# Package sort

Go/docs/latest/sort/index

# Package sort

## Overview

Package sort provides primitives for sorting slices and user-defined collections.

#### Example

Code:

``````package sort_test

import (
"fmt"
"sort"
)

type Person struct {
Name string
Age  int
}

func (p Person) String() string {
return fmt.Sprintf("%s: %d", p.Name, p.Age)
}

// ByAge implements sort.Interface for []Person based on
// the Age field.
type ByAge []Person

func (a ByAge) Len() int           { return len(a) }
func (a ByAge) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
func (a ByAge) Less(i, j int) bool { return a[i].Age < a[j].Age }

func Example() {
people := []Person{
{"Bob", 31},
{"John", 42},
{"Michael", 17},
{"Jenny", 26},
}

fmt.Println(people)
// There are two ways to sort a slice. First, one can define
// a set of methods for the slice type, as with ByAge, and
// call sort.Sort. In this first example we use that technique.
sort.Sort(ByAge(people))
fmt.Println(people)

// The other way is to use sort.Slice with a custom Less
// function, which can be provided as a closure. In this
// case no methods are needed. (And if they exist, they
// are ignored.) Here we re-sort in reverse order: compare
// the closure with ByAge.Less.
sort.Slice(people, func(i, j int) bool {
return people[i].Age > people[j].Age
})
fmt.Println(people)

// Output:
// [Bob: 31 John: 42 Michael: 17 Jenny: 26]
// [Michael: 17 Jenny: 26 Bob: 31 John: 42]
// [John: 42 Bob: 31 Jenny: 26 Michael: 17]
}``````

#### Example (SortKeys)

ExampleSortKeys demonstrates a technique for sorting a struct type using programmable sort criteria.

Code:

``````package sort_test

import (
"fmt"
"sort"
)

// A couple of type definitions to make the units clear.
type earthMass float64
type au float64

// A Planet defines the properties of a solar system object.
type Planet struct {
name     string
mass     earthMass
distance au
}

// By is the type of a "less" function that defines the ordering of its Planet arguments.
type By func(p1, p2 *Planet) bool

// Sort is a method on the function type, By, that sorts the argument slice according to the function.
func (by By) Sort(planets []Planet) {
ps := &planetSorter{
planets: planets,
by:      by, // The Sort method's receiver is the function (closure) that defines the sort order.
}
sort.Sort(ps)
}

// planetSorter joins a By function and a slice of Planets to be sorted.
type planetSorter struct {
planets []Planet
by      func(p1, p2 *Planet) bool // Closure used in the Less method.
}

// Len is part of sort.Interface.
func (s *planetSorter) Len() int {
return len(s.planets)
}

// Swap is part of sort.Interface.
func (s *planetSorter) Swap(i, j int) {
s.planets[i], s.planets[j] = s.planets[j], s.planets[i]
}

// Less is part of sort.Interface. It is implemented by calling the "by" closure in the sorter.
func (s *planetSorter) Less(i, j int) bool {
return s.by(&s.planets[i], &s.planets[j])
}

var planets = []Planet{
{"Mercury", 0.055, 0.4},
{"Venus", 0.815, 0.7},
{"Earth", 1.0, 1.0},
{"Mars", 0.107, 1.5},
}

// ExampleSortKeys demonstrates a technique for sorting a struct type using programmable sort criteria.
func Example_sortKeys() {
// Closures that order the Planet structure.
name := func(p1, p2 *Planet) bool {
return p1.name < p2.name
}
mass := func(p1, p2 *Planet) bool {
return p1.mass < p2.mass
}
distance := func(p1, p2 *Planet) bool {
return p1.distance < p2.distance
}
decreasingDistance := func(p1, p2 *Planet) bool {
return distance(p2, p1)
}

// Sort the planets by the various criteria.
By(name).Sort(planets)
fmt.Println("By name:", planets)

By(mass).Sort(planets)
fmt.Println("By mass:", planets)

By(distance).Sort(planets)
fmt.Println("By distance:", planets)

By(decreasingDistance).Sort(planets)
fmt.Println("By decreasing distance:", planets)

