• egg

## sequences

### Introduction

Operations over generic or user-defined sequences.

Note: this is currently under review - the API might still change

### Usage

`(require-extension sequences)`

fast-generic

### Documentation

A sequence is a collection of objects and may be either one of the built-in types vector, list or string, or the result of the sequence-constructors make-linear-sequence and make-random-access-sequences. A linear sequence is a sequence that only allows element-by-element access (i.e. a list), a random access sequences allows access to arbitrary elements through an index (i.e. vectors or strings).

An iterator is an object that designates a particular position in a linear or random-access sequence.

#### Basic sequence operations

##### size
[procedure] (size S)

Returns the number of elements in the sequence S. For linear sequences, this operation traverses all elements.

##### elt
[procedure] (elt S I)

Returns the I-th element of S. I may be an exact integer or an iterator (see below).

A sequence-element can be modified with (set! (elt S I) X).

If I is an iterator, then S must be the same sequence that had been used to construct the iterator.

##### rev
[procedure] (rev S)

Returns a new sequence of the same type with the elements of S in reverse order.

##### foldl
[procedure] (foldl PROC SEED S)

Performs a left "fold" over the sequence S, where the procedure PROC is applied to its previous result (or SEED for the first element) and each sequence-element.

##### foldr
[procedure] (foldr PROC SEED S)

A right "fold" over sequence S, PROC is applied to each sequence element and the result of its last invocation (or SEED for the first element).

##### sub
[procedure] (sub S START [END])

Returns a new sequence with the elements of S, starting at position START up to but not including the element at position END. If END is not given, all remaining elements are returned. START and END may be exact integers or iterators.

A range of elements may be modified by executing (set! (sub S1 START [END]) S2), which assigns the elements of S2 to the designated locations of sequence S1.

##### pos
[procedure] (pos PRED S)

Returns the index of the first element in S that for which the one argument procedure PRED returns true. If PRED returns false for all arguments, #f is returned.

##### take
[procedure] (take PRED S)

Returns a new sequence of the same type as S with the elements up to but not including the first element for which the one-argument procedure PRED returns #f.

##### drop
[procedure] (drop PRED S)

Returns a new sequence of the same type as S with the elements from the first element for which the one-argument procedure PRED returns #f.

##### split
[procedure] (split PRED S)

Returns two sequences of the same type as S holding the elements split at the first position for which the one-argument procedure PRED returns #f.

##### partition
[procedure] (partition PRED S)

Returns two sequences of the same type as S holding those elements for which the one-argument procedure PRED returns true and false, respectively.

##### fill!
[procedure] (fill! PROC S [START [END]])

Calls PROC with the sequence S and an iterator object over the elements in S starting at position START up to but not including END and returns the modified sequence.

##### all?
[procedure] (all? PROC S)

Returns true if PROC returns true for all elements of S.

##### thereis?
[procedure] (thereis? PROC S)

Returns #t if S contains an element for which PROC returns true.

##### empty?
[procedure] (empty? S)

Returns true if S is of size 0.

##### peek
[procedure] (peek S)

Returns the first element of S.

##### pop
[procedure] (pop S)

Returns all but the first element of S.

##### filter
[procedure] (filter PROTO PROC S)

Returns a new sequence of the same type as PROTO with all elements of S for which PROC returns true.

#### Set-operations

##### intersection
[procedure] (intersection PROTO COMPARE S1 ...)

Returns the intersection of sequences S1 ... using the two-argument procedure COMPARE to compare the elements. The returned sequence is of the same type as PROTO.

##### difference
[procedure] (difference PROTO COMPARE S1 S2 ...)

Returns the set-difference of sequences S2 ... taken from S1 using the two-argument procedure COMPARE to compare the elements. The returned sequence is of the same type as PROTO.

##### union
[procedure] (union PROTO COMPARE S1 ...)

Returns the union of sequences S1 ... using the two-argument procedure COMPARE to compare the elements. The returned sequence is of the same type as PROTO.

#### Predicates over sequence types

##### sequence?
[procedure] (sequence? X)

Returns #t if X is a sequence or #f otherwise.

##### linear-sequence?
[procedure] (linear-sequence? X)

Reurns #t if X is a list or a sequence created with make-linear-sequence or #f otherwise.

