SEQUENCE``LESS?`procedureReturns true if the list or vector

`SEQUENCE`is already sorted.

#### topological-sort

`topological-sort``DAG``PRED`procedureSorts the directed acyclic graph dag

`DAG`so that for every edge from vertex u to v, u will come before v in the resulting list of vertices.`DAG`is a list of sublists. The car of each sublist is a vertex. The cdr is the adjacency list of that vertex, i.e. a list of all vertices to which there exists an edge from the car vertex.`pred`is procedure of two arguments that should compare vertices for equality.Time complexity: O (|V| + |E|)

(topological-sort '((shirt tie belt) (tie jacket) (belt jacket) (watch) (pants shoes belt) (undershorts pants shoes) (socks shoes)) eq?) => (socks undershorts pants shoes watch shirt belt tie jacket)

If a cycle is detected during the sorting process, an exception of the condition kinds

`(exn runtime cycle)`is thrown.

### Strings

#### conc

`conc``X``...`procedureReturns a string with the string-represenation of all arguments concatenated together.

`conc`could be implemented as(define (conc . args) (apply string-append (map ->string args)) )

#### ->string

`->string``X`procedureReturns a string-representation of

`X`.

#### string-chop

`string-chop``STRING``LENGTH`procedureReturns a list of substrings taken by

*chopping*`STRING`every`LENGTH`characters:(string-chop "one two three" 4) ==> ("one " "two " "thre" "e")

#### string-chomp

`string-chomp``STRING``#!optional``SUFFIX`procedureIf

`STRING`ends with`SUFFIX`, then this procedure returns a copy of its first argument with the suffix removed, otherwise returns`STRING`unchanged.`SUFFIX`defaults to`"\n"`.

#### string-compare3

`string-compare3``STRING1``STRING2`procedure`string-compare3-ci``STRING1``STRING2`procedurePerform a three-way comparison between the

`STRING1`and`STRING2`, returning either`-1`if`STRING1`is lexicographically less than`STRING2`,`0`if it is equal, or`1`if it s greater.`string-compare3-ci`performs a case-insensitive comparison.

#### string-intersperse

`string-intersperse``LIST``#!optional``STRING`procedureReturns a string that contains all strings in

`LIST`concatenated together.`STRING`is placed between each concatenated string and defaults to`" "`.(string-intersperse '("one" "two") "three")

is equivalent to

(apply string-append (intersperse '("one" "two") "three"))

#### string-split

`string-split``STRING``#!optional``DELIMITER-STRING``KEEPEMPTY`procedureSplit string into substrings delimited by any of the characters given in the delimiter string. If no delimiters are specified, a string comprising the tab, newline and space characters is assumed. If the parameter

`KEEPEMPTY`is given and not`#f`, then empty substrings are retained:(string-split "one two three") ==> ("one" "two" "three") (string-split "foo:bar::baz:" ":" #t) ==> ("foo" "bar" "" "baz" "") (string-split "foo:bar:baz,quux,zot" ":," ) ==> ("foo" "bar" "baz" "quux" "zot")

#### string-translate

`string-translate``STRING``FROM``#!optional``TO`procedureReturns a fresh copy of

`STRING`with characters matching`FROM`translated to`TO`. If`TO`is omitted, then matching characters are removed.`FROM`and`TO`may be a character, a string or a list. If both`FROM`and`TO`are strings, then the character at the same position in`TO`as the matching character in`FROM`is substituted.

#### string-translate*

`string-translate*``STRING``SMAP`procedureSubstitutes elements of

`STRING`according to`SMAP`.`SMAP`should be an association-list where each element of the list is a pair of the form`(MATCH . REPLACEMENT)`. Every occurrence of the string`MATCH`in`STRING`will be replaced by the string`REPLACEMENT`:(string-translate* "<h1>this is a \"string\"</h1>" '(("<" . "<") (">" . ">") ("\"" . """)) ) => "<h1>this is a "string"</h1>"

#### substring=?

`substring=?``STRING1``STRING2``#!optional``START1``START2``LENGTH`procedure`substring-ci=?``STRING1``STRING2``#!optional``START1``START2``LENGTH`procedureReturns

`#t`if the strings`STRING1`and`STRING2`are equal, or`#f`otherwise. The comparison starts at the positions`START1`and`START2`(which default to 0), comparing`LENGTH`characters (which defaults to the minimum of the remaining length of both strings).

#### substring-index

`substring-index``WHICH``WHERE``#!optional``START`procedure`substring-index-ci``WHICH``WHERE``#!optional``START`procedureSearches for first index in string

`WHERE`where string`WHICH`occurs. If the optional argument`START`is given, then the search starts at that index.`substring-index-ci`is a case-insensitive version of`substring-index`.

#### reverse-string-append

`reverse-string-append``LIST`procedure`(apply string-append (reverse LIST))`

### Combinators

#### any?

`any?``X`procedureIgnores its argument and always returns

`#t`. This is actually useful sometimes.

#### constantly

`constantly``X``...`procedureReturns a procedure that always returns the values

`X ...`regardless of the number and value of its arguments.(constantly X) <=> (lambda args X)

#### complement

`complement``PROC`procedureReturns a procedure that returns the boolean inverse of

`PROC`.(complement PROC) <=> (lambda (x) (not (PROC x)))

#### compose

`compose``PROC1``PROC2``...`procedureReturns a procedure that represents the composition of the argument-procedures

`PROC1 PROC2 ...`.(compose F G) <=> (lambda args (call-with-values (lambda () (apply G args)) F))

`(compose)`is equivalent to`values`.

#### conjoin

`conjoin``PRED``...`procedureReturns a procedure that returns

`#t`if its argument satisfies the predicates`PRED ...`.((conjoin odd? positive?) 33) ==> #t ((conjoin odd? positive?) -33) ==> #f

#### disjoin

`disjoin``PRED``...`procedureReturns a procedure that returns

`#t`if its argument satisfies any predicate`PRED ...`.((disjoin odd? positive?) 32) ==> #t ((disjoin odd? positive?) -32) ==> #f

#### each

`each``PROC``...`procedureReturns a procedure that applies

`PROC ...`to its arguments, and returns the result(s) of the last procedure application. For example(each pp eval)

is equivalent to

(lambda args (apply pp args) (apply eval args) )

`(each PROC)`is equivalent to`PROC`and`(each)`is equivalent to`void`.

#### flip

`flip``PROC`procedureReturns a two-argument procedure that calls

`PROC`with its arguments swapped:(flip PROC) <=> (lambda (x y) (PROC y x))

#### identity

`identity``X`procedureReturns its sole argument

`X`.

#### list-of?

`list-of?``PRED`procedureReturns a procedure of one argument that returns

`#t`when applied to a list of elements that all satisfy the predicate procedure`PRED`, or`#f`otherwise.((list-of? even?) '(1 2 3)) ==> #f ((list-of? number?) '(1 2 3)) ==> #t

#### o

`o``PROC``...`procedureA single value version of

`compose`(slightly faster).`(o)`is equivalent to`identity`.

### Binary searching

#### binary-search

`binary-search``SEQUENCE``PROC`procedurePerforms a binary search in

`SEQUENCE`, which should be a sorted list or vector.`PROC`is called to compare items in the sequence, should accept a single argument and return an exact integer: zero if the searched value is equal to the current item, negative if the searched value is*less*than the current item, and positive otherwise. Returns the index of the found value or`#f`otherwise.

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