Scala for the Impatients---(3)Working with Arrays

Fixed-Length Arrays

scala> val nums = new Array[Int](10)// An array of ten integers, all initialized with zero
nums: Array[Int] = Array(0, 0, 0, 0, 0, 0, 0, 0, 0, 0)

scala> val a = new Array[String](10)// A string array with ten elements, all initialized with null
a: Array[String] = Array(null, null, null, null, null, null, null, null, null, null)

scala> val s = Array("Hello", "World")// An Array[String] of length 2—the type is inferred
s: Array[String] = Array(Hello, World)

Variable-Length Arrays: Array Buffers

scala> import scala.collection.mutable.ArrayBuffer
import scala.collection.mutable.ArrayBuffer

scala> val b = ArrayBuffer[Int]()
b: scala.collection.mutable.ArrayBuffer[Int] = ArrayBuffer()

scala> val b = new ArrayBuffer[Int]
b: scala.collection.mutable.ArrayBuffer[Int] = ArrayBuffer()

scala> b += 1
res1: b.type = ArrayBuffer(1)

scala> b += (1, 2, 3, 5)
res2: b.type = ArrayBuffer(1, 1, 2, 3, 5)

scala> b ++= Array(8, 13, 21)\you can append any collection with the ++= operator
res3: b.type = ArrayBuffer(1, 1, 2, 3, 5, 8, 13, 21)

scala> b.trimEnd(5)

scala> b
res5: scala.collection.mutable.ArrayBuffer[Int] = ArrayBuffer(1, 1, 2)

scala> val a=b.toArray//arraybuffer to array
a: Array[Int] = Array(1, 1, 2)

scala> a.toBuffer//array to arraybuffer
res8: scala.collection.mutable.Buffer[Int] = ArrayBuffer(1, 1, 2)

Traversing Arrays and Array Buffers

scala> for (i <- 0 until b.length)
     | println(i + ": " + b(i))
0: 1
1: 1
2: 6
3: 6
4: 6
5: 2

The variable i goes from 0 to a.length - 1 .

scala> 0 until 10
res15: scala.collection.immutable.Range = Range(0, 1, 2, 3, 4, 5, 6, 7, 8, 9)

scala> 0 to 10
res16: scala.collection.immutable.Range.Inclusive = Range(0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10)

To visit every second element, let i traverse this

scala> for (i <- 0 until (b.length,2))
     | println(i + ": " + b(i))
0: 1
2: 6
4: 6

If you don’t need the array index in the loop body, visit the array elements directly, like this:

scala> for (elem <- b)
     | println(elem)
1
1
6
6
6
2

Transforming Arrays

Transformations don’t modify the original array, but they yield a new one.

scala> val a = Array(2, 3, 5, 7, 11)
a: Array[Int] = Array(2, 3, 5, 7, 11)

scala> val result = for (elem <- a) yield 2 * elem
result: Array[Int] = Array(4, 6, 10, 14, 22)

Oftentimes, when you traverse a collection, you only want to process the elements that match a particular condition. This is achieved with a guard: an if inside the for:

scala> for (elem<-a if elem%2==0) yield 2 * elem
res30: Array[Int] = Array(4)

Given a sequence of integers, we want to remove all but the first negative number:

val indexes = for (i <- 0 until a.length if a(i) < 0) yield i
for (j <- (1 until indexes.length).reverse) a.remove(indexes(j))

Common Algorithms

scala> Array(1, 7, 2, 9).sum
res31: Int = 19

scala> ArrayBuffer("Mary", "had", "a", "little", "lamb").max
res1: String = little

scala> val b = ArrayBuffer(1, 7, 2, 9)
b: scala.collection.mutable.ArrayBuffer[Int] = ArrayBuffer(1, 7, 2, 9)

scala> val bSorted = b.sorted
bSorted: scala.collection.mutable.ArrayBuffer[Int] = ArrayBuffer(1, 2, 7, 9)

scala> val bSorted = b.sortWith(_ > _)
bSorted: scala.collection.mutable.ArrayBuffer[Int] = ArrayBuffer(9, 7, 2, 1)

scala> b.mkString(" and ")
res2: String = 1 and 7 and 2 and 9

Multidimensional Arrays

Multidimensional arrays are implemented as arrays of arrays. A two-dimensional array of Double values has the type Array[Array[Double]], to construct such an array, use the ofDim method:

scala> val matrix = Array.ofDim[Double](3, 4) // Three rows, four columns
matrix: Array[Array[Double]] = Array(Array(0.0, 0.0, 0.0, 0.0), Array(0.0, 0.0, 0.0, 0.0), Array(0.0, 0.0, 0.0, 0.0))

Access an element: matrix(row)(column) = 42

With varying row lengths:

scala> val triangle = new Array[Array[Int]](10)\只指定了10列，和每一列的类型，即Array[Int]，但是没有指定每一列的元素个数。
triangle: Array[Array[Int]] = Array(null, null, null, null, null, null, null, null, null, null)

scala> for (i <- 0 until triangle.length)
     | triangle(i) = new Array[Int](i + 1)

scala> triangle
res2: Array[Array[Int]] = Array(Array(0), Array(0, 0), Array(0, 0, 0), Array(0, 0, 0, 0), Array(0, 0, 0, 0, 0), Array(0, 0, 0, 0, 0, 0), Array(0, 0, 0, 0, 0, 0, 0), Array(0, 0, 0, 0, 0, 0, 0, 0), Array(0, 0, 0, 0, 0, 0, 0, 0, 0), Array(0, 0, 0, 0, 0, 0, 0, 0, 0, 0))