scala.math

Ordering

trait Ordering[T] extends Comparator[T] with PartialOrdering[T] with Serializable

Ordering is a trait whose instances each represent a strategy for sorting instances of a type.

Ordering's companion object defines many implicit objects to deal with subtypes of AnyVal (e.g. Int, Double), String, and others.

To sort instances by one or more member variables, you can take advantage of these built-in orderings using Ordering.by and Ordering.on:

import scala.util.Sorting
val pairs = Array(("a", 5, 2), ("c", 3, 1), ("b", 1, 3))

// sort by 2nd element
Sorting.quickSort(pairs)(Ordering.by[(String, Int, Int), Int](_._2))

// sort by the 3rd element, then 1st
Sorting.quickSort(pairs)(Ordering[(Int, String)].on(x => (x._3, x._1)))

An Ordering[T] is implemented by specifying compare(a:T, b:T), which decides how to order to instances a and b. Instances of Ordering[T] can be used by things like scala.util.Sorting to sort collections like Array[T].

For example:

import scala.util.Sorting

case class Person(name:String, age:Int)
val people = Array(Person("bob", 30), Person("ann", 32), Person("carl", 19))

// sort by age
object AgeOrdering extends Ordering[Person] {
  def compare(a:Person, b:Person) = a.age compare b.age
}
Sorting.quickSort(people)(AgeOrdering)

This trait and scala.math.Ordered both provide this same functionality, but in different ways. A type T can be given a single way to order itself by extending Ordered. Using Ordering, this same type may be sorted in many other ways. Ordered and Ordering both provide implicits allowing them to be used interchangeably.

You can import scala.math.Ordering.Implicits to gain access to other implicit orderings.

Self Type
Ordering[T]
Annotations
@implicitNotFound( msg = ... )
Source
Ordering.scala
Version

0.9.5, 2008-04-15

Since

2.7

See also

scala.math.Ordered, scala.util.Sorting

Linear Supertypes
PartialOrdering[T], Equiv[T], Serializable, java.io.Serializable, Comparator[T], AnyRef, Any
Known Subclasses
Ordering
  1. Alphabetic
  2. By inheritance
Inherited
  1. Ordering
  2. PartialOrdering
  3. Equiv
  4. Serializable
  5. Serializable
  6. Comparator
  7. AnyRef
  8. Any
Implicitly
  1. by any2stringadd
  2. by any2stringfmt
  3. by any2ArrowAssoc
  4. by any2Ensuring
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Visibility
  1. Public
  2. All

Type Members

  1. class Ops extends AnyRef

    This inner class defines comparison operators available for T.

Abstract Value Members

  1. abstract def compare(x: T, y: T): Int

    Returns an integer whose sign communicates how x compares to y.

    Returns an integer whose sign communicates how x compares to y.

    The result sign has the following meaning:

    • negative if x < y
    • positive if x > y
    • zero otherwise (if x == y)
    Definition Classes
    Ordering → Comparator

Concrete Value Members

  1. final def !=(arg0: AnyRef): Boolean

    Definition Classes
    AnyRef
  2. final def !=(arg0: Any): Boolean

    Test two objects for inequality.

    Test two objects for inequality.

    returns

    true if !(this == that), false otherwise.

    Definition Classes
    Any
  3. final def ##(): Int

    Equivalent to x.hashCode except for boxed numeric types and null.

    Equivalent to x.hashCode except for boxed numeric types and null. For numerics, it returns a hash value which is consistent with value equality: if two value type instances compare as true, then ## will produce the same hash value for each of them. For null returns a hashcode where null.hashCode throws a NullPointerException.

    returns

    a hash value consistent with ==

    Definition Classes
    AnyRef → Any
  4. def +(other: String): String

    Implicit information
    This member is added by an implicit conversion from Ordering[T] to StringAdd performed by method any2stringadd in scala.Predef.
    Definition Classes
    StringAdd
  5. def ->[B](y: B): (Ordering[T], B)

    Implicit information
    This member is added by an implicit conversion from Ordering[T] to ArrowAssoc[Ordering[T]] performed by method any2ArrowAssoc in scala.Predef.
    Definition Classes
    ArrowAssoc
    Annotations
    @inline()
  6. final def ==(arg0: AnyRef): Boolean

    Definition Classes
    AnyRef
  7. final def ==(arg0: Any): Boolean

    Test two objects for equality.