##### random-access-sequence?
[procedure] (random-access-sequence? X)

Returns #t if X is a vector, a string or a sequence created with make-random-access-sequence, or #f otherwise.

#### Sequence constructors

##### make-random-access-sequence
[procedure] (make-random-access-sequence MAKE ELT SIZE)

Returns an object representing a sequence that allows random access to its elements. MAKE should be a procedure of two arguments, a size count and an initial value and should return a collection of elements which will be stored as "data" in the sequence object. ELT should be a procedure of two arguments receiving the "data" and an exact integer index and should return the element inside the data collection at the given position. SIZE should be a procedure that receives the data and returns the number of elements in that collection.

Note that the "data" may be anything - the operators fully define how it is interpreted.

##### make-linear-sequence
[procedure] (make-linear-sequence MAKE ELT NEXT)

Returns an object representing a sequence that only allows sequential "on-at-a-time" access to its elements. MAKE should be a procedure of two arguments, a size count and an initial value and should return a collection of elements which will be stored as "state" in the sequence object. ELT should be a procedure of one argument receiving the "state" and should return the element inside the collection that is represented by the currently stored state. NEXT should be a procedure that receives the current state and returns a new state representing the underlying collection that will make the next element accessible via ELT. If the collection has run out of elements, NEXT should return #f.

##### make
[procedure] (make S LENGTH INIT)

Creates a sequence of the same type as S with LENGTH elements that have the initial value INIT.

##### sequence
[procedure] (sequence S X1 ...)

Creates a sequence of the same type as S with X1, ... as its initial elements.

#### Iterators

##### iterator?
[procedure] (iterator? X)

Returns #t if X is an iterator object or #f otherwise.

##### linear-iterator?
[procedure] (linear-iterator? X)

Returns #t if X is an iterator on a linear sequence or #f otherwise.

##### random-access-iterator?
[procedure] (random-access-iterator? X)

Returns #t if X is an iterator on a random-access sewuence or #f otherwise.

##### iterator
[procedure] (iterator S [INDEX])

Returns an iterator object over sequence S, optionally starting at osition INDEX (an exact integer).

##### at-end?
[procedure] (at-end? ITERATOR)

Returns #t if ITERATOR points past the lat element of its associated sequence or #f otherwise.

Returns a new iterator (or modifies the given iterator in case of advance!) pointing to the next element of the associated sequence or to the element at the position I + STEPS, where I is the current index of ITERATOR.

##### index
[procedure] (index ITERATOR)

Returns the exact integer index of the position to which ITERATOR points to.

#### Iteration constructs

##### for*
[procedure] (for PROC S)
[procedure] (for* PROC S)

Invokes PROC for each element in sequence and returns an undefined result. for* operates as for but invokes PROC with the sequence S and an iterator pointing to the current element.

##### smap*
[procedure] (smap S1 PROC S2)
[procedure] (smap S1 PROC S2)

Applies PROC to each element in the sequence S2 and returns a new sequence of the same type as S1 constructed of the results returned by PROC.

#### Other operations

##### coerce
[procedure] (coerce S1 S2)

Returns a new sequence of the same type as S1 containing the elements of S2.

##### copy
[procedure] (copy S)

Returns a copy of the sequence S.

##### is?
[procedure] (is? X)

Returns a single-argument procedure that returns #t if the argument is equal? to X or #f otherwise.

#### SRFI-42 comprehensions

(This code was kindly contributed by Thomas Chust)

SRFI-42 comprehensions for sequences are provided using the :seq generator, here an example:

` (string-ec (:seq x "aAbBcC") (if (char-lower-case? x)) x)  ==>  "abc"`

This is mostly useful with user-defined sequences created by make-linear-sequence and make-random-access-sequence.

To use this feature, execute

` (require-extension sequence-comprehensions)`

felix

### Repository

This egg is hosted on the CHICKEN Subversion repository:

https://anonymous@code.call-cc.org/svn/chicken-eggs/release/5/sequences

If you want to check out the source code repository of this egg and you are not familiar with Subversion, see this page.

```Copyright (c) 2010-2011, Felix L. Winkelmann and Thomas Chust

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### Version History

0.5.1
Fix segfault when calling certain procedures with non-fixnum as indices and step sizes (fixes #1631)
0.5
ported to CHICKEN 5
0.4
removed replicate, renamed contains? to thereis?, performance tuning
0.3