c# barcode generator library open source defined class MPerson in Font

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defined class MPerson
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CHAPTER 5 PATTERN MATCHING
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scala> val mp = MPerson("Jorge", 24)
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mp: MPerson = MPerson(Jorge,24)
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scala> mp.age = 25 scala> mp
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res3: MPerson = MPerson(Jorge,25)
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So far, this is just some syntactic sugar. How, you ask, does it work with pattern matching Pattern matching against case classes is syntactically pleasing and very powerful. We can match against our Person class, and we get the extractors for free:
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def older(p: Person): Option[String] = p match { case Person(name, age, true) if age > 35 => Some(name) case _ => None }
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Our method matches against instances of Person. If the valid field is true, the age is extracted and compared against a guard. If the guard succeeds, the Person s name is returned, otherwise None is returned. Let s try it out:
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scala> older(p)
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res4: Option[String] = Some(David)
scala> older(Person("Fred", 73, false))
res5: Option[String] = None
scala> older(Person("Jorge", 24, true))
res6: Option[String] = None
CHAPTER 5 PATTERN MATCHING
Pattern Matching in Lists
As we saw in 3 s Roman numeral example (Listing 3-5), Scala s pattern matching can also be applied to Lists. Scala s List collection is implemented as a linked list where the head of the list is called a cons cell.2 It contains a reference to its contents and another reference to the tail of the list, which may be another cons cell or the Nil object. Lists are immutable, so the same tail can be shared by many different heads. In Scala, the cons cell is represented by the :: case class. Perhaps you have just said, Ah hah! Creating a List is Scala is as simple as this:
1 :: Nil :: is the name of the method and the name of a case class. By keeping the creation method, ::, and the case class name the same, we can construct and pattern match Lists in a syntactically pleasing way. And as we ve just seen, case classes can be used in pattern matching to either compare or extract values. This holds for Lists as well and leads to some very pleasing syntax. We construct a List with scala> val x = 1
x: Int = 1
scala> val rest = List(2,3,4)
rest: List[Int] = List(2, 3, 4)
scala> x :: rest
res1: List[Int] = List(1, 2, 3, 4)
2. The naming of the cons cell traces its roots back to Lisp and came from the act of constructing a list. One constructs a list by linking a cons cell to the head of the list.
CHAPTER 5 PATTERN MATCHING
scala> (x :: rest) match { // note the symmetry between creation and matching case xprime :: restprime => println(xprime); println(restprime) }
1 List(2, 3, 4)
Then we can extract the head (x) and tail (rest) of the List in pattern matching.
Pattern Matching and Lists
Pattern matching and Lists go hand in hand. We can start off using pattern matching to sum up all the odd Ints in a List[Int].
def sumOdd(in: List[Int]): Int = in match { case Nil => 0 case x :: rest if x % 2 == 1 => x + sumOdd(rest) case _ :: rest => sumOdd(rest) }
If the list is empty, Nil, then we return 0. The next case extracts the first element from the list and tests it to see whether it s odd. If it is, we add it to the sum of the rest of the odd numbers in the list. The default case is to ignore the first element of the list (a match with the _ wildcard) and return the sum of the odd numbers in the rest of the list. Extracting the head of a list is useful, but when pattern matching against List, we can match against any number of elements in the List. In this example, we will replace any number of contiguous identical items with just one instance of that item:
def noPairs[T](in: List[T]): List[T] = in match { case Nil => Nil case a :: b :: rest if a == b => noPairs(a :: rest) // the first two elements in the list are the same, so we ll // call noPairs with a List that excludes the duplicate element case a :: rest => a :: noPairs(rest) // return a List of the first element followed by noPairs // run on the rest of the List }
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