Scala in Impatient 习题解答4 映射和元组A1

15 July 2013

本章中文版有一小错误。 4.5小节包名collection是没有s的，参考了下英文原版，是正确的。翻译后反而多了个s，比较奇怪。

val scores = scala.collections.immutable.SortedMap("Alice" ->10,
                             ^ 正确的应该是没有s

scala> val items = Map("iphone" -> 4999, "mx2" -> 2499, "pad" -> 3800)
items: scala.collection.immutable.Map[java.lang.String,Int] = Map(iphone ->
4999, mx2 -> 2499, pad -> 3800)

scala> val items2 = for ((k,v) <- items) yield (k, v * 0.9)
items2: scala.collection.immutable.Map[java.lang.String,Double] = Map(iphone ->
4499.1, mx2 -> 2249.1, pad -> 3420.0)

import scala.collection.mutable.HashMap

val in = new java.util.Scanner(new java.io.File("README.md"))
val result = new HashMap[String,Long]
while (in.hasNext()) {
    val str = in.next()
    if (result.contains(str)) result(str) += 1 
    else result += (str -> 1)
}
in.close()

print(result)

重复前一个练习，这次用不可变的映射

 val in = new java.util.Scanner(new java.io.File("README.md"))
 var result = Map[String,Long]()
 while (in.hasNext()) {
     val str = in.next()
     if (result.contains(str)) {
         val c_v = result(str)
         result = result - str
         result = result + (str -> (c_v + 1))
     } else
         result = result + (str -> 1)
 }
 in.close()

 print(result)

import scala.collection.immutable.SortedMap

val in = new java.util.Scanner(new java.io.File("README.md"))
var result = SortedMap[String,Long]()
while (in.hasNext()) {
    val str = in.next()
    if (result.contains(str)) {
        val c_v = result(str)
        result = result - str
        result = result + (str -> (c_v + 1))
    } else
        result = result + (str -> 1)
}
in.close()

print(result)

use TreeMap in scala

 import scala.collection.JavaConversions.mapAsScalaMap

 val in = new java.util.Scanner(new java.io.File("README.md"))
 val result: scala.collection.mutable.Map[String, Long] = 
     new java.util.TreeMap[String,Long]
 while (in.hasNext()) {
     val str = in.next()
     if (result.contains(str)) result(str) += 1 
     else result += (str -> 1)
 }
 in.close()

 print(result)

scala> val months = scala.collection.mutable.LinkedHashMap("Monday" ->
java.util.Calendar.MONDAY)
months: scala.collection.mutable.LinkedHashMap[java.lang.String,Int] =
Map(Monday -> 2)

scala> months += "Sunday" -> java.util.Calendar.SUNDAY
res5: months.type = Map(Monday -> 2, Sunday -> 1)

scala> months += "Friday" -> java.util.Calendar.FRIDAY
res6: months.type = Map(Monday -> 2, Sunday -> 1, Friday -> 6)

scala> import scala.collection.JavaConversions.propertiesAsScalaMap
import scala.collection.JavaConversions.propertiesAsScalaMap

scala> val props: scala.collection.Map[String, String] = System.getProperties()

scala> var maxLen = 0; for (i <- props.keys) if (i.length > maxLen) maxLen =
i.length
maxLen: Int = 29

scala> for ((k,v) <- props) println(k + " " * (maxLen+2-k.length) + "|" + v)
env.emacs                      |
java.runtime.name              |Java(TM) SE Runtime Environment
sun.boot.library.path          |/usr/lib/jvm/java-7-sun/jre/lib/i386
java.vm.version                |23.21-b01
java.vm.vendor                 |Oracle Corporation
java.vendor.url                |http://java.oracle.com/
path.separator                 |:
java.vm.name                   |Java HotSpot(TM) Server VM
file.encoding.pkg              |sun.io
user.country                   |US
sun.java.launcher              |SUN_STANDARD

def minmax(values: Array[Int]) = {
    (values.min, values.max)
}

scala> val a = minmax(Array(1,2,3,4,-5))
a: (Int, Int) = (-5,4)

def lteqgt(values: Array[Int], v: Int) = {
    val lt = values.filter(_ < v).length
    val eq = values.filter(_ == v).length
    (lt, eq, (values.length - lt - eq))
}

scala> val (lt, eq, gt) = lteqgt(Array(1,2,3,4,5,6), 4)
lt: Int = 3
eq: Int = 1
gt: Int = 2

执行"Hello".zip("World")的结果是，生成了字母相对的tuple。一个现实中可能的用途？捆绑一起然后随机排列，以便能保持相同顺序？
```
scala> "Hello".zip("World")
res18: scala.collection.immutable.IndexedSeq[(Char, Char)] = Vector((H,W),
(e,o), (l,r), (l,l), (o,d))
```

use Scala 2.9.1

Scala 22