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Scala 3/scala/scala.math/Ordering

Ordering

scala.math.Ordering
See theOrdering companion object
trait Ordering[T] extends Comparator[T], PartialOrdering[T], 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 the compare method, compare(a: T, b: T): Int, which decides how to order two 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.

Type parameters

T

the type of objects that this ordering can compare

Attributes

See also
Companion
object
Source
Ordering.scala
Graph
Supertypes
trait PartialOrdering[T]
trait Equiv[T]
trait Serializable
trait Comparator[T]
class Object
trait Matchable
class Any
Show all
Known subtypes
trait Numeric[T]
trait Fractional[T]
trait Integral[T]
object IntIsIntegral
object BigDecimal
object BigInt
object Boolean
trait ByteOrdering
object Byte
trait CachedReverse[T]
object Int
trait CharOrdering
object Char
trait IeeeOrdering
object IeeeOrdering
object TotalOrdering
trait IeeeOrdering
object IeeeOrdering
object TotalOrdering
trait IntOrdering
trait LongOrdering
object Long
trait OptionOrdering[T]
object Short
object String
object Symbol
trait UnitOrdering
object Unit
Show all
Self type

Members list

Type members

Classlikes

class OrderingOps(lhs: Ordering.this.T)
Implicitly added by Iterable

This inner class defines comparison operators available for T.

This inner class defines comparison operators available for T.

It can't extend AnyVal because it is not a top-level class or a member of a statically accessible object.

Value parameters

lhs

the left-hand side value for infix comparison operations

Attributes

Source
Ordering.scala
Supertypes
class Object
trait Matchable
class Any
class OrderingOps(lhs: Ordering.this.T)
Implicitly added by Option

This inner class defines comparison operators available for T.

This inner class defines comparison operators available for T.

It can't extend AnyVal because it is not a top-level class or a member of a statically accessible object.

Value parameters

lhs

the left-hand side value for infix comparison operations

Attributes

Source
Ordering.scala
Supertypes
class Object
trait Matchable
class Any
class OrderingOps(lhs: Ordering.this.T)

This inner class defines comparison operators available for T.

This inner class defines comparison operators available for T.

It can't extend AnyVal because it is not a top-level class or a member of a statically accessible object.

Value parameters

lhs

the left-hand side value for infix comparison operations

Attributes

Source
Ordering.scala
Supertypes
class Object
trait Matchable
class Any

Value members

Abstract methods

def compare(x: T, y: T): Int
Implicitly added by Iterable

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)

Value parameters

x

the first value to compare

y

the second value to compare

Attributes

Returns

a negative integer, zero, or a positive integer as x is less than, equal to, or greater than y

Source
Ordering.scala
def compare(x: T, y: T): Int
Implicitly added by Option

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)

Value parameters

x

the first value to compare

y

the second value to compare

Attributes

Returns

a negative integer, zero, or a positive integer as x is less than, equal to, or greater than y

Source
Ordering.scala
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)

Value parameters

x

the first value to compare

y

the second value to compare

Attributes

Returns

a negative integer, zero, or a positive integer as x is less than, equal to, or greater than y

Source
Ordering.scala

Concrete methods

override def equiv(x: T, y: T): Boolean

Returns true if x == y in the ordering.

Returns true if x == y in the ordering.

Value parameters

x

the first value to compare

y

the second value to compare

Attributes

Definition Classes
Source
Ordering.scala
override def gt(x: T, y: T): Boolean

Returns true if x > y in the ordering.

Returns true if x > y in the ordering.

Value parameters

x

the first value to compare

y

the second value to compare

Attributes

Definition Classes
Source
Ordering.scala
override def gteq(x: T, y: T): Boolean

Returns true if x >= y in the ordering.

Returns true if x >= y in the ordering.

Value parameters

x

the first value to compare

y

the second value to compare

Attributes

Definition Classes
Source
Ordering.scala
def isReverseOf(other: Ordering[_]): Boolean
Implicitly added by Iterable

Returns whether or not the other ordering is the opposite ordering of this one.

Returns whether or not the other ordering is the opposite ordering of this one.

Equivalent to other == this.reverse.

Implementations should only override this method if they are overriding reverse as well.

Value parameters

other

the ordering to check

Attributes

Returns

true if other is the reverse of this ordering, false otherwise

Source
Ordering.scala
def isReverseOf(other: Ordering[_]): Boolean
Implicitly added by Option

Returns whether or not the other ordering is the opposite ordering of this one.

Returns whether or not the other ordering is the opposite ordering of this one.

Equivalent to other == this.reverse.

Implementations should only override this method if they are overriding reverse as well.

Value parameters

other

the ordering to check

Attributes

Returns

true if other is the reverse of this ordering, false otherwise

Source
Ordering.scala
def isReverseOf(other: Ordering[_]): Boolean

Returns whether or not the other ordering is the opposite ordering of this one.

