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Sequence

Sequence is an abstraction of the order defined on a collection of elements of some type. The only operation that is allowed on a sequence is iterating its elements from first to last. A sequence is immutable. All operations defined in the following subsections and declared to return a sequence, always return a new instance of a sequence or the original sequence.

Although it is possible to create a sequence that produces infinite number of elements, it is not recommended. Some operations may require one or two full traversals of the sequence in order to compute, and invoking such an operation on an infinite sequence would never yield result.

Sequence type

sequence<Type>

Subtypes

Supertypes

Comparable types

list<Type>
set<Type>

none

java.lang.Iterable<Type>

Creation

new sequence

Parameter type

Result type

{ => sequence<Type> }

sequence<Type>

Sequence can be created with initializer.

closure invocation

Result type

sequence<Type>

A sequence may be returned from a closure (see Closures).

array as a sequence

Operand type

Parameter type

Result type

Type[]

none

sequence<Type>

An array can be used as a sequence.

A list, a set and a map are sequences, too. All operations defined on a sequence are also available on an instance of any of these types.

Sequence type is assignable to a variable of type java.lang.Iterable. The opposite is also true.

Operations on sequence

Iteration and querying
foreach statement

Loop statement

is equivalent to

forEach

Operand type

Parameter type

Result type

sequence<Type>

{ Type => void }

void

The code passed as a parameter (as a closure literal or by reference) is executed once for each element.

size

Operand type

Parameter type

Result type

sequence<Type>

none

int

Gives number of elements in a sequence.

isEmpty

Operand type

Parameter type

Result type

sequence<Type>

none

boolean

Test whether a sequence is empty, that is its size is 0.

isNotEmpty

Operand type

Parameter type

Result type

sequence<Type>

none

boolean

Test whether a sequence contains any elements.

indexOf

Operand type

Parameter type

Result type

sequence<Type>

Type

int

Gives the index of a first occurrence in a sequence of an element that is passed to it as a parameter.

contains

Operand type

Parameter type

Result type

sequence<Type>

Type

boolean

Produces boolean value, indicating whether or not a sequence contains the specified element.

any / all

Operand type

Parameter type

Result type

sequence<Type>

{ Type => boolean }

boolean

Produces boolean value that indicates whether any (in case of any operation) or all (in case of all) of the elements in the input sequence match the condition specified by the closure.

iterator

Operand type

Parameter type

Result type

sequence<Type>

none

iterator<Type>

Produces an #iterator.

enumerator

Operand type

Parameter type

Result type

sequence<Type>

none

enumerator<Type>

Produces an #enumerator.

Selection and filtering
first

Operand type

Parameter type

Result type

sequence<Type>

none

Type

Yields the first element.

last

Operand type

Parameter type

Result type

sequence<Type>

none

Type

Yields the last element.

take

Operand type

Parameter type

Result type

sequence<Type>

int

sequence<Type>

Produces a sequence that is sub-sequence of the original one, starting from first element and of size count.

skip

Operand type

Parameter type

Result type

sequence<Type>

int

sequence<Type>

Produces a sequence that is sub-sequence of the original one, containing all elements starting with the element at index count.

cut

Operand type

Parameter type

Result type

sequence<Type>

int

sequence<Type>

Produces a sequence that is a sub-sequence of the original one, containing all elements starting with first and up to (but not including) the element at index size minus count. In other words, this operation returns a sequence with all elements from the original one except the last count elements.

tail

Operand type

Parameter type

Result type

sequence<Type>

int

sequence<Type>

Produces a sequence that is a sub-sequence of the original one, containing all elements starting with the element at index size minus count. In other words, this operations returns a sequence with count elements from the end of the original sequence, in the original order.

page

Operand type

Parameter type

Result type

sequence<Type>

int
int

sequence<Type>

Results in a sequence that is a sub-sequence of the original one, containing all elements starting with the element at index start and up to (but not including) the element at index end.


This is equivalent to

Where skip = start, count = end - start .

where

Operand type

Parameter type

Result type

sequence<Type>

{ Type => boolean }

sequence<Type>

Produces a sequence that is a sub-sequence of the original one, with all elements for which the code passed as a parameter returns true.

findFirst

Operand type

Parameter type

Result type

sequence<Type>

{ Type => boolean }

Type

Results in the first element that matches the parameter closure.

findLast

Operand type

Parameter type

Result type

sequence<Type>

{ Type => boolean }

Type

Results in the last element that matches the parameter closure.

