Package sort

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]
}

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]

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}]
}

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}]

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
// exercise for the reader.
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}]

}

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}]

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)
    }
}

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)

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]

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

Code:

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

Output:

[1 2 3 4 5 6]

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

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 {
    fmt.Printf("%d not found in %v\n", x, a)
}

Output:

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

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 {
    fmt.Printf("%d not found in %v\n", x, a)
}

Output:

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

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}]

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}]

Code:

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

Output:

[Alpha Bravo Delta Go Gopher Grin]

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]