# Source file src/sort/example_keys_test.go

## Documentation: sort

```     1  // Copyright 2013 The Go Authors. All rights reserved.
2  // Use of this source code is governed by a BSD-style
4
5  package sort_test
6
7  import (
8  	"fmt"
9  	"sort"
10  )
11
12  // A couple of type definitions to make the units clear.
13  type earthMass float64
14  type au float64
15
16  // A Planet defines the properties of a solar system object.
17  type Planet struct {
18  	name     string
19  	mass     earthMass
20  	distance au
21  }
22
23  // By is the type of a "less" function that defines the ordering of its Planet arguments.
24  type By func(p1, p2 *Planet) bool
25
26  // Sort is a method on the function type, By, that sorts the argument slice according to the function.
27  func (by By) Sort(planets []Planet) {
28  	ps := &planetSorter{
29  		planets: planets,
30  		by:      by, // The Sort method's receiver is the function (closure) that defines the sort order.
31  	}
32  	sort.Sort(ps)
33  }
34
35  // planetSorter joins a By function and a slice of Planets to be sorted.
36  type planetSorter struct {
37  	planets []Planet
38  	by      func(p1, p2 *Planet) bool // Closure used in the Less method.
39  }
40
41  // Len is part of sort.Interface.
42  func (s *planetSorter) Len() int {
43  	return len(s.planets)
44  }
45
46  // Swap is part of sort.Interface.
47  func (s *planetSorter) Swap(i, j int) {
48  	s.planets[i], s.planets[j] = s.planets[j], s.planets[i]
49  }
50
51  // Less is part of sort.Interface. It is implemented by calling the "by" closure in the sorter.
52  func (s *planetSorter) Less(i, j int) bool {
53  	return s.by(&s.planets[i], &s.planets[j])
54  }
55
56  var planets = []Planet{
57  	{"Mercury", 0.055, 0.4},
58  	{"Venus", 0.815, 0.7},
59  	{"Earth", 1.0, 1.0},
60  	{"Mars", 0.107, 1.5},
61  }
62
63  // ExampleSortKeys demonstrates a technique for sorting a struct type using programmable sort criteria.
64  func Example_sortKeys() {
65  	// Closures that order the Planet structure.
66  	name := func(p1, p2 *Planet) bool {
67  		return p1.name < p2.name
68  	}
69  	mass := func(p1, p2 *Planet) bool {
70  		return p1.mass < p2.mass
71  	}
72  	distance := func(p1, p2 *Planet) bool {
73  		return p1.distance < p2.distance
74  	}
75  	decreasingDistance := func(p1, p2 *Planet) bool {
76  		return distance(p2, p1)
77  	}
78
79  	// Sort the planets by the various criteria.
80  	By(name).Sort(planets)
81  	fmt.Println("By name:", planets)
82
83  	By(mass).Sort(planets)
84  	fmt.Println("By mass:", planets)
85
86  	By(distance).Sort(planets)
87  	fmt.Println("By distance:", planets)
88
89  	By(decreasingDistance).Sort(planets)
90  	fmt.Println("By decreasing distance:", planets)
91
92  	// Output: By name: [{Earth 1 1} {Mars 0.107 1.5} {Mercury 0.055 0.4} {Venus 0.815 0.7}]
93  	// By mass: [{Mercury 0.055 0.4} {Mars 0.107 1.5} {Venus 0.815 0.7} {Earth 1 1}]
94  	// By distance: [{Mercury 0.055 0.4} {Venus 0.815 0.7} {Earth 1 1} {Mars 0.107 1.5}]
95  	// By decreasing distance: [{Mars 0.107 1.5} {Earth 1 1} {Venus 0.815 0.7} {Mercury 0.055 0.4}]
96  }
97
```

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