// Output: By name: [{Earth 1 1} {Mars 0.107 1.5} {Mercury 0.055 0.4} {Venus 0.815 0.7}]
// By mass: [{Mercury 0.055 0.4} {Mars 0.107 1.5} {Venus 0.815 0.7} {Earth 1 1}]
// By distance: [{Mercury 0.055 0.4} {Venus 0.815 0.7} {Earth 1 1} {Mars 0.107 1.5}]
// By decreasing distance: [{Mars 0.107 1.5} {Earth 1 1} {Venus 0.815 0.7} {Mercury 0.055 0.4}]
}``````

#### Example (SortMultiKeys)

ExampleMultiKeys demonstrates a technique for sorting a struct type using different sets of multiple fields in the comparison. We chain together "Less" functions, each of which compares a single field.

Code:

``````package sort_test

import (
"fmt"
"sort"
)

// A Change is a record of source code changes, recording user, language, and delta size.
type Change struct {
user     string
language string
lines    int
}

type lessFunc func(p1, p2 *Change) bool

// multiSorter implements the Sort interface, sorting the changes within.
type multiSorter struct {
changes []Change
less    []lessFunc
}

// Sort sorts the argument slice according to the less functions passed to OrderedBy.
func (ms *multiSorter) Sort(changes []Change) {
ms.changes = changes
sort.Sort(ms)
}

// OrderedBy returns a Sorter that sorts using the less functions, in order.
// Call its Sort method to sort the data.
func OrderedBy(less ...lessFunc) *multiSorter {
return &multiSorter{
less: less,
}
}

// Len is part of sort.Interface.
func (ms *multiSorter) Len() int {
return len(ms.changes)
}

// Swap is part of sort.Interface.
func (ms *multiSorter) Swap(i, j int) {
ms.changes[i], ms.changes[j] = ms.changes[j], ms.changes[i]
}

// Less is part of sort.Interface. It is implemented by looping along the
// less functions until it finds a comparison that discriminates between
// the two items (one is less than the other). Note that it can call the
// less functions twice per call. We could change the functions to return
// -1, 0, 1 and reduce the number of calls for greater efficiency: an
func (ms *multiSorter) Less(i, j int) bool {
p, q := &ms.changes[i], &ms.changes[j]
// Try all but the last comparison.
var k int
for k = 0; k < len(ms.less)-1; k++ {
less := ms.less[k]
switch {
case less(p, q):
// p < q, so we have a decision.
return true
case less(q, p):
// p > q, so we have a decision.
return false
}
// p == q; try the next comparison.
}
// All comparisons to here said "equal", so just return whatever
// the final comparison reports.
return ms.less[k](p, q)
}

var changes = []Change{
{"gri", "Go", 100},
{"ken", "C", 150},
{"glenda", "Go", 200},
{"rsc", "Go", 200},
{"r", "Go", 100},
{"ken", "Go", 200},
{"dmr", "C", 100},
{"r", "C", 150},
{"gri", "Smalltalk", 80},
}

// ExampleMultiKeys demonstrates a technique for sorting a struct type using different
// sets of multiple fields in the comparison. We chain together "Less" functions, each of
// which compares a single field.
func Example_sortMultiKeys() {
// Closures that order the Change structure.
user := func(c1, c2 *Change) bool {
return c1.user < c2.user
}
language := func(c1, c2 *Change) bool {
return c1.language < c2.language
}
increasingLines := func(c1, c2 *Change) bool {
return c1.lines < c2.lines
}
decreasingLines := func(c1, c2 *Change) bool {
return c1.lines > c2.lines // Note: > orders downwards.
}

// Simple use: Sort by user.
OrderedBy(user).Sort(changes)
fmt.Println("By user:", changes)

// More examples.
OrderedBy(user, increasingLines).Sort(changes)
fmt.Println("By user,<lines:", changes)

OrderedBy(user, decreasingLines).Sort(changes)
fmt.Println("By user,>lines:", changes)

OrderedBy(language, increasingLines).Sort(changes)
fmt.Println("By language,<lines:", changes)

OrderedBy(language, increasingLines, user).Sort(changes)
fmt.Println("By language,<lines,user:", changes)

// Output:
// By user: [{dmr C 100} {glenda Go 200} {gri Go 100} {gri Smalltalk 80} {ken C 150} {ken Go 200} {r Go 100} {r C 150} {rsc Go 200}]
// By user,<lines: [{dmr C 100} {glenda Go 200} {gri Smalltalk 80} {gri Go 100} {ken C 150} {ken Go 200} {r Go 100} {r C 150} {rsc Go 200}]
// By user,>lines: [{dmr C 100} {glenda Go 200} {gri Go 100} {gri Smalltalk 80} {ken Go 200} {ken C 150} {r C 150} {r Go 100} {rsc Go 200}]
// By language,<lines: [{dmr C 100} {ken C 150} {r C 150} {r Go 100} {gri Go 100} {ken Go 200} {glenda Go 200} {rsc Go 200} {gri Smalltalk 80}]
// By language,<lines,user: [{dmr C 100} {ken C 150} {r C 150} {gri Go 100} {r Go 100} {glenda Go 200} {ken Go 200} {rsc Go 200} {gri Smalltalk 80}]

}``````

#### Example (SortWrapper)

Code:

``````package sort_test

import (
"fmt"
"sort"
)

type Grams int

func (g Grams) String() string { return fmt.Sprintf("%dg", int(g)) }

type Organ struct {
Name   string
Weight Grams
}

type Organs []*Organ

func (s Organs) Len() int      { return len(s) }
func (s Organs) Swap(i, j int) { s[i], s[j] = s[j], s[i] }

// ByName implements sort.Interface by providing Less and using the Len and
// Swap methods of the embedded Organs value.
type ByName struct{ Organs }

func (s ByName) Less(i, j int) bool { return s.Organs[i].Name < s.Organs[j].Name }

// ByWeight implements sort.Interface by providing Less and using the Len and
// Swap methods of the embedded Organs value.
type ByWeight struct{ Organs }

func (s ByWeight) Less(i, j int) bool { return s.Organs[i].Weight < s.Organs[j].Weight }

func Example_sortWrapper() {
s := []*Organ{
{"brain", 1340},
{"heart", 290},
{"liver", 1494},
{"pancreas", 131},
{"prostate", 62},
{"spleen", 162},
}

sort.Sort(ByWeight{s})
fmt.Println("Organs by weight:")
printOrgans(s)

sort.Sort(ByName{s})
fmt.Println("Organs by name:")
printOrgans(s)

// Output:
// Organs by weight:
// prostate (62g)
// pancreas (131g)
// spleen   (162g)
// heart    (290g)
// brain    (1340g)
// liver    (1494g)
// Organs by name:
// brain    (1340g)
// heart    (290g)
// liver    (1494g)
// pancreas (131g)
// prostate (62g)
// spleen   (162g)
}

func printOrgans(s []*Organ) {
for _, o := range s {
fmt.Printf("%-8s (%v)\n", o.Name, o.Weight)
}
}``````

## Index

### Package files

search.go slice.go slice_go113.go sort.go zfuncversion.go

## func Float64s

``func Float64s(x []float64)``

Float64s sorts a slice of float64s in increasing order. Not-a-number (NaN) values are ordered before other values.

#### Example

Code:

``````s := []float64{5.2, -1.3, 0.7, -3.8, 2.6} // unsorted
sort.Float64s(s)
fmt.Println(s)

s = []float64{math.Inf(1), math.NaN(), math.Inf(-1), 0.0} // unsorted
sort.Float64s(s)
fmt.Println(s)``````

Output:

``````[-3.8 -1.3 0.7 2.6 5.2]
[NaN -Inf 0 +Inf]``````

## func Float64sAreSorted

``func Float64sAreSorted(x []float64) bool``

Float64sAreSorted reports whether the slice x is sorted in increasing order, with not-a-number (NaN) values before any other values.

#### Example

Code:

``````s := []float64{0.7, 1.3, 2.6, 3.8, 5.2} // sorted ascending
fmt.Println(sort.Float64sAreSorted(s))

s = []float64{5.2, 3.8, 2.6, 1.3, 0.7} // sorted descending
fmt.Println(sort.Float64sAreSorted(s))

s = []float64{5.2, 1.3, 0.7, 3.8, 2.6} // unsorted
fmt.Println(sort.Float64sAreSorted(s))``````

Output:

``````true
false
false``````

## func Ints

``func Ints(x []int)``

Ints sorts a slice of ints in increasing order.

