快学Scala 第三章习题答案
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1.编写一段代码,将a设置为一个n歌随机整数的数组,要求随机数介于0(包含)和n(不包含)之间。
scala> def randomArray(n:Int)={ | for(i <- 0 until n) yield (scala.math.random * i).toInt | }randomArray: (n: Int)scala.collection.immutable.IndexedSeq[Int]scala> print(randomArray(10).mkString(","))0,0,0,1,1,4,0,1,3,3
2.编写一个循环,将整数数组中相邻的元素置换。例如,Array(1,2,3,4,5)经过置换后变为Array(2,1,4,3,5).
scala> def between(arr:Array[Int]):Array[Int]={ | var t = arr.toBuffer | for(i <- 0 until (arr.length,2)) if(i+1 < arr.length){ | val a = t(i) | val b = t(i+1) | t.remove(i,2) | t.insert(i,b) | t.insert(i+1,a) | } | t.toArray | }between: (arr: Array[Int])Array[Int]scala> val arr = Array(1,2,3,4,5)arr: Array[Int] = Array(1, 2, 3, 4, 5)scala> between(arr)res0: Array[Int] = Array(2, 1, 4, 3, 5)
3.重复前一个练习,不过这一次生成一个新的值交换过得数组。用for/yield。
scala> def between(arr:Array[Int]):Array[Int]={ | var t = arr.toBuffer | val result = for(i <- 0 until arr.length) yield { | if(i == t.length-1 && i % 2 == 0) t(i) | else if(i % 2 == 0 && i < t.length-1) t(i+1) | else t(i-1) | } | result.toArray | }between: (arr: Array[Int])Array[Int]scala> print(between(Array(1,2,3,4,5)).mkString(","))2,1,4,3,5
上面那道题目,我打了好几遍,因为用的是scala-shell,所以每次一个地方出错,必须要全部重打,累死了,光是int大小写的问题我就打了好几遍了,哎。。。。
4.给定一个整数数组,产出一个新的数组,包含元数组中的所有正值,以原有顺序排列,之后的元素是所有零或负值,以原有顺序排列。
scala> val a = Array[Int](0,1,-2,3,-4,5,-6,6,-7)a: Array[Int] = Array(0, 1, -2, 3, -4, 5, -6, 6, -7)scala> for(i <- 0 until a.length) { | if(a(i) > 0) positive += i | else negtive += i | }scala> for(i <- 0 until positive.length) result += a(positive(i))scala> for(i <- 0 until negtive.length) result += a(negtive(i))scala> resultres11: scala.collection.mutable.ArrayBuffer[Int] = ArrayBuffer(1, 3, 5, 6, 0, -2, -4, -6, -7)
5.如何计算Array[Double]的平均值?
scala> def avg(arr:Array[Double]):Double={ | val sum = arr.sum | val count = arr.length | arr.sum / arr.length | }avg: (arr: Array[Double])Doublescala> avg(Array(1,2,3,4,5,3.5))res12: Double = 3.0833333333333335
6.如何重新组织Array[Int]的元素将他们以反序排列?对于ArrayBuffer[Int]你又会怎么做呢?
scala> def ArrayReserve(arr:Array[Int]):Array[Int]={ | arr.reverse | }ArrayReserve: (arr: Array[Int])Array[Int]scala> val b = ArrayBuffer[Int](1,2,3,4,5,6)b: scala.collection.mutable.ArrayBuffer[Int] = ArrayBuffer(1, 2, 3, 4, 5, 6)scala> b.reverseres18: scala.collection.mutable.ArrayBuffer[Int] = ArrayBuffer(6, 5, 4, 3, 2, 1)
7.编写一段代码,产出数组中的所有值,去掉重复项。(提示查看Scaladoc)
scala> def DistinctArray(arr:Array[Int])={ | print(arr.distinct.mkString(",")) | }DistinctArray: (arr: Array[Int])Unitscala> DistinctArray(Array(1,2,3,1,3,6,7))1,2,3,6,7
8.重新编写3.4节结尾的实例。手收集负值元素的下标,反序,去掉最后一个下标,然后对每一个下标调用a.remove(i)。比较这样做的效率和3.4节中另外两种方法的效率。
scala> def Negtive(arr:Array[Int])={ | val t = arr.toBuffer | val negtive = new ArrayBuffer[Int] | for(i <- 0 until t.length){ | if(t(i)<0) negtive +=i | } | negtive.remove(0) | var neg_reverse = negtive.reverse | for(i <- 0 until neg_reverse.length){ | t.remove(neg_reverse(i)) | } | t.toArray | }Negtive: (arr: Array[Int])Array[Int]scala> Negtive(Array(1,2,-3,4,-5,-6,7,-8))res33: Array[Int] = Array(1, 2, -3, 4, 7)
这个编出来真不容易啊,犯了很多错误:
1).经过reverse以后,返回的是一个新的数组,并不会更改本身数组的属性