Returns whether or not the other ordering is the opposite ordering of this one.

Equivalent to other == this.reverse.

Implementations should only override this method if they are overriding reverse as well.

Value parameters

other

the ordering to check

Attributes

Returns

true if other is the reverse of this ordering, false otherwise

Source
Ordering.scala
override def lt(x: T, y: T): Boolean

Returns true if x < y in the ordering.

Returns true if x < y in the ordering.

Value parameters

x

the first value to compare

y

the second value to compare

Attributes

Definition Classes
Source
Ordering.scala
override def lteq(x: T, y: T): Boolean

Returns true if x <= y in the ordering.

Returns true if x <= y in the ordering.

Value parameters

x

the first value to compare

y

the second value to compare

Attributes

Definition Classes
Source
Ordering.scala
def max[U <: T](x: U, y: U): U
Implicitly added by Iterable

Returns x if x >= y, otherwise y.

Returns x if x >= y, otherwise y.

Type parameters

U

a subtype of T, used to preserve the specific type in the return value

Value parameters

x

the first candidate value

y

the second candidate value

Attributes

Source
Ordering.scala
def max[U <: T](x: U, y: U): U
Implicitly added by Option

Returns x if x >= y, otherwise y.

Returns x if x >= y, otherwise y.

Type parameters

U

a subtype of T, used to preserve the specific type in the return value

Value parameters

x

the first candidate value

y

the second candidate value

Attributes

Source
Ordering.scala
def max[U <: T](x: U, y: U): U

Returns x if x >= y, otherwise y.

Returns x if x >= y, otherwise y.

Type parameters

U

a subtype of T, used to preserve the specific type in the return value

Value parameters

x

the first candidate value

y

the second candidate value

Attributes

Source
Ordering.scala
def min[U <: T](x: U, y: U): U
Implicitly added by Iterable

Returns x if x <= y, otherwise y.

Returns x if x <= y, otherwise y.

Type parameters

U

a subtype of T, used to preserve the specific type in the return value

Value parameters

x

the first candidate value

y

the second candidate value

Attributes

Source
Ordering.scala
def min[U <: T](x: U, y: U): U
Implicitly added by Option

Returns x if x <= y, otherwise y.

Returns x if x <= y, otherwise y.

Type parameters

U

a subtype of T, used to preserve the specific type in the return value

Value parameters

x

the first candidate value

y

the second candidate value

Attributes

Source
Ordering.scala
def min[U <: T](x: U, y: U): U

Returns x if x <= y, otherwise y.

Returns x if x <= y, otherwise y.

Type parameters

U

a subtype of T, used to preserve the specific type in the return value

Value parameters

x

the first candidate value

y

the second candidate value

Attributes

Source
Ordering.scala
def on[U](f: U => T): Ordering[U]
Implicitly added by Iterable

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))

Type parameters

U

the type of the values to be ordered

Value parameters

f

the function to extract a T value from a U value

Attributes

Returns

an Ordering[U] that orders values by applying f and comparing the results using this ordering

Source
Ordering.scala
def on[U](f: U => T): Ordering[U]
Implicitly added by Option

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))

Type parameters

U

the type of the values to be ordered

Value parameters

f

the function to extract a T value from a U value

Attributes

Returns

an Ordering[U] that orders values by applying f and comparing the results using this ordering

Source
Ordering.scala
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))

Type parameters

U

the type of the values to be ordered

Value parameters

f

the function to extract a T value from a U value

Attributes

Returns

an Ordering[U] that orders values by applying f and comparing the results using this ordering

Source
Ordering.scala
def orElse(other: Ordering[T]): Ordering[T]
Implicitly added by Iterable

Creates an Ordering[T] whose compare function returns the result of this Ordering's compare function, if it is non-zero, or else the result of others compare function.

Creates an Ordering[T] whose compare function returns the result of this Ordering's compare function, if it is non-zero, or else the result of others compare function.

Value parameters

other

an Ordering to use if this Ordering returns zero

Attributes

Returns

an Ordering[T] that uses this ordering first, falling back to other when values are equal

Example 
case class Pair(a: Int, b: Int)
val pairOrdering = Ordering.by[Pair, Int](_.a)
                          .orElse(Ordering.by[Pair, Int](_.b))
Source
Ordering.scala
def orElse(other: Ordering[T]): Ordering[T]
Implicitly added by Option

Creates an Ordering[T] whose compare function returns the result of this Ordering's compare function, if it is non-zero, or else the result of others compare function.

Creates an Ordering[T] whose compare function returns the result of this Ordering's compare function, if it is non-zero, or else the result of others compare function.