#### Example

Code:

``````s := []int{5, 2, 6, 3, 1, 4} // unsorted
sort.Ints(s)
fmt.Println(s)``````

Output:

``[1 2 3 4 5 6]``

## func IntsAreSorted

``func IntsAreSorted(x []int) bool``

IntsAreSorted reports whether the slice x is sorted in increasing order.

#### Example

Code:

``````s := []int{1, 2, 3, 4, 5, 6} // sorted ascending
fmt.Println(sort.IntsAreSorted(s))

s = []int{6, 5, 4, 3, 2, 1} // sorted descending
fmt.Println(sort.IntsAreSorted(s))

s = []int{3, 2, 4, 1, 5} // unsorted
fmt.Println(sort.IntsAreSorted(s))``````

Output:

``````true
false
false``````

## func IsSorted

``func IsSorted(data Interface) bool``

IsSorted reports whether data is sorted.

## func Search

``func Search(n int, f func(int) bool) int``

Search uses binary search to find and return the smallest index i in [0, n) at which f(i) is true, assuming that on the range [0, n), f(i) == true implies f(i+1) == true. That is, Search requires that f is false for some (possibly empty) prefix of the input range [0, n) and then true for the (possibly empty) remainder; Search returns the first true index. If there is no such index, Search returns n. (Note that the "not found" return value is not -1 as in, for instance, strings.Index.) Search calls f(i) only for i in the range [0, n).

A common use of Search is to find the index i for a value x in a sorted, indexable data structure such as an array or slice. In this case, the argument f, typically a closure, captures the value to be searched for, and how the data structure is indexed and ordered.

For instance, given a slice data sorted in ascending order, the call Search(len(data), func(i int) bool { return data[i] >= 23 }) returns the smallest index i such that data[i] >= 23. If the caller wants to find whether 23 is in the slice, it must test data[i] == 23 separately.

Searching data sorted in descending order would use the <= operator instead of the >= operator.

To complete the example above, the following code tries to find the value x in an integer slice data sorted in ascending order:

``````x := 23
i := sort.Search(len(data), func(i int) bool { return data[i] >= x })
if i < len(data) && data[i] == x {
// x is present at data[i]
} else {
// x is not present in data,
// but i is the index where it would be inserted.
}``````

As a more whimsical example, this program guesses your number:

``````func GuessingGame() {
var s string
fmt.Printf("Pick an integer from 0 to 100.\n")
answer := sort.Search(100, func(i int) bool {
fmt.Printf("Is your number <= %d? ", i)
fmt.Scanf("%s", &s)
return s != "" && s[0] == 'y'
})
}``````

#### Example

This example demonstrates searching a list sorted in ascending order.

Code:

``````a := []int{1, 3, 6, 10, 15, 21, 28, 36, 45, 55}
x := 6

i := sort.Search(len(a), func(i int) bool { return a[i] >= x })
if i < len(a) && a[i] == x {
fmt.Printf("found %d at index %d in %v\n", x, i, a)
} else {
}``````

Output:

``found 6 at index 2 in [1 3 6 10 15 21 28 36 45 55]``

#### Example (DescendingOrder)

This example demonstrates searching a list sorted in descending order. The approach is the same as searching a list in ascending order, but with the condition inverted.

Code:

``````a := []int{55, 45, 36, 28, 21, 15, 10, 6, 3, 1}
x := 6

i := sort.Search(len(a), func(i int) bool { return a[i] <= x })
if i < len(a) && a[i] == x {
fmt.Printf("found %d at index %d in %v\n", x, i, a)
} else {
}``````

Output:

``found 6 at index 7 in [55 45 36 28 21 15 10 6 3 1]``

## func SearchFloat64s

``func SearchFloat64s(a []float64, x float64) int``

SearchFloat64s searches for x in a sorted slice of float64s and returns the index as specified by Search. The return value is the index to insert x if x is not present (it could be len(a)). The slice must be sorted in ascending order.

#### Example

This example demonstrates searching for float64 in a list sorted in ascending order.

Code:

``````a := []float64{1.0, 2.0, 3.3, 4.6, 6.1, 7.2, 8.0}

x := 2.0
i := sort.SearchFloat64s(a, x)
fmt.Printf("found %g at index %d in %v\n", x, i, a)

x = 0.5
i = sort.SearchFloat64s(a, x)
fmt.Printf("%g not found, can be inserted at index %d in %v\n", x, i, a)``````

