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Source file src/sort/example_keys_test.go

Documentation: sort

  // Copyright 2013 The Go Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style
  // license that can be found in the LICENSE file.
  
  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(p1, p2)
  	}
  
  	// 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}]
  }
  

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