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CHAPTER 2 SCALA SYNTAX, SCRIPTS, AND YOUR FIRST SCALA PROGRAMS
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while executes its code block as long as its expression evaluates to true, just like Java. In
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practice, using recursion, a method calling itself, provides more readable code and enforces the concept of transforming input to output rather than changing, mutating, variables. Recursive methods can be as efficient as a while loop.6
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while (exp) println("Working...") while (exp) { println("Working...") }
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The for comprehension
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At first blush, Scala s for comprehension looks a lot like Java s for loop. However, they are significantly different. The basic form implements flow of control, executing the body one time for each value:
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scala> for {i <- 1 to 3} println(i)
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Scala s for comprehension allows for nesting of expressions. You don t have multiple for comprehensions nested in order to have nested expressions, thus there s an entire iteration of the inner variable, j, for each value of the outer variable, i.
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scala> for {i <- 1 to 3 j <- 1 to 3} println(i * j)
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6. See http://en.wikipedia.org/wiki/Tail_recursion.
CHAPTER 2 SCALA SYNTAX, SCRIPTS, AND YOUR FIRST SCALA PROGRAMS
The for comprehension also supports guards:
scala> def isOdd(in: Int) = in % 2 == 1 isOdd: (Int)Boolean scala> for {i <- 1 to 5 if isOdd(i)} println(i)
1 3 5
And the guards can be part of the expression nesting:
scala> for {i <- 1 to 5 j <- 1 to 5 if isOdd(i * j)} println(i * j)
1 3 5 3 9 15 5 15 25
I primarily use the for comprehension to transform a collection or set of collections into a new collection. Let s define some data:
scala> val lst = (1 to 18 by 3).toList
lst: List[Int] = List(1, 4, 7, 10, 13, 16)
CHAPTER 2 SCALA SYNTAX, SCRIPTS, AND YOUR FIRST SCALA PROGRAMS
If the yield keyword introduces the for comprehension s body, the for comprehension returns a collection rather than simply calling the code block. Thus:
scala> for {i <- lst if isOdd(i)} yield i
res11: List[Int] = List(1, 7, 13)
And the nesting rules and guards work the same way:
scala> for {i <- lst; j <- lst if isOdd(i * j)} yield i * j
res12: List[Int] = List(1, 7, 13, 7, 49, 91, 13, 91, 169)
We ve explored a little bit of the syntax of the for comprehension. In 3, we ll see how it can be used with a variety of collections to provide powerful, syntactically pleasing, concise data transformations.
throw, try/catch/finally, and synchronized
Throwing exceptions is the same in Scala and Java:
throw new Exception("Working...")
The try/finally construct is the same:
try { throw new Exception("Working...") } finally { println("This will always be printed") }
The try/catch construct is different. First, it s an expression that results in a value. This makes it possible to wrap a call in a try/catch and assign a default value if the call fails. Second, the exception is pattern matched in the catch block rather than having separate
CHAPTER 2 SCALA SYNTAX, SCRIPTS, AND YOUR FIRST SCALA PROGRAMS
catch clauses for each different exception. Guards are legal in the patterns as well. Here s a basic try/catch expression: try { file.write(stuff) } catch { case e: java.io.IOException => // handle IO Exception case n: NullPointerException => // handle null pointer }
Here s an example of calling Integer.parseInt and defaulting to 0 if an exception is thrown:
scala> try{Integer.parseInt("dog")} catch {case _ => 0}
res16: Int = 0
scala> try{Integer.parseInt("44")} catch {case _ => 0}
res17: Int = 44
To synchronize based on an instance, call the instance s synchronized method with a code block:
obj.synchronized { // do something that needs to be serialized }
There s no synchronized modifier for methods. Synchronize a method this way:
def foo(): Int = synchronized { 42 }
CHAPTER 2 SCALA SYNTAX, SCRIPTS, AND YOUR FIRST SCALA PROGRAMS
Comments
Scala comments are much like Java and C++ comments. Multiline comments are started with /* and ended with */.
/* This is a multi-line comment */
A single-line comment is started with // and continues to the end of the line:
// This is a single line comment
In Scala, you can nest multiline comments:
/* This is an outer comment /* And this comment is nested */ Outer comment */
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