Output:

``````found 2 at index 1 in [1 2 3.3 4.6 6.1 7.2 8]
0.5 not found, can be inserted at index 0 in [1 2 3.3 4.6 6.1 7.2 8]``````

## func SearchInts

``func SearchInts(a []int, x int) int``

SearchInts searches for x in a sorted slice of ints and returns the index as specified by Search. The return value is the index to insert x if x is not present (it could be len(a)). The slice must be sorted in ascending order.

#### Example

This example demonstrates searching for int in a list sorted in ascending order.

Code:

``````a := []int{1, 2, 3, 4, 6, 7, 8}

x := 2
i := sort.SearchInts(a, x)
fmt.Printf("found %d at index %d in %v\n", x, i, a)

x = 5
i = sort.SearchInts(a, x)
fmt.Printf("%d not found, can be inserted at index %d in %v\n", x, i, a)``````

Output:

``````found 2 at index 1 in [1 2 3 4 6 7 8]
5 not found, can be inserted at index 4 in [1 2 3 4 6 7 8]``````

## func SearchStrings

``func SearchStrings(a []string, x string) int``

SearchStrings searches for x in a sorted slice of strings and returns the index as specified by Search. The return value is the index to insert x if x is not present (it could be len(a)). The slice must be sorted in ascending order.

## func Slice 1.8

``func Slice(x interface{}, less func(i, j int) bool)``

Slice sorts the slice x given the provided less function. It panics if x is not a slice.

The sort is not guaranteed to be stable: equal elements may be reversed from their original order. For a stable sort, use SliceStable.

The less function must satisfy the same requirements as the Interface type's Less method.

#### Example

Code:

``````people := []struct {
Name string
Age  int
}{
{"Gopher", 7},
{"Alice", 55},
{"Vera", 24},
{"Bob", 75},
}
sort.Slice(people, func(i, j int) bool { return people[i].Name < people[j].Name })
fmt.Println("By name:", people)

sort.Slice(people, func(i, j int) bool { return people[i].Age < people[j].Age })
fmt.Println("By age:", people)``````

Output:

``````By name: [{Alice 55} {Bob 75} {Gopher 7} {Vera 24}]
By age: [{Gopher 7} {Vera 24} {Alice 55} {Bob 75}]``````

## func SliceIsSorted 1.8

``func SliceIsSorted(x interface{}, less func(i, j int) bool) bool``

SliceIsSorted reports whether the slice x is sorted according to the provided less function. It panics if x is not a slice.

## func SliceStable 1.8

``func SliceStable(x interface{}, less func(i, j int) bool)``

SliceStable sorts the slice x using the provided less function, keeping equal elements in their original order. It panics if x is not a slice.

The less function must satisfy the same requirements as the Interface type's Less method.

#### Example

Code:

``````people := []struct {
Name string
Age  int
}{
{"Alice", 25},
{"Elizabeth", 75},
{"Alice", 75},
{"Bob", 75},
{"Alice", 75},
{"Bob", 25},
{"Colin", 25},
{"Elizabeth", 25},
}

// Sort by name, preserving original order
sort.SliceStable(people, func(i, j int) bool { return people[i].Name < people[j].Name })
fmt.Println("By name:", people)

// Sort by age preserving name order
sort.SliceStable(people, func(i, j int) bool { return people[i].Age < people[j].Age })
fmt.Println("By age,name:", people)``````

Output:

``````By name: [{Alice 25} {Alice 75} {Alice 75} {Bob 75} {Bob 25} {Colin 25} {Elizabeth 75} {Elizabeth 25}]
By age,name: [{Alice 25} {Bob 25} {Colin 25} {Elizabeth 25} {Alice 75} {Alice 75} {Bob 75} {Elizabeth 75}]``````

## func Sort

``func Sort(data Interface)``

Sort sorts data. It makes one call to data.Len to determine n and O(n*log(n)) calls to data.Less and data.Swap. The sort is not guaranteed to be stable.

## func Stable 1.2

``func Stable(data Interface)``

Stable sorts data while keeping the original order of equal elements.

It makes one call to data.Len to determine n, O(n*log(n)) calls to data.Less and O(n*log(n)*log(n)) calls to data.Swap.

## func Strings

``func Strings(x []string)``

Strings sorts a slice of strings in increasing order.

#### Example

Code:

``````s := []string{"Go", "Bravo", "Gopher", "Alpha", "Grin", "Delta"}
sort.Strings(s)
fmt.Println(s)``````

Output:

``[Alpha Bravo Delta Go Gopher Grin]``

## func StringsAreSorted

``func StringsAreSorted(x []string) bool``

StringsAreSorted reports whether the slice x is sorted in increasing order.