2).通过ArrayBuffer的remove操作以后返回的是删除的数的值,而不是返回数组,这两个错误导致我的代码经常出错。
9.创建一个由java.util.TimeZone.getAvailableIDs返回的时区集合,判断条件是它们在美洲。去掉“America”前缀并排序。
scala> val americaTimeZone = java.util.TimeZone.getAvailableIDsamericaTimeZone: Array[String] = Array(Etc/GMT+12, Etc/GMT+11, Pacific/Midway, Pacific/Niue, Pacific/Pago_Pago, Pacific/Samoa, US/Samoa, America/Adak, America/Atka, Etc/GMT+10, HST, Pacific/Honolulu, Pacific/Johnston, Pacific/Rarotonga, Pacific/Tahiti, SystemV/HST10, US/Aleutian, US/Hawaii, Pacific/Marquesas, AST, America/Anchorage, America/Juneau, America/Nome, America/Sitka, America/Yakutat, Etc/GMT+9, Pacific/Gambier, SystemV/YST9, SystemV/YST9YDT, US/Alaska, America/Dawson, America/Ensenada, America/Los_Angeles, America/Metlakatla, America/Santa_Isabel, America/Tijuana, America/Vancouver, America/Whitehorse, Canada/Pacific, Canada/Yukon, Etc/GMT+8, Mexico/BajaNorte, PST, PST8PDT, Pacific/Pitcairn, SystemV/PST8, SystemV/PST8PDT, US/Pacific, US/Pacific-New, America/Boise, America/Camb...scala> val americatimezone = java.util.TimeZone.getAvailableIDs.filter(_.take(8)=="America/")americatimezone: Array[String] = Array(America/Adak, America/Atka, America/Anchorage, America/Juneau, America/Nome, America/Sitka, America/Yakutat, America/Dawson, America/Ensenada, America/Los_Angeles, America/Metlakatla, America/Santa_Isabel, America/Tijuana, America/Vancouver, America/Whitehorse, America/Boise, America/Cambridge_Bay, America/Chihuahua, America/Creston, America/Dawson_Creek, America/Denver, America/Edmonton, America/Hermosillo, America/Inuvik, America/Mazatlan, America/Ojinaga, America/Phoenix, America/Shiprock, America/Yellowknife, America/Bahia_Banderas, America/Belize, America/Cancun, America/Chicago, America/Costa_Rica, America/El_Salvador, America/Guatemala, America/Indiana/Knox, America/Indiana/Tell_City, America/Knox_IN, America/Managua, America/Matamoros, Amer...scala> val sortedamericaTimeZone = americaTimeZone.map(_.drop(8)).sortedsortedamericaTimeZone: Array[String] = Array("", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", -Nera, /Azores, /Bermuda, /Canary, /Cape_Verde, /Faeroe, /Faroe, /Jan_Mayen, /Madeira, /Reykjavik, /South_Georgia, /St_Helena, /Stanley, 0, 0, 1, 1, 10, 10, 11, 11, 12, 12, 13, 14, 2, 2, 3, 3, 4, 4, 5, 5, 6, 6, 7, 7, 8, 8, 9, 9, AST4, AST4ADT, Adak, Anchorage, Anguilla, Antigua, Apia, Araguaina, Argentina/Buenos_Aires, Argentina/Catamarca, Argentina/ComodRivadavia, Argentina/Cordoba, Argentina/Jujuy, Argentina/La_Rioja, Argentina/Mendoza, Argentina/Rio_Gallegos, Argentina/Sal...
10.引入java.awt.datatransfer._并构建一个类型为SystemFlavorMap()类型的对象:
val flavors = SystemFlavorMap.getDefaultMap().asInstanceOf[SystemFlavorMap]
然后以DataFlavor.imageFlavor为参数调用getNativesForFlavor方法,以Scala缓冲保存返回值。(为什么用这样一个晦涩难懂的类?因为在java标准类库中很难找得到使用java.util.List的代码)
scala> val flavors = SystemFlavorMap.getDefaultFlavorMap().asInstanceOf[SystemFlavorMap]flavors: java.awt.datatransfer.SystemFlavorMap = java.awt.datatransfer.SystemFlavorMap@8827dc8scala> print(flavors.getNativesForFlavor(DataFlavor.imageFlavor).toArray.toBuffer)ArrayBuffer(image/png, image/jpeg, image/x-png, image/gif, PNG, JFIF)
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