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> | 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 | 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
. It is a requirement that start
is no greater than 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.
ofType
Operand type | Parameter type | Result type |
---|---|---|
sequence<Type> | OtherType | sequence<OtherType> |
Filters the sequence by the type of the elements. Returns a sub-sequence of the original one, with all elements which match the provided type. The collection is cast to the new element type.
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 } | 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 } | 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 } |
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
All operations in this section have the semantics of operations on multisets. This means that the elements of both the operand and the parameter sequences are taken with their respective cardinalities (that is, how many times an element appears in the sequence), and then the cardinality of matched elements in the output is the result of an arithmetic operation on these values. These operations keep the order of elements (first operand's, then the parameter's).
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. More precisely, the cardinality in the output of an element appearing in both the operand and the parameter sequences, is the minimum of its cardinalities in the input.
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. More precisely, the cardinality in the output of an element appearing in both the operand and the parameter sequences, is the maximum of 0 and the result of subtracting the parameter's cardinality from the operand's cardinality.
union
Operand type | Parameter type | Result type |
---|---|---|
sequence<Type> | sequence<Type> | sequence<Type> |
Produces a sequence containing elements either from the original sequence or the one passed as a parameter. More precisely, the cardinality in the output of an element appearing in both the operand and the parameter sequences, is the maximum of its cardinalities in the input.
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. This operation equivalent to:
(operand.except(param)).concat(parameter.except(operand))
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 | { Z, Type => Z } | Z | foldLeft |
seed | { 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.