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This article mainly introduces the relevant knowledge of "the method of Scala pattern matching". The editor shows you the operation process through the actual case. The operation method is simple, fast and practical. I hope this article "the method of Scala pattern matching" can help you solve the problem.

Simple matching

Pattern matching of Scala is most commonly used in match statements. The following is a simple example of matching integer values.

Val colorNum = 1val colorStr = colorNum match {case 1 = > "red" case 2 = > "green" case 3 = > "yellow" case _ = > "Not Allowed"} println (colorStr)

To test the above code, you can put the above code directly into the "/ usr/local/scala/mycode/test.scala" file, and then execute the following command from the Linux system's Shell command prompt:

Scala test.scala

Shell command

In addition, variables can be used in pattern matching case statements.

Val colorNum = 4val colorStr = colorNum match {case 1 = > "red" case 2 = > "green" case 3 = > "yellow" case unexpected = > unexpected + "is Not Allowed"} println (colorStr)

Following the method given earlier, the test executes the above code in the test.scala file and then outputs it on the screen:

4 is Not Allowed

That is, when colorNum=4, a value of 4 is passed to the unexpected variable.

Type pattern

Scala can match the type of expression.

For (elem "Spark", "Hadoop", 'Hello)) {val str = elem match {case I: Int = > I + "is an int value." case d: Double = > d + "is a double value." case "Spark" = > "Spark is found." case s: String = > s + "is a string value." case _ = > "This is an unexpected value."} println (str)}

After testing and executing the above code in the test.scala file, it will be output on the screen:

9 is an int value.12.3 is a double value.Spark is found.Hadoop is a string value.This is an unexpected value. "guard" statement

You can add some necessary processing logic to pattern matching.

For (elem case _ if (elem 2 = = 0) = > println (elem + "is even.") Case _ = > println (elem + "is odd.")}}

The parentheses of the conditional expression after if in the above code can be left out. After executing the above code, you can get the following output:

1 patterns in is odd.2 is even.3 is odd.4 is even.for expressions

We have actually touched on patterns in for expressions when we introduced "mapping" before. Taking the mapping we mentioned earlier as an example, the mapping we created is as follows:

Val university = Map ("XMU"-> "Xiamen University", "THU"-> "Tsinghua University", "PKU"-> "Peking University")

The basic format of a loop traversal map is:

For ((KBLING v)

For each value obtained by the traversal process, it is bound to the two variables k and v, that is, the "key" in the map is bound to the variable k, and the "value" in the mapping is bound to the variable v.

Here are some examples that have been introduced before:

For ((kpene v) printf ("Code is:% s and name is:% s\ n", kpene v)

The execution result of the above code is as follows:

Matching of Code is: XMU and name is: Xiamen UniversityCode is: THU and name is: Tsinghua UniversityCode is: PKU and name is: Peking Universitycase classes

The case class is a special class that is optimized to be used for pattern matching.

Case class Car (brand: String, price: Int) val myBYDCar = new Car ("BYD", 89000) val myBMWCar = new Car ("BMW", 1200000) val myBenzCar = new Car ("Benz", 1500000) for (car case Car ("BYD", 89000) = > println ("Hello, BYD!") Case Car ("BMW" 1200000) = > println ("Hello, BMW!") Case Car (brand, price) = > println ("Brand:" + brand + ", Price:" + price+ ", do you want it?")}}

Put the above code into the test.scala file, and run the "scala test.scala" command to get the following results:

Hello, BYDuring Hello, BMWH Brand: Benz, Price:1500000, do you want it?Option type

The Option type in the standard class library uses the case class to represent values that may or may not exist. In general, for every language, there is a keyword to indicate that an object refers to "none", and null is used in Java. Scala incorporates the functional programming style, so it is recommended that you use the Option type when you expect that the return value of a variable or function may not refer to any value. The Option class contains a subclass Some, which can be included using Some when there is a value that can be referenced, such as Some ("Hadoop"). None, on the other hand, is declared as an object, not a class, indicating that it has no value. An example is given below.

/ / first, let's create a mapping scala > val books=Map ("hadoop"-> 5, "spark"-> 10, "hbase"-> 7) books: scala.collection.immutable.Map [String,Int] = Map (hadoop-> 5, spark-> 10, hbase-> 7) / / Let's take the value corresponding to "hadoop" from the mapping. This key exists and can be taken. And the value will be included in Some to return scala > books.get ("hadoop") res0: Option [Int] = Some (5) / / Let's take the value corresponding to "hive" from the mapping. This key does not exist, so the value taken is None object scala > books.get ("hive") res1: Option [Int] = None.

Scala

The Option type also provides the getOrElse method, which returns the corresponding value when the Option is an instance of Some and the parameters passed in when it is an instance of None. For example:

Scala > val sales=books.get ("hive") sales: Option [Int] = None scala > sales.getOrElse ("No Such Book") res3: Any = No Such Book scala > println (sales.getOrElse ("No Such Book")) No Such Book

Scala

As you can see, when we use the getOrElse method, if we take the "hive" without a corresponding value, we can display the "No Such Book" we specified instead of the None. In Scala, Option is used very frequently. In Scala, the option [T] type is often used, where T can be Sting or Int or various other data types. Option [T] is actually a container, and we can think of it as a collection, but the collection either contains only one element (returned in Some), or there is no element (returning None). Since it is a collection, we can certainly use methods such as map, foreach, or filter on it. For example:

Scala > books.get ("hive") .foreach (println)

Scala

You can find that after the above code is executed, nothing is displayed on the screen, because when the foreach traversal encounters None, it does nothing and naturally does not perform the println operation.

This is the end of the introduction to the method of Scala pattern matching. Thank you for reading. If you want to know more about the industry, you can follow the industry information channel. The editor will update different knowledge points for you every day.

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