scala-problem16-20

P16 Drop every Nth element from a list
要求

方案

P17 Split a list into two parts
要求

方案

P18 Extract a slice from a list
要求

方案

P19 Rotate a list N places to the left
要求

方案

P20 Remove the Kth element from a list
要求

方案

* 声明*

该系列文章来自：http://aperiodic.net/phil/scala/s-99/

大部分内容和原文相同,加入了部分自己的代码。

如有侵权，请及时联系本人。本人将立即删除相关内容。

P16 (**) Drop every Nth element from a list.

要求

Example:

scala> drop(3, List('a, 'b, 'c, 'd, 'e, 'f, 'g, 'h, 'i, 'j, 'k))
res0: List[Symbol] = List('a, 'b, 'd, 'e, 'g, 'h, 'j, 'k)

方案

(1) 递归

def drop[T](n: Int, list: List[T]): List[T] = {
def dropR(x: Int, ls: List[T]): List[T] = (x, ls) match {
case (_, Nil)          => Nil
case (1, head :: tail) => dropR(n, tail)
case (_, head :: tail) => head :: dropR(x - 1, tail)
}
dropR(n, list)
}

P17 (*) Split a list into two parts.

要求

The length of the first part is given. Use a Tuple for your result.

Example:

scala> split(3, List('a, 'b, 'c, 'd, 'e, 'f, 'g, 'h, 'i, 'j, 'k))
res0: (List[Symbol], List[Symbol]) = (List('a, 'b, 'c),List('d, 'e, 'f, 'g, 'h, 'i, 'j, 'k))

方案

(1) take(n) + drop(n)

def split[T](x: Int, list: List[T]): (List[T], List[T]) = (list.take(x), list.drop(x))

(2) take(n) + drop(n) == splitAt(n)

def split2[T](x: Int, list: List[T]): (List[T], List[T]) = list.splitAt(x)

(3) 普通递归

def splitRecursive[T](x: Int, list: List[T]): (List[T], List[T]) =(x, list) match {
case (_, Nil) => (Nil, Nil)
case (0, ls)  => (Nil, ls)
case (n, head :: tail) => {
val (pre, post) = splitRecursive(n - 1, tail)
return (head :: pre, post)
}
}

P18 (**) Extract a slice from a list.

要求

Given two indices, I and K, the slice is the list containing the elements from and including the Ith element up to but not including the Kth element of the original list. Start counting the elements with 0.

Example:

scala> slice(3, 7, List('a, 'b, 'c, 'd, 'e, 'f, 'g, 'h, 'i, 'j, 'k))
res0: List[Symbol] = List('d, 'e, 'f, 'g)

方案

(1) List内置slice(废话)

def slice[T](from: Int, to: Int, list: List[T]): List[T] =
list.slice(from, to)

(2) take + drop (实际上，内置slice就是这么实现的)

def slice2[T](from: Int, to: Int, list: List[T]): List[T] =
list.drop(from).take(to - from)

P19 (**) Rotate a list N places to the left.

要求

Examples:

scala> rotate(3, List('a, 'b, 'c, 'd, 'e, 'f, 'g, 'h, 'i, 'j, 'k))
res0: List[Symbol] = List('d, 'e, 'f, 'g, 'h, 'i, 'j, 'k, 'a, 'b, 'c)

scala> rotate(-2, List('a, 'b, 'c, 'd, 'e, 'f, 'g, 'h, 'i, 'j, 'k))
res1: List[Symbol] = List('j, 'k, 'a, 'b, 'c, 'd, 'e, 'f, 'g, 'h, 'i)

方案

(1) drop + take

def rotate[T](index: Int, list: List[T]): List[T] = {
if (index < 0) {
val i = list.length + index
list.drop(i) ::: list.take(i)
} else {
list.drop(index) ::: list.take(index)
}
}

def rotate2[T](index: Int, list: List[T]): List[T] = index match {
case i if i < 0 => {

4000
val i = list.length + index
list.drop(i) ::: list.take(i)
}
case _ => list.drop(index) ::: list.take(index)
}

P20 (*) Remove the Kth element from a list.

要求

Return the list and the removed element in a Tuple. Elements are numbered from 0.

Example:

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

方案

(1) aplitAt

def removeAt[T](index: Int, list: List[T]): (List[T], T) = {
if (index < 0) throw new NoSuchElementException
val (prev, tail) = list.splitAt(index + 1)
(prev.init ::: tail, prev.last)
}

另一写法:

def removeAt2[T](n: Int, ls: List[T]): (List[T], T) = ls.splitAt(n) match {
case (Nil, _) if n < 0 => throw new NoSuchElementException
case (pre, e :: post)  => (pre ::: post, e)
case (pre, Nil)        => throw new NoSuchElementException
}

(2) 递归(这个写的很恶心)

def removeAt3[T](index: Int, list: List[T]): (List[T], T) = {
if (index < 0) throw new NoSuchElementException

(index, list) match {
case (_, Nil)          => throw new NoSuchElementException
case (0, head :: tail) => (tail, head)
case (i, head :: tail) => (head :: removeAt3(i - 1, tail)._1, removeAt3(i - 1, tail)._2)
}
}

(3) 递归

def removeAt4[T](index: Int, list: List[T]): (List[T], T) = {
if (index < 0) throw new NoSuchElementException

(index, list) match {
case (_, Nil)          => throw new NoSuchElementException
case (0, head :: tail) => (tail, head)
case (_, head :: tail) => {
val (prev, e) = removeAt4(index - 1, tail)
(list.head :: prev, e)
}
}
}