container/heap: update example code

- use Init to establish heap invariant on
  a non-empty heap
- use Fix to update heap after an element's
  properties have been changed

(The old code used Init where it wasn't needed,
 and didn't use Fix because Fix was added after
 the example was written.)

LGTM=bradfitz
R=adonovan, bradfitz
CC=golang-codereviews
https://golang.org/cl/94520043

src/pkg/container/heap/example_pq_test.go

@@ -52,13 +52,12 @@ func (pq *PriorityQueue) Pop() interface{} {
 
 // update modifies the priority and value of an Item in the queue.
 func (pq *PriorityQueue) update(item *Item, value string, priority int) {
-	heap.Remove(pq, item.index)
 	item.value = value
 	item.priority = priority
-	heap.Push(pq, item)
+	heap.Fix(pq, item.index)
 }
 
-// This example inserts some items into a PriorityQueue, manipulates an item,
+// 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.
@@ -66,28 +65,31 @@ func Example_priorityQueue() {
 		"banana": 3, "apple": 2, "pear": 4,
 	}
 
-	// Create a priority queue and put the items in it.
-	pq := &PriorityQueue{}
-	heap.Init(pq)
+	// 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 {
-		item := &Item{
+		pq[i] = &Item{
 			value:    value,
 			priority: priority,
+			index:    i,
 		}
-		heap.Push(pq, item)
+		i++
 	}
+	heap.Init(&pq)
 
 	// Insert a new item and then modify its priority.
 	item := &Item{
 		value:    "orange",
 		priority: 1,
 	}
-	heap.Push(pq, item)
+	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)
+		item := heap.Pop(&pq).(*Item)
 		fmt.Printf("%.2d:%s ", item.priority, item.value)
 	}
 	// Output: