Week3: TweetSet Class
最近一直在跟Coursera上的scala和cloud computing课，干脆开博客记录一下遇到的问题和解决方法。Scala作为函数式编程，如何熟练掌握function的传递，tail recursion的设计是这门课前半部分的重点。这次的作业是设计一个TweetSet，Tweet是一个记录了user,string,Retweeted三个field的class，要求TweetSet以binary tree形式存储，同时满足特定函数。
Aim: To implement a TweetSet Class, including its fundamental functions.Eg: union, filter(p: Tweet => Boolean), mostRetweeted, descendingByRetweet
Note:
filter and mostRetweeted are both implemented using tail recursion(filterAcc and mostAcc)
注意tail recursion的逻辑，一般都需要传递一个中间结果（1作为累加值或者2作为比较值），从而使下次递归可以直接比较之前完成部分的结果。注意递归顺序，这里因为是binary tree，一般都是先elem，再left，最后right。
filterAcc records a TweetSet acc that represents the accumulative tweetSet which satisfy the predicate pif (p(elem)) then include current tweet into the tweetSet
mostAcc records a tweet object with maximum retweets. At beginning, it is initialized as new tweet("ini","ini",0)retweet number is 0 at first!
Difficulties:
Remember how to use function to implement traverse of the list!!! Just as follows.
google.exists(x => y.text.contains(x))
val google = List("android", "Android", "galaxy", "Galaxy", "nexus", "Nexus")
val apple = List("ios", "iOS", "iphone", "iPhone", "ipad", "iPad")lazy
val googleTweets: TweetSet = TweetReader.allTweets.filter(y => google.exists(x => y.text.contains(x)))lazy
val appleTweets: TweetSet = TweetReader.allTweets.filter(y => apple.exists(x => y.text.contains(x)))
----------Codes-------------
package objsets
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 elements 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 {
/**
* 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.filterAcc(p, new Empty())
/**
* 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 = mostAcc(new Tweet("ini_max", "ini_max", 0))
def mostAcc(max: Tweet): 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 filterAcc(p: Tweet => Boolean, acc: TweetSet): TweetSet = acc
def union(that: TweetSet): TweetSet = that
def mostAcc(max: Tweet): Tweet = max
def descendingByRetweet: TweetList = Nil
/**
* 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 filterAcc(p: Tweet => Boolean, acc: TweetSet): TweetSet =
if (p(elem)) right.filterAcc(p, left.filterAcc(p, acc.incl(elem)))
else right.filterAcc(p, left.filterAcc(p, acc))
def union(that: TweetSet): TweetSet =
((left union right) union that) incl elem
def mostAcc(max: Tweet): Tweet =
if (elem.retweets > max.retweets) right.mostAcc(left.mostAcc(elem))
else right.mostAcc(left.mostAcc(max))
def descendingByRetweet: TweetList =
new Cons(this.mostRetweeted, this.remove(this.mostRetweeted).descendingByRetweet)
/**
* 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 = TweetReader.allTweets.filter(y => google.exists(x => y.text.contains(x)))
lazy val appleTweets: TweetSet = TweetReader.allTweets.filter(y => apple.exists(x => y.text.contains(x)))
/**
* A list of all tweets mentioning a keyword from either apple or google,
* sorted by the number of retweets.
*/
lazy val trending: TweetList = (googleTweets.union(appleTweets)).descendingByRetweet
}
object Main extends App {
// Print the trending tweets
GoogleVsApple.trending foreach println
}