    Test two objects for equality. The expression x == that is equivalent to if (x eq null) that eq null else x.equals(that).

    returns

    true if the receiver object is equivalent to the argument; false otherwise.

    Definition Classes
    Any
  8. final def asInstanceOf[T0]: T0

    Cast the receiver object to be of type T0.

    Cast the receiver object to be of type T0.

    Note that the success of a cast at runtime is modulo Scala's erasure semantics. Therefore the expression 1.asInstanceOf[String] will throw a ClassCastException at runtime, while the expression List(1).asInstanceOf[List[String]] will not. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the requested type.

    returns

    the receiver object.

    Definition Classes
    Any
    Exceptions thrown
    ClassCastException

    if the receiver object is not an instance of the erasure of type T0.

  9. def clone(): AnyRef

    Create a copy of the receiver object.

    Create a copy of the receiver object.

    The default implementation of the clone method is platform dependent.

    returns

    a copy of the receiver object.

    Attributes
    protected[java.lang]
    Definition Classes
    AnyRef
    Annotations
    @throws( ... )
    Note

    not specified by SLS as a member of AnyRef

  10. def ensuring(cond: (Ordering[T]) ⇒ Boolean, msg: ⇒ Any): Ordering[T]

    Implicit information
    This member is added by an implicit conversion from Ordering[T] to Ensuring[Ordering[T]] performed by method any2Ensuring in scala.Predef.
    Definition Classes
    Ensuring
  11. def ensuring(cond: (Ordering[T]) ⇒ Boolean): Ordering[T]

    Implicit information
    This member is added by an implicit conversion from Ordering[T] to Ensuring[Ordering[T]] performed by method any2Ensuring in scala.Predef.
    Definition Classes
    Ensuring
  12. def ensuring(cond: Boolean, msg: ⇒ Any): Ordering[T]

    Implicit information
    This member is added by an implicit conversion from Ordering[T] to Ensuring[Ordering[T]] performed by method any2Ensuring in scala.Predef.
    Definition Classes
    Ensuring
  13. def ensuring(cond: Boolean): Ordering[T]

    Implicit information
    This member is added by an implicit conversion from Ordering[T] to Ensuring[Ordering[T]] performed by method any2Ensuring in scala.Predef.
    Definition Classes
    Ensuring
  14. final def eq(arg0: AnyRef): Boolean

    Tests whether the argument (arg0) is a reference to the receiver object (this).

    Tests whether the argument (arg0) is a reference to the receiver object (this).

    The eq method implements an equivalence relation on non-null instances of AnyRef, and has three additional properties:

    • It is consistent: for any non-null instances x and y of type AnyRef, multiple invocations of x.eq(y) consistently returns true or consistently returns false.
    • For any non-null instance x of type AnyRef, x.eq(null) and null.eq(x) returns false.
    • null.eq(null) returns true.

    When overriding the equals or hashCode methods, it is important to ensure that their behavior is consistent with reference equality. Therefore, if two objects are references to each other (o1 eq o2), they should be equal to each other (o1 == o2) and they should hash to the same value (o1.hashCode == o2.hashCode).

    returns

    true if the argument is a reference to the receiver object; false otherwise.

    Definition Classes
    AnyRef
  15. def equals(arg0: Any): Boolean

    The equality method for reference types.

    The equality method for reference types. Default implementation delegates to eq.

    See also equals in scala.Any.

    returns

    true if the receiver object is equivalent to the argument; false otherwise.

    Definition Classes
    AnyRef → Any
  16. def equiv(x: T, y: T): Boolean

    Return true if x == y in the ordering.

    Return true if x == y in the ordering.

    Definition Classes
    OrderingPartialOrderingEquiv
  17. def finalize(): Unit

    Called by the garbage collector on the receiver object when there are no more references to the object.

    Called by the garbage collector on the receiver object when there are no more references to the object.

    The details of when and if the finalize method is invoked, as well as the interaction between finalize and non-local returns and exceptions, are all platform dependent.

    Attributes
    protected[java.lang]
    Definition Classes
    AnyRef
    Annotations
    @throws( classOf[java.lang.Throwable] )
    Note

    not specified by SLS as a member of AnyRef

  18. def formatted(fmtstr: String): String

    Returns string formatted according to given format string.

    Returns string formatted according to given format string. Format strings are as for String.format (@see java.lang.String.format).

    Implicit information
    This member is added by an implicit conversion from Ordering[T] to StringFormat performed by method any2stringfmt in scala.Predef.
    Definition Classes
    StringFormat
    Annotations
    @inline()
  19. final def getClass(): Class[_]

    A representation that corresponds to the dynamic class of the receiver object.

    A representation that corresponds to the dynamic class of the receiver object.

    The nature of the representation is platform dependent.

    returns

    a representation that corresponds to the dynamic class of the receiver object.

    Definition Classes
    AnyRef → Any
    Note

    not specified by SLS as a member of AnyRef

  20. def gt(x: T, y: T): Boolean

    Return true if x > y in the ordering.

    Return true if x > y in the ordering.

    Definition Classes
    OrderingPartialOrdering
  21. def gteq(x: T, y: T): Boolean

    Return true if x >= y in the ordering.

    Return true if x >= y in the ordering.

    Definition Classes
    OrderingPartialOrdering
  22. def hashCode(): Int

    The hashCode method for reference types.

    The hashCode method for reference types. See hashCode in scala.Any.

    returns

    the hash code value for this object.

    Definition Classes
    AnyRef → Any
  23. final def isInstanceOf[T0]: Boolean

    Test whether the dynamic type of the receiver object is T0.

    Test whether the dynamic type of the receiver object is T0.

    Note that the result of the test is modulo Scala's erasure semantics. Therefore the expression 1.isInstanceOf[String] will return false, while the expression List(1).isInstanceOf[List[String]] will return true. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the specified type.

    returns

    true if the receiver object is an instance of erasure of type T0; false otherwise.

    Definition Classes
    Any
  24. def lt(x: T, y: T): Boolean

    Return true if x < y in the ordering.

    Return true if x < y in the ordering.

    Definition Classes
    OrderingPartialOrdering
  25. def lteq(x: T, y: T): Boolean

    Return true if x <= y in the ordering.

    Return true if x <= y in the ordering.

    Definition Classes
    OrderingPartialOrdering
  26. def max(x: T, y: T): T

    Return x if x >= y, otherwise y.

  27. def min(x: T, y: T): T

    Return x if x <= y, otherwise y.

  28. implicit def mkOrderingOps(lhs: T): Ops

    This implicit method augments T with the comparison operators defined in scala.math.Ordering.Ops.

  29. final def ne(arg0: AnyRef): Boolean

    Equivalent to !(this eq that).

    Equivalent to !(this eq that).

    returns

    true if the argument is not a reference to the receiver object; false otherwise.

    Definition Classes
    AnyRef
  30. final def notify(): Unit

    Wakes up a single thread that is waiting on the receiver object's monitor.

    Wakes up a single thread that is waiting on the receiver object's monitor.

    Definition Classes
    AnyRef
    Note

    not specified by SLS as a member of AnyRef

  31. final def notifyAll(): Unit

    Wakes up all threads that are waiting on the receiver object's monitor.

    Wakes up all threads that are waiting on the receiver object's monitor.

    Definition Classes
    AnyRef
    Note

    not specified by SLS as a member of AnyRef

  32. def on[U](f: (U) ⇒ T): Ordering[U]

    Given f, a function from U into T, creates an Ordering[U] whose compare function is equivalent to:

    Given f, a function from U into T, creates an Ordering[U] whose compare function is equivalent to:

    def compare(x:U, y:U) = Ordering[T].compare(f(x), f(y))
  33. def reverse: Ordering[T]

    Return the opposite ordering of this one.

    Return the opposite ordering of this one.

