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Source file src/container/heap/example_pq_test.go

Documentation: container/heap

  // Copyright 2012 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.
  // This example demonstrates a priority queue built using the heap interface.
  package heap_test
  import (
  // An Item is something we manage in a priority queue.
  type Item struct {
  	value    string // The value of the item; arbitrary.
  	priority int    // The priority of the item in the queue.
  	// The index is needed by update and is maintained by the heap.Interface methods.
  	index int // The index of the item in the heap.
  // A PriorityQueue implements heap.Interface and holds Items.
  type PriorityQueue []*Item
  func (pq PriorityQueue) Len() int { return len(pq) }
  func (pq PriorityQueue) Less(i, j int) bool {
  	// We want Pop to give us the highest, not lowest, priority so we use greater than here.
  	return pq[i].priority > pq[j].priority
  func (pq PriorityQueue) Swap(i, j int) {
  	pq[i], pq[j] = pq[j], pq[i]
  	pq[i].index = i
  	pq[j].index = j
  func (pq *PriorityQueue) Push(x interface{}) {
  	n := len(*pq)
  	item := x.(*Item)
  	item.index = n
  	*pq = append(*pq, item)
  func (pq *PriorityQueue) Pop() interface{} {
  	old := *pq
  	n := len(old)
  	item := old[n-1]
  	item.index = -1 // for safety
  	*pq = old[0 : n-1]
  	return item
  // update modifies the priority and value of an Item in the queue.
  func (pq *PriorityQueue) update(item *Item, value string, priority int) {
  	item.value = value
  	item.priority = priority
  	heap.Fix(pq, item.index)
  // This example creates a PriorityQueue with some items, adds and manipulates an item,
  // and then removes the items in priority order.
  func Example_priorityQueue() {
  	// Some items and their priorities.
  	items := map[string]int{
  		"banana": 3, "apple": 2, "pear": 4,
  	// Create a priority queue, put the items in it, and
  	// establish the priority queue (heap) invariants.
  	pq := make(PriorityQueue, len(items))
  	i := 0
  	for value, priority := range items {
  		pq[i] = &Item{
  			value:    value,
  			priority: priority,
  			index:    i,
  	// Insert a new item and then modify its priority.
  	item := &Item{
  		value:    "orange",
  		priority: 1,
  	heap.Push(&pq, item)
  	pq.update(item, item.value, 5)
  	// Take the items out; they arrive in decreasing priority order.
  	for pq.Len() > 0 {
  		item := heap.Pop(&pq).(*Item)
  		fmt.Printf("%.2d:%s ", item.priority, item.value)
  	// Output:
  	// 05:orange 04:pear 03:banana 02:apple

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