## type Float64Slice

Float64Slice implements Interface for a []float64, sorting in increasing order, with not-a-number (NaN) values ordered before other values.

``type Float64Slice []float64``

### func (Float64Slice) Len

``func (x Float64Slice) Len() int``

### func (Float64Slice) Less

``func (x Float64Slice) Less(i, j int) bool``

Less reports whether x[i] should be ordered before x[j], as required by the sort Interface. Note that floating-point comparison by itself is not a transitive relation: it does not report a consistent ordering for not-a-number (NaN) values. This implementation of Less places NaN values before any others, by using:

``x[i] < x[j] || (math.IsNaN(x[i]) && !math.IsNaN(x[j]))``

### func (Float64Slice) Search

``func (p Float64Slice) Search(x float64) int``

Search returns the result of applying SearchFloat64s to the receiver and x.

### func (Float64Slice) Sort

``func (x Float64Slice) Sort()``

Sort is a convenience method: x.Sort() calls Sort(x).

### func (Float64Slice) Swap

``func (x Float64Slice) Swap(i, j int)``

## type IntSlice

IntSlice attaches the methods of Interface to []int, sorting in increasing order.

``type IntSlice []int``

### func (IntSlice) Len

``func (x IntSlice) Len() int``

### func (IntSlice) Less

``func (x IntSlice) Less(i, j int) bool``

### func (IntSlice) Search

``func (p IntSlice) Search(x int) int``

Search returns the result of applying SearchInts to the receiver and x.

### func (IntSlice) Sort

``func (x IntSlice) Sort()``

Sort is a convenience method: x.Sort() calls Sort(x).

### func (IntSlice) Swap

``func (x IntSlice) Swap(i, j int)``

## type Interface

An implementation of Interface can be sorted by the routines in this package. The methods refer to elements of the underlying collection by integer index.

``````type Interface interface {
// Len is the number of elements in the collection.
Len() int

// Less reports whether the element with index i
// must sort before the element with index j.
//
// If both Less(i, j) and Less(j, i) are false,
// then the elements at index i and j are considered equal.
// Sort may place equal elements in any order in the final result,
// while Stable preserves the original input order of equal elements.
//
// Less must describe a transitive ordering:
//  - if both Less(i, j) and Less(j, k) are true, then Less(i, k) must be true as well.
//  - if both Less(i, j) and Less(j, k) are false, then Less(i, k) must be false as well.
//
// Note that floating-point comparison (the < operator on float32 or float64 values)
// is not a transitive ordering when not-a-number (NaN) values are involved.
// See Float64Slice.Less for a correct implementation for floating-point values.
Less(i, j int) bool

// Swap swaps the elements with indexes i and j.
Swap(i, j int)
}``````

### func Reverse 1.1

``func Reverse(data Interface) Interface``

Reverse returns the reverse order for data.

#### Example

Code:

``````s := []int{5, 2, 6, 3, 1, 4} // unsorted
sort.Sort(sort.Reverse(sort.IntSlice(s)))
fmt.Println(s)``````

Output:

``[6 5 4 3 2 1]``

## type StringSlice

StringSlice attaches the methods of Interface to []string, sorting in increasing order.

``type StringSlice []string``

### func (StringSlice) Len

``func (x StringSlice) Len() int``

### func (StringSlice) Less

``func (x StringSlice) Less(i, j int) bool``

### func (StringSlice) Search

``func (p StringSlice) Search(x string) int``

Search returns the result of applying SearchStrings to the receiver and x.

### func (StringSlice) Sort

``func (x StringSlice) Sort()``

Sort is a convenience method: x.Sort() calls Sort(x).

### func (StringSlice) Swap

``func (x StringSlice) Swap(i, j int)``