Overriding the Pop() method

php Editor Banana today will introduce to you how to rewrite the Pop() method. In programming, Pop() method is used to remove and return the last element of an array. However, sometimes we need to customize the Pop() method to meet specific needs. By overriding the Pop() method, we can add additional logic or modify the returned elements to better fit our code. This article will introduce in detail how to override the Pop() method and give some examples to help understand. let's start!

Question content

Under the go installation, they have an example of a priority queue in container/heap/example_pq_test.go I paste the contents of the entire file so I can ask the pop() method.

// 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 (
    "container/heap"
    "fmt"
)

// 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 any) {
    n := len(*pq)
    item := x.(*item)
    item.index = n
    *pq = append(*pq, item)
}

func (pq *priorityqueue) pop() any {
    old := *pq
    n := len(old)
    item := old[n-1]
    old[n-1] = nil  // avoid memory leak
    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,
        }
        i++
    }
    heap.init(&pq)

    // 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
}

If I have a pop() method like the following (without creating a deep copy of the original slice), what harm could it cause or is there a fallacy

func (pq *PriorityQueue) Pop2() any {
    n := len(*pq)
    item := (*pq)[n-1]
    (*pq)[n-1] = nil  // avoid memory leak
    item.index = -1 // for safety
    *pq = (*pq)[: n-1]
    return item
}

I believe the original pop() method, this line slices old := *pq to create a deep copy (allocate a new underlying array). is this real?

Solution

makeThe objects created by the function, here are map and slice, are more like pointing to the data location pointer, not the data itself.

So old := *pq behaves more like alias than data copying.

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