scala-problem21-25

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P21 (*) Insert an element at a given position into a list.

要求

Example:

scala> insertAt('new, 1, List('a, 'b, 'c, 'd))res0: List[Symbol] = List('a, 'new, 'b, 'c, 'd)

方案

• (1) take + newElement+ drop

def insertAt[T](e: T, index: Int, list: List[T]): List[T] = {    if (index < 0 || list == Nil) throw new IllegalArgumentException    list.take(index) ::: List(e) ::: list.drop(index)}

• (2)

def insertAt2[A](e: A, n: Int, ls: List[A]): List[A] = ls.splitAt(n) match {    case (pre, post) => pre ::: e :: post}

P22 (*) Create a list containing all integers within a given range.

要求

Example:

scala> range(4, 9)res0: List[Int] = List(4, 5, 6, 7, 8, 9)

方案

• (1) 内置Range(废话)

def range(start: Int, end: Int): List[Int] =     Range(start, end + 1).toList

• (2) 指令式编程(scala中并不推荐)

def range2(start: Int, end: Int): List[Int] = {    var i = 0    var ret = scala.collection.mutable.ListBuffer[Int]()    while (i <= end - start) {        ret.append(start + i)        i += 1    }    return ret.toList}

P23 (**) Extract a given number of randomly selected elements from a list.

要求

Example:

scala> randomSelect(3, List('a, 'b, 'c, 'd, 'f, 'g, 'h))res0: List[Symbol] = List('e, 'd, 'a)Hint: Use the solution to problem P20

方案

• (1) java.util.Random + p20.removeAt

def randomSelect[T](count: Int, list: List[T]): List[T] = {    if (count <= 0) Nil    else {        val r = new java.util.Random()        val (rest, removedElement) = removeAt(r.nextInt(list.length), list)        return removedElement :: randomSelect(count - 1, rest)    }}

def randomSelect2[T](n: Int, ls: List[T]): List[T] = {    def randomSelectR(n: Int, ls: List[T], r: util.Random): List[T] =        if (n <= 0) Nil        else {            val (rest, e) = removeAt(r.nextInt(ls.length), ls)            e :: randomSelectR(n - 1, rest, r)        }    randomSelectR(n, ls, new util.Random)}  

P24 (*) Lotto: Draw N different random numbers from the set 1..M.

要求

Example:

scala> lotto(6, 49)res0: List[Int] = List(23, 1, 17, 33, 21, 37)

方案

• (1) p23.randomSelect + Range()

def lotto(count: Int, max: Int): List[Int] =    randomSelect(count, List.range(1, max + 1))

P25 (*) Generate a random permutation of the elements of a list.

要求

Hint: Use the solution of problem P23.

Example:

scala> randomPermute(List('a, 'b, 'c, 'd, 'e, 'f))res0: List[Symbol] = List('b, 'a, 'd, 'c, 'e, 'f)

方案

• (1) P23.randomeSelect(太没效率了)

def randomPermute[T](list: List[T]): List[T] = {    import Problem23.randomSelect    randomSelect(list.length, list)}

• (2) 在mutable的数组中随机交换元素

def randomPermute2[T](list: List[T]): List[T] = {    val arr = scala.collection.mutable.ArrayBuffer[T]()    //第一个for只想把list转换为Array    //无奈List.toArray总是出错,只好用这个了    for (e <- list) {        arr.append(e)    }    //这里才是正题    val r = new java.util.Random()    for (i <- list.length - 1 until 0 by -1) {        val index = r.nextInt(i)        val tmp = arr(index)        arr(index) = arr(i)        arr(i) = tmp    }    arr.toList}