我正在尝试构建一个简单的程序，但看起来好像我遇到了一个我根本不理解的小问题：
我有以下代码：
package objsets
import common._
import TweetReader._
/**
* A class to represent tweets.
*/
class Tweet(val user: String, val text: String, val retweets: Int) {
override def toString: String =
"User: " + user + "\n" +
"Text: " + text + " [" + retweets + "]"
}
/**
* This represents a set of objects of type `Tweet` in the form of a binary search
* tree. Every branch in the tree has two children (two `TweetSet`s). There is an
* invariant which always holds: for every branch `b`, all elements in the left
* subtree are smaller than the tweet at `b`. The eleemnts in the right subtree are
* larger.
*
* Note that the above structure requires us to be able to compare two tweets (we
* need to be able to say which of two tweets is larger, or if they are equal). In
* this implementation, the equality / order of tweets is based on the tweet's text
* (see `def incl`). Hence, a `TweetSet` could not contain two tweets with the same
* text from different users.
*
*
* The advantage of representing sets as binary search trees is that the elements
* of the set can be found quickly. If you want to learn more you can take a look
* at the Wikipedia page [1], but this is not necessary in order to solve this
* assignment.
*
* [1] http://en.wikipedia.org/wiki/Binary_search_tree
*/
abstract class TweetSet {
def isEmpty: Boolean
/**
* This method takes a predicate and returns a subset of all the elements
* in the original set for which the predicate is true.
*
* Question: Can we implment this method here, or should it remain abstract
* and be implemented in the subclasses?
*/
def filter(p: Tweet => Boolean): TweetSet
/**
* This is a helper method for `filter` that propagetes the accumulated tweets.
*/
def filterAcc(p: Tweet => Boolean, acc: TweetSet): TweetSet
/**
* Returns a new `TweetSet` that is the union of `TweetSet`s `this` and `that`.
*
* Question: Should we implment this method here, or should it remain abstract
* and be implemented in the subclasses?
*/
def union(that: TweetSet): TweetSet;
/**
* Returns the tweet from this set which has the greatest retweet count.
*
* Calling `mostRetweeted` on an empty set should throw an exception of
* type `java.util.NoSuchElementException`.
*
* Question: Should we implment this method here, or should it remain abstract
* and be implemented in the subclasses?
*/
def mostRetweeted: Tweet = ???
/**
* Returns a list containing all tweets of this set, sorted by retweet count
* in descending order. In other words, the head of the resulting list should
* have the highest retweet count.
*
* Hint: the method `remove` on TweetSet will be very useful.
* Question: Should we implment this method here, or should it remain abstract
* and be implemented in the subclasses?
*/
def descendingByRetweet: TweetList
/**
* The following methods are already implemented
*/
/**
* Returns a new `TweetSet` which contains all elements of this set, and the
* the new element `tweet` in case it does not already exist in this set.
*
* If `this.contains(tweet)`, the current set is returned.
*/
def incl(tweet: Tweet): TweetSet
/**
* Returns a new `TweetSet` which excludes `tweet`.
*/
def remove(tweet: Tweet): TweetSet
/**
* Tests if `tweet` exists in this `TweetSet`.
*/
def contains(tweet: Tweet): Boolean
/**
* This method takes a function and applies it to every element in the set.