Value parameters

other

an Ordering to use if this Ordering returns zero

Attributes

Returns

an Ordering[T] that uses this ordering first, falling back to other when values are equal

Example 
case class Pair(a: Int, b: Int)
val pairOrdering = Ordering.by[Pair, Int](_.a)
                          .orElse(Ordering.by[Pair, Int](_.b))
Source
Ordering.scala
def orElse(other: Ordering[T]): Ordering[T]

Creates an Ordering[T] whose compare function returns the result of this Ordering's compare function, if it is non-zero, or else the result of others compare function.

Creates an Ordering[T] whose compare function returns the result of this Ordering's compare function, if it is non-zero, or else the result of others compare function.

Value parameters

other

an Ordering to use if this Ordering returns zero

Attributes

Returns

an Ordering[T] that uses this ordering first, falling back to other when values are equal

Example 
case class Pair(a: Int, b: Int)
val pairOrdering = Ordering.by[Pair, Int](_.a)
                          .orElse(Ordering.by[Pair, Int](_.b))
Source
Ordering.scala
def orElseBy[S](f: T => S)(implicit ord: Ordering[S]): Ordering[T]
Implicitly added by Iterable

Given f, a function from T into S, creates an Ordering[T] whose compare function returns the result of this Ordering's compare function, if it is non-zero, or else a result equivalent to:

Given f, a function from T into S, creates an Ordering[T] whose compare function returns the result of this Ordering's compare function, if it is non-zero, or else a result equivalent to:

Ordering[S].compare(f(x), f(y))

This function is equivalent to passing the result of Ordering.by(f) to orElse.

Type parameters

S

the type returned by the extraction function f

Value parameters

f

the function to extract a comparison key from a T value

ord

the implicit ordering for the extracted key type S

Attributes

Returns

an Ordering[T] that uses this ordering first, falling back to comparing by f when values are equal

Example 
case class Pair(a: Int, b: Int)
val pairOrdering = Ordering.by[Pair, Int](_.a)
                          .orElseBy[Int](_.b)
Source
Ordering.scala
def orElseBy[S](f: T => S)(implicit ord: Ordering[S]): Ordering[T]
Implicitly added by Option

Given f, a function from T into S, creates an Ordering[T] whose compare function returns the result of this Ordering's compare function, if it is non-zero, or else a result equivalent to:

Given f, a function from T into S, creates an Ordering[T] whose compare function returns the result of this Ordering's compare function, if it is non-zero, or else a result equivalent to:

Ordering[S].compare(f(x), f(y))

This function is equivalent to passing the result of Ordering.by(f) to orElse.

Type parameters

S

the type returned by the extraction function f

Value parameters

f

the function to extract a comparison key from a T value

ord

the implicit ordering for the extracted key type S

Attributes

Returns

an Ordering[T] that uses this ordering first, falling back to comparing by f when values are equal

Example 
case class Pair(a: Int, b: Int)
val pairOrdering = Ordering.by[Pair, Int](_.a)
                          .orElseBy[Int](_.b)
Source
Ordering.scala
def orElseBy[S](f: T => S)(implicit ord: Ordering[S]): Ordering[T]

Given f, a function from T into S, creates an Ordering[T] whose compare function returns the result of this Ordering's compare function, if it is non-zero, or else a result equivalent to:

Given f, a function from T into S, creates an Ordering[T] whose compare function returns the result of this Ordering's compare function, if it is non-zero, or else a result equivalent to:

Ordering[S].compare(f(x), f(y))

This function is equivalent to passing the result of Ordering.by(f) to orElse.

Type parameters

S

the type returned by the extraction function f

Value parameters

f

the function to extract a comparison key from a T value

ord

the implicit ordering for the extracted key type S

Attributes

Returns

an Ordering[T] that uses this ordering first, falling back to comparing by f when values are equal

Example 
case class Pair(a: Int, b: Int)
val pairOrdering = Ordering.by[Pair, Int](_.a)
                          .orElseBy[Int](_.b)
Source
Ordering.scala
override def reverse: Ordering[T]

Returns the opposite ordering of this one.

Returns the opposite ordering of this one.

Implementations overriding this method MUST override isReverseOf as well if they change the behavior at all (for example, caching does not require overriding it).

Attributes

Returns

an Ordering[T] that compares elements in the reverse order of this ordering

Definition Classes
Source
Ordering.scala
def tryCompare(x: T, y: T): Some[Int]
Implicitly added by Iterable

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).

Value parameters

x

the first value to compare

y

the second value to compare

Attributes

Source
Ordering.scala
def tryCompare(x: T, y: T): Some[Int]
Implicitly added by Option

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).

Value parameters

x

the first value to compare

y

the second value to compare

Attributes

Source
Ordering.scala
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).