    Definition Classes
    OrderingPartialOrdering
  34. final def synchronized[T0](arg0: ⇒ T0): T0

    Definition Classes
    AnyRef
  35. def toString(): String

    Creates a String representation of this object.

    Creates a String representation of this object. The default representation is platform dependent. On the java platform it is the concatenation of the class name, "@", and the object's hashcode in hexadecimal.

    returns

    a String representation of the object.

    Definition Classes
    AnyRef → Any
  36. def tryCompare(x: T, y: T): Some[Int]

    Returns whether a comparison between x and y is defined, and if so the result of compare(x, y).

    Returns whether a comparison between x and y is defined, and if so the result of compare(x, y).

    Definition Classes
    OrderingPartialOrdering
  37. final def wait(): Unit

    Definition Classes
    AnyRef
    Annotations
    @throws( ... )
  38. final def wait(arg0: Long, arg1: Int): Unit

    Definition Classes
    AnyRef
    Annotations
    @throws( ... )
  39. final def wait(arg0: Long): Unit

    Definition Classes
    AnyRef
    Annotations
    @throws( ... )
  40. def [B](y: B): (Ordering[T], B)

    Implicit information
    This member is added by an implicit conversion from Ordering[T] to ArrowAssoc[Ordering[T]] performed by method any2ArrowAssoc in scala.Predef.
    Definition Classes
    ArrowAssoc

Shadowed Implicit Value Members

  1. val self: Any

    Implicit information
    This member is added by an implicit conversion from Ordering[T] to StringAdd performed by method any2stringadd in scala.Predef.
    Shadowing
    This implicitly inherited member is ambiguous. One or more implicitly inherited members have similar signatures, so calling this member may produce an ambiguous implicit conversion compiler error.
    To access this member you can use a type ascription:
    (ordering: StringAdd).self
    Definition Classes
    StringAdd
  2. val self: Any

    Implicit information
    This member is added by an implicit conversion from Ordering[T] to StringFormat performed by method any2stringfmt in scala.Predef.
    Shadowing
    This implicitly inherited member is ambiguous. One or more implicitly inherited members have similar signatures, so calling this member may produce an ambiguous implicit conversion compiler error.
    To access this member you can use a type ascription:
    (ordering: StringFormat).self
    Definition Classes
    StringFormat

Deprecated Value Members

  1. def x: Ordering[T]

    Implicit information
    This member is added by an implicit conversion from Ordering[T] to ArrowAssoc[Ordering[T]] performed by method any2ArrowAssoc in scala.Predef.
    Shadowing
    This implicitly inherited member is ambiguous. One or more implicitly inherited members have similar signatures, so calling this member may produce an ambiguous implicit conversion compiler error.
    To access this member you can use a type ascription:
    (ordering: ArrowAssoc[Ordering[T]]).x
    Definition Classes
    ArrowAssoc
    Annotations
    @deprecated
    Deprecated

    (Since version 2.10.0) Use leftOfArrow instead

  2. def x: Ordering[T]

    Implicit information
    This member is added by an implicit conversion from Ordering[T] to Ensuring[Ordering[T]] performed by method any2Ensuring in scala.Predef.
    Shadowing
    This implicitly inherited member is ambiguous. One or more implicitly inherited members have similar signatures, so calling this member may produce an ambiguous implicit conversion compiler error.
    To access this member you can use a type ascription:
    (ordering: Ensuring[Ordering[T]]).x
    Definition Classes
    Ensuring
    Annotations
    @deprecated
    Deprecated

    (Since version 2.10.0) Use resultOfEnsuring instead

Inherited from PartialOrdering[T]

Inherited from Equiv[T]

Inherited from Serializable

Inherited from java.io.Serializable

Inherited from Comparator[T]

Inherited from AnyRef

Inherited from Any

Inherited by implicit conversion any2stringadd from Ordering[T] to StringAdd

Inherited by implicit conversion any2stringfmt from Ordering[T] to StringFormat

Inherited by implicit conversion any2ArrowAssoc from Ordering[T] to ArrowAssoc[Ordering[T]]

Inherited by implicit conversion any2Ensuring from Ordering[T] to Ensuring[Ordering[T]]

Ungrouped