*/
def foreach(f: Tweet => Unit): Unit
}
class Empty extends TweetSet {
def descendingByRetweet() = Nil
def union(that: TweetSet): TweetSet = that
def isEmpty = true
def filter(p: Tweet=> Boolean): TweetSet = new Empty()
def filterAcc(p: Tweet => Boolean, acc: TweetSet): TweetSet = new Empty()
/**
* The following methods are already implemented
*/
def contains(tweet: Tweet): Boolean = false
def incl(tweet: Tweet): TweetSet = new NonEmpty(tweet, new Empty, new Empty)
def remove(tweet: Tweet): TweetSet = this
def foreach(f: Tweet => Unit): Unit = ()
}
class NonEmpty(elem: Tweet, left: TweetSet, right: TweetSet) extends TweetSet {
def descendingByRetweet = {
def iterateAndAddList(list: TweetList,first: Boolean): TweetList = {
val current: Tweet = greatestCurrent()
if(current != Nil && first != true){
list.head = current
list.isEmpty = false
list.tail = Nil
remove(current)
iterateAndAddList(list,false)
}else list
}
def greatestCurrent():Tweet = {
new Tweet("a","b",21)
}
iterateAndAddList(Nil,true)
}
def union(that: TweetSet): TweetSet = (left.union(right)).union(that).incl(elem)
val isEmpty = false
def filter(p: Tweet => Boolean): TweetSet = filterAcc(p,new Empty)
def filterAcc(p: Tweet => Boolean, acc: TweetSet): TweetSet = {
if(left.isEmpty && right.isEmpty) acc
else if(p(elem)){ left.filterAcc(p,acc.incl(elem)).union(right.filterAcc(p,acc.incl(elem)))}
else left.filterAcc(p,acc).union(right.filterAcc(p,acc))
}
/**
* The following methods are already implemented
*/
def contains(x: Tweet): Boolean =
if (x.text < elem.text) left.contains(x)
else if (elem.text < x.text) right.contains(x)
else true
def incl(x: Tweet): TweetSet = {
if (x.text < elem.text) new NonEmpty(elem, left.incl(x), right)
else if (elem.text < x.text) new NonEmpty(elem, left, right.incl(x))
else this
}
def remove(tw: Tweet): TweetSet =
if (tw.text < elem.text) new NonEmpty(elem, left.remove(tw), right)
else if (elem.text < tw.text) new NonEmpty(elem, left, right.remove(tw))
else left.union(right)
def foreach(f: Tweet => Unit): Unit = {
f(elem)
left.foreach(f)
right.foreach(f)
}
}
//--------------------------------------------------------------------------------------------
trait TweetList {
def head: Tweet
def tail: TweetList
def isEmpty: Boolean
def foreach(f: Tweet => Unit): Unit =
if (!isEmpty) {
f(head)
tail.foreach(f)
}
}
object Nil extends TweetList {
def head = throw new java.util.NoSuchElementException("head of EmptyList")
def tail = throw new java.util.NoSuchElementException("tail of EmptyList")
def isEmpty = true
}
class Cons(val head: Tweet, val tail: TweetList) extends TweetList {
def isEmpty = false
}
object GoogleVsApple {
val google = List("android", "Android", "galaxy", "Galaxy", "nexus", "Nexus")
val apple = List("ios", "iOS", "iphone", "iPhone", "ipad", "iPad")
lazy val googleTweets: TweetSet = ???
lazy val appleTweets: TweetSet = ???
/**
* A list of all tweets mentioning a keyword from either apple or google,
* sorted by the number of retweets.
*/
lazy val trending: TweetList = ???
}
object Main extends App {
// Print the trending tweets
GoogleVsApple.trending foreach println
}
（这是来自scala coursera课程。这个问题与课程无关。我不是要求解决方案。）
我的问题是在helper函数中的descendingByOrder函数中（在NonEmpty类中）
list.head
list.isEmpty
list.tail
出于某种原因似乎隐藏了？这个属性不应该可以访问吗？
答案 0 :(得分：1)
不确定appearing hidden是什么意思，但其中一个问题可能就是
def descendingByRetweet = {
def iterateAndAddList(list: TweetList,first: Boolean): TweetList = {
val current: Tweet = greatestCurrent()
if(current != Nil && first != true){
list.head = current
list.isEmpty = false
list.tail = Nil
remove(current)
iterateAndAddList(list,false)
}
else list
}
def greatestCurrent():Tweet = {
new Tweet("a","b",21)
}
iterateAndAddList(Nil,true)
}
您正在重新分配给def，TweetList是不可变的，您无法为它们分配属性，而是可以创建一个新元素，然后可以将其传递给函数并保持递归：
def descendingByRetweet = {
def iterateAndAddList(list: TweetList,first: Boolean): TweetList = {
val current: Tweet = greatestCurrent()
if(current != Nil && first != true) iterateAndAddList(new Cons(current, Nil), false)
else list
}
def greatestCurrent():Tweet = {
new Tweet("a","b",21)
}
iterateAndAddList(Nil,true)
}
这是在Scala中递归不可变集合的方法，您总是需要创建一个新对象。