Transformation and sorting
select

Operand type

Parameter type

Result type

sequence<Type>

{ Type => Type2 }

sequence<Type2>

Results in a sequence consisting of elements, each of which is the result of applying the parameter function to each element of the original sequence in turn.

selectMany

Operand type

Parameter type

Result type

sequence<Type>

{ Type => sequence<Type2> }

sequence<Type2>

Produces a sequence that is a concatenation of all sequences, which are all the results of applying the parameter closure to each element of the original sequence in turn. The statements skip and stop are available within the parameter closure.

distinct

Operand type

Parameter type

Result type

sequence<Type>

none

sequence<Type>

Produces a sequence, which contains all elements from the original sequence in the original order, with all the elements having cardinality exactly 1. Of all occurrences of an element in the original sequence only the first occurrence is included in the resulting sequence.

sortBy

Operand type

Parameter type

Result type

sequence<Type>

{ Type => Type2 }
boolean

sequence<Type>

Produces a sequence with all elements from the original one in the order, which corresponds to an order induced by an imaginary sequence produced by applying the selector function to each element in the original sequence in turn. The selector function can be thought of as returning a key, which is used to sort elements in a sequence. The ascending parameter controls the sort order.

alsoSortBy

Operand type

Parameter type

Result type

sequence<Type>

{ Type => Type2 }
boolean

sequence<Type>

Equivalent to sortBy, unless used as a chain operation immediately following sortBy or another alsoSortBy. The result is a sequence sorted with a compound key, with the first component taken from previous sortBy or alsoSortBy (which is also a compound key), and the last component taken from this operation.

sort

Operand type

Parameter type

Result type

sequence<Type>

{ Type, Type => int }
boolean sequence<Type>

Produces a sequence containing all elements from the original one in the order produced by applying the comparator function (passed as a closure literal or by reference) to a list with elements from the original sequence. The ascending parameter controls the sort order (order is reversed if the value is false).

Binary operations
intersect

Operand type

Parameter type

Result type

sequence<Type>

sequence<Type>

sequence<Type>

Produces a sequence containing elements contained both by the original sequence and the parameter sequence.

except

Operand type

Parameter type

Result type

sequence<Type>

sequence<Type>

sequence<Type>

Produces a sequence containing all elements from the original sequence that are not also members of the parameter sequence.

union

Operand type

Parameter type

Result type

sequence<Type>

sequence<Type>

sequence<Type>

Produces a sequence containing elements both from the original sequence and the one passed as a parameter.

disjunction

Operand type

Parameter type

Result type

sequence<Type>

sequence<Type>

sequence<Type>

Produces exclusive disjunction (symmetric difference) of the original sequence and the one passed as a parameter.

concat

Operand type

Parameter type

Result type

sequence<Type>

sequence<Type> }

sequence<Type>

Produces a sequence, which is a concatenation of the original one with the sequence passed as a parameter

Conversion
reduceLeft / reduceRight

Operand type

Parameter type

Result type

sequence<Type>

{ Type, Type => Type }

Type

Operation reduceLeft/reduceRight applies the combinator function passed as a parameter to all elements of the sequence in turn. One of the function parameters is a sequence element, and another is the result of previous application of the function. Operation reduceLeft takes first two sequence elements and applies the function to them, then takes the result of the first application and the third sequence element, etc. Operation reduceRight does the same, but moving from the sequence's tail backwards.

  • reduceLeft
  • reduceRight
foldLeft / foldRight

Operand type

Parameter type

Result type

Applicable for

seed
sequence<Type>

{ Z, Type => Z }

Z

foldLeft

seed
sequence<Type>

{ Type, Z => Z }

Z

foldRight

Operation foldLeft/foldRight behaves similarly to reduceLeft/reduceRight, with the difference that it also accepts a seed value. Also the combinator function is asymmetric (it takes a Type and a Z parameters and returns a Z value). The result of the operation is of type Z.

  • foldLeft
  • foldRight
join

Operand type

Parameter type

Result type

sequence< ? extends string >

string (optional)

string

This operation is only available on a sequence of strings. The result is a string that is produced by concatenating all elements with the optional separator. The default separator is " " (single space).

toList

Operand type

Parameter type

Result type

sequence<Type>

none

list<Type>

Returns new list containing all the elements from the original sequence.

toArray

Operand type

Parameter type

Result type

sequence<Type>

none

Type*[]*

Returns new array containing all the elements from the original sequence.

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