Value parameters

x

the first value to compare

y

the second value to compare

Attributes

Source
Ordering.scala

Inherited methods

def reversed(): Comparator[T]
Implicitly added by Iterable

Attributes

Inherited from:
Comparator
def reversed(): Comparator[T]
Implicitly added by Option

Attributes

Inherited from:
Comparator
def reversed(): Comparator[T]

Attributes

Inherited from:
Comparator
def thenComparing[U <: Comparable[_ >: U <: <FromJavaObject>]](x$0: Function[_ >: T <: <FromJavaObject>, _ <: U]): Comparator[T]
Implicitly added by Iterable

Attributes

Inherited from:
Comparator
def thenComparing[U <: <FromJavaObject>](x$0: Function[_ >: T <: <FromJavaObject>, _ <: U], x$1: Comparator[_ >: U <: <FromJavaObject>]): Comparator[T]
Implicitly added by Iterable

Attributes

Inherited from:
Comparator
def thenComparing(x$0: Comparator[_ >: T <: <FromJavaObject>]): Comparator[T]
Implicitly added by Iterable

Attributes

Inherited from:
Comparator
def thenComparing[U <: Comparable[_ >: U <: <FromJavaObject>]](x$0: Function[_ >: T <: <FromJavaObject>, _ <: U]): Comparator[T]
Implicitly added by Option

Attributes

Inherited from:
Comparator
def thenComparing[U <: <FromJavaObject>](x$0: Function[_ >: T <: <FromJavaObject>, _ <: U], x$1: Comparator[_ >: U <: <FromJavaObject>]): Comparator[T]
Implicitly added by Option

Attributes

Inherited from:
Comparator
def thenComparing(x$0: Comparator[_ >: T <: <FromJavaObject>]): Comparator[T]
Implicitly added by Option

Attributes

Inherited from:
Comparator
def thenComparing[U <: Comparable[_ >: U <: <FromJavaObject>]](x$0: Function[_ >: T <: <FromJavaObject>, _ <: U]): Comparator[T]

Attributes

Inherited from:
Comparator
def thenComparing[U <: <FromJavaObject>](x$0: Function[_ >: T <: <FromJavaObject>, _ <: U], x$1: Comparator[_ >: U <: <FromJavaObject>]): Comparator[T]

Attributes

Inherited from:
Comparator
def thenComparing(x$0: Comparator[_ >: T <: <FromJavaObject>]): Comparator[T]

Attributes

Inherited from:
Comparator
def thenComparingDouble(x$0: ToDoubleFunction[_ >: T <: <FromJavaObject>]): Comparator[T]
Implicitly added by Iterable

Attributes

Inherited from:
Comparator
def thenComparingDouble(x$0: ToDoubleFunction[_ >: T <: <FromJavaObject>]): Comparator[T]
Implicitly added by Option

Attributes

Inherited from:
Comparator
def thenComparingDouble(x$0: ToDoubleFunction[_ >: T <: <FromJavaObject>]): Comparator[T]

Attributes

Inherited from:
Comparator
def thenComparingInt(x$0: ToIntFunction[_ >: T <: <FromJavaObject>]): Comparator[T]
Implicitly added by Iterable

Attributes

Inherited from:
Comparator
def thenComparingInt(x$0: ToIntFunction[_ >: T <: <FromJavaObject>]): Comparator[T]
Implicitly added by Option

Attributes

Inherited from:
Comparator
def thenComparingInt(x$0: ToIntFunction[_ >: T <: <FromJavaObject>]): Comparator[T]

Attributes

Inherited from:
Comparator
def thenComparingLong(x$0: ToLongFunction[_ >: T <: <FromJavaObject>]): Comparator[T]
Implicitly added by Iterable

Attributes

Inherited from:
Comparator
def thenComparingLong(x$0: ToLongFunction[_ >: T <: <FromJavaObject>]): Comparator[T]
Implicitly added by Option

Attributes

Inherited from:
Comparator
def thenComparingLong(x$0: ToLongFunction[_ >: T <: <FromJavaObject>]): Comparator[T]

Attributes

Inherited from:
Comparator

Implicits

Implicits

implicit def mkOrderingOps(lhs: T): OrderingOps
Implicitly added by Iterable

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

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

Value parameters

lhs

the value to enrich with ordering operators

Attributes

Source
Ordering.scala
implicit def mkOrderingOps(lhs: T): OrderingOps
Implicitly added by Option

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

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

Value parameters

lhs

the value to enrich with ordering operators

Attributes

Source
Ordering.scala
implicit def mkOrderingOps(lhs: T): OrderingOps

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

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

Value parameters

lhs

the value to enrich with ordering operators

Attributes

Source
Ordering.scala
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Copyright (c) 2002-2026, LAMP/EPFL
Copyright (c) 2002-2026, LAMP/EPFL