英文:
How to modify a slice within for loop?
问题
type Article struct {
FeedURL string
URL string // should be unique
// ... more data
}
func unsubscribe(articleList []Article, url string) []Article {
var result []Article
for _, article := range articleList {
if article.FeedURL != url {
result = append(result, article)
}
}
return result
}
func main() {
myArticleList := []Article{
Article{"http://blog.golang.org/feed.atom", "http://blog.golang.org/race-detector"},
Article{"http://planet.python.org/rss20.xml", "http://archlinux.me/dusty/2013/06/29/creating-an-application-in-kivy-part-3/"},
Article{"http://planet.python.org/rss20.xml", "http://feedproxy.google.com/~r/cubicweborg/~3/BncbP-ap0n0/2957378"},
// ... much more examples
}
myArticleList = unsubscribe(myArticleList, "http://planet.python.org/rss20.xml")fmt.Printf("%+v", myArticleList)
}
英文:
I've got a slice of articles in my reading list. Each Article has the attribute "FeedURL" that has the URL of the feed the article came from. When I unsubscribe from a feed, I want to be able to remove every Article that contains that Feed's URL.
type Article struct {FeedURL stringURL string // should be unique// ... more data}func unsubscribe(articleList []Article, url string) []Article {// how do I remove every Article from articleList that contains url?}func main() {myArticleList := []Article{Article{"http://blog.golang.org/feed.atom", "http://blog.golang.org/race-detector"},Article{"http://planet.python.org/rss20.xml", "http://archlinux.me/dusty/2013/06/29/creating-an-application-in-kivy-part-3/"},Article{"http://planet.python.org/rss20.xml", "http://feedproxy.google.com/~r/cubicweborg/~3/BncbP-ap0n0/2957378"},// ... much more examples}myArticleList = unsubscribe(myArticleList, "http://planet.python.org/rss20.xml")fmt.Printf("%+v", myArticleList)}
What is the efficient way of solving this problem?
At first my code looked like this for unsubscribe:
func unsubscribe(articleList []Article, url string) []Article {for _, article := range articleList {if article.FeedURL == url {articleList = append(articleList[:i], articleList[i+1:]...)}}return articleList}
But then I realized that this would change the slice and make the for loop unpredictable.
What is an efficient and pretty way to accomplish this?
答案1
得分: 5
为了提高效率：
- 使用指向Article的指针切片，这样我们将移动指针而不是结构体值。
- 如果列表中Article的顺序不重要，使用无序算法；它减少了指针的移动。否则，使用有序算法。无论如何，都要尽量减少指针的移动。
- 不要在列表末尾留下悬空指针。垃圾收集器会认为它们仍在使用；它查看的是切片的容量而不是切片的长度。
- 尽量减少内存分配。
例如，
package mainimport "fmt"type Article struct {FeedURL stringURL string // 应该是唯一的// ... 更多数据}// 从articleList中删除包含url的每个Article，不保留顺序。func unsubscribeUnordered(a []*Article, url string) []*Article {for i := 0; i < len(a); i++ {if a[i].FeedURL == url {a[len(a)-1], a[i], a = nil, a[len(a)-1], a[:len(a)-1]i--}}return a}// 从articleList中删除包含url的每个Article，保留顺序。func unsubscribeOrdered(a []*Article, url string) []*Article {j := 0for i := 0; i < len(a); i++ {if a[i].FeedURL == url {continue}if i != j {a[j] = a[i]}j++}for k := j; k < len(a); k++ {a[k] = nil}return a[:j]}func NewArticleList() []*Article {return []*Article{&Article{"http://blog.golang.org/feed.atom", "http://blog.golang.org/race-detector"},&Article{"http://planet.python.org/rss20.xml", "http://archlinux.me/dusty/2013/06/29/creating-an-application-in-kivy-part-3/"},&Article{"http://planet.python.org/rss20.xml", "http://feedproxy.google.com/~r/cubicweborg/~3/BncbP-ap0n0/2957378"},// ... 更多示例}}func PrintArticleList(a []*Article) {fmt.Print("[")for _, e := range a {fmt.Printf("%+v", *e)}fmt.Println("]")}func main() {PrintArticleList(NewArticleList())ao := unsubscribeOrdered(NewArticleList(), "http://planet.python.org/rss20.xml")PrintArticleList(ao)auo := unsubscribeUnordered(NewArticleList(), "http://planet.python.org/rss20.xml")PrintArticleList(auo)}
输出：
[{FeedURL:http://blog.golang.org/feed.atom URL:http://blog.golang.org/race-detector}{FeedURL:http://planet.python.org/rss20.xml URL:http://archlinux.me/dusty/2013/06/29/creating-an-application-in-kivy-part-3/}{FeedURL:http://planet.python.org/rss20.xml URL:http://feedproxy.google.com/~r/cubicweborg/~3/BncbP-ap0n0/2957378}][{FeedURL:http://blog.golang.org/feed.atom URL:http://blog.golang.org/race-detector}][{FeedURL:http://blog.golang.org/feed.atom URL:http://blog.golang.org/race-detector}]
英文:
To be efficient:
- Use a slice of pointers to Articles, then we will be moving pointers
to structures instead of structure values. - If the order of the Articles in the list is not important, use the
unordered algorithm; it reduces pointer movement. Otherwise, use the
ordered algorithm. In any case, minimize pointer movement. - Don't leave dangling pointers at the end of the list. The garbage
collector will think they are still in use; it looks at the slice
capacity not the slice length. - Minimize memory allocations.
