# Documentation

Std.Data.Ord

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Swaps less and greater ordering results

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@[macro_inline]

If o₁ and o₂ are Ordering, then o₁.then o₂ returns o₁ unless it is .eq, in which case it returns o₂. Additionally, it has "short-circuiting" semantics similar to boolean x && y: if o₁ is not .eq then the expression for o₂ is not evaluated. This is a useful primitive for constructing lexicographic comparator functions:

structure Person where
name : String
age : Nat

instance : Ord Person where
compare a b := (compare a.name b.name).then (compare b.age a.age)


This example will sort people first by name (in ascending order) and will sort people with the same name by age (in descending order). (If all fields are sorted ascending and in the same order as they are listed in the structure, you can also use deriving Ord on the structure definition for the same effect.)

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Check whether the ordering is 'equal'.

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• = match x with | Ordering.eq => true | x => false

Check whether the ordering is 'not equal'.

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• = match x with | Ordering.eq => false | x => true

Check whether the ordering is 'less than'.

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• = match x with | Ordering.lt => true | x => false

Check whether the ordering is 'greater than'.

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• = match x with | Ordering.gt => true | x => false

Check whether the ordering is 'greater than or equal'.

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• = match x with | Ordering.lt => false | x => true
@[inline]
def compareLex {α : Sort u_1} {β : Sort u_2} (cmp₁ : αβOrdering) (cmp₂ : αβOrdering) (a : α) (b : β) :

Compare a and b lexicographically by cmp₁ and cmp₂. a and b are first compared by cmp₁. If this returns 'equal', a and b are compared by cmp₂ to break the tie.

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@[inline]
def compareOn {β : Type u_1} {α : Sort u_2} [ord : Ord β] (f : αβ) (x : α) (y : α) :

Compare x and y by comparing f x and f y.

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def Ord.toBEq {α : Type u_1} (ord : Ord α) :
BEq α

Derive a BEq instance from an Ord instance.

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def Ord.toLT {α : Type u_1} :
Ord αLT α

Derive an LT instance from an Ord instance.

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• = ltOfOrd
def Ord.toLE {α : Type u_1} :
Ord αLE α

Derive an LE instance from an Ord instance.

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• = leOfOrd
def Ord.opposite {α : Type u_1} (ord : Ord α) :
Ord α

Invert the order of an Ord instance.

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def Ord.on {β : Type u_1} {α : Type u_2} (ord : Ord β) (f : αβ) :
Ord α

ord.on f compares x and y by comparing f x and f y according to ord.

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def Ord.lex {α : Type u_1} {β : Type u_2} :
Ord αOrd βOrd (α × β)

Derive the lexicographic order on products α × β× β from orders for α and β.

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def Ord.lex' {α : Type u_1} (ord₁ : Ord α) (ord₂ : Ord α) :
Ord α

Create an order which compares elements first by ord₁ and then, if this returns 'equal', by ord₂.

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