For example,
package mainimport "fmt"type Article struct {FeedURL stringURL string // should be unique// ... more data}// Remove every Article from an articleList that contains url without preserving order.func unsubscribeUnordered(a []*Article, url string) []*Article {for i := 0; i < len(a); i++ {if a[i].FeedURL == url {a[len(a)-1], a[i], a = nil, a[len(a)-1], a[:len(a)-1]i--}}return a}// Remove every Article from an articleList that contains url while preserving order.func unsubscribeOrdered(a []*Article, url string) []*Article {j := 0for i := 0; i < len(a); i++ {if a[i].FeedURL == url {continue}if i != j {a[j] = a[i]}j++}for k := j; k < len(a); k++ {a[k] = nil}return a[:j]}func NewArticleList() []*Article {return []*Article{&Article{"http://blog.golang.org/feed.atom", "http://blog.golang.org/race-detector"},&Article{"http://planet.python.org/rss20.xml", "http://archlinux.me/dusty/2013/06/29/creating-an-application-in-kivy-part-3/"},&Article{"http://planet.python.org/rss20.xml", "http://feedproxy.google.com/~r/cubicweborg/~3/BncbP-ap0n0/2957378"},// ... much more examples}}func PrintArticleList(a []*Article) {fmt.Print("[")for _, e := range a {fmt.Printf("%+v", *e)}fmt.Println("]")}func main() {PrintArticleList(NewArticleList())ao := unsubscribeOrdered(NewArticleList(), "http://planet.python.org/rss20.xml")PrintArticleList(ao)auo := unsubscribeUnordered(NewArticleList(), "http://planet.python.org/rss20.xml")PrintArticleList(auo)}
Output:
[{FeedURL:http://blog.golang.org/feed.atom URL:http://blog.golang.org/race-detector}{FeedURL:http://planet.python.org/rss20.xml URL:http://archlinux.me/dusty/2013/06/29/creating-an-application-in-kivy-part-3/}{FeedURL:http://planet.python.org/rss20.xml URL:http://feedproxy.google.com/~r/cubicweborg/~3/BncbP-ap0n0/2957378}][{FeedURL:http://blog.golang.org/feed.atom URL:http://blog.golang.org/race-detector}][{FeedURL:http://blog.golang.org/feed.atom URL:http://blog.golang.org/race-detector}]
答案2
得分: 0
PeterSO的答案完成了工作，并且效率很高。
但是，我可能会选择像这样简单的方法
func unsubscribe(articleList []Article, url string) (filtered []Article) {filtered = articleList[:0] // 可选的。重用已分配的内存。for _, article := range articleList {if article.FeedURL != url {filtered = append(filtered, article)}}return}
只需要大约两秒钟就可以阅读和理解。
这个想法对于文章的指针也适用，就像PeterSO说的那样，如果你的Article结构体很大，这可能是一个好主意。
英文:
PeterSO's answer is gets the job done, and with efficiency.
But, I might go with something simple like this
func unsubscribe(articleList []Article, url string) (filtered []Article) {filtered = articleList[:0] // optional. reuses already-allocated memory.for _, article := range articleList {if article.FeedURL != url {filtered = append(filtered, article)}}return}
which only takes about a two seconds to read, and comprehend.
The idea works fine with pointers to articles too and, like PeterSO said, if your Article struct is big, that may be a good thing to do.
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