Scala Learning Essays - Control Statements

Scala only has a few built-in program control statements: if, while, for, try catch, and function calls. This is because function literals have been included since the birth of Scala, and the Scala kernel does not define too many control structures. Instead, the program's control structures can be extended with additional libraries.

1. if expression

Like other languages, if in Scala is also an option (branch), and unlike other languages, each expression can return a value.

var age = 25
val  result = if(age > 20) "worker"  else  "student"
println(result)

2. while loop

There are two forms, while(...){} and do{}while(...)

In Scala, while and do-while are called loops instead of expressions, because they have no return value, return Unit or ()

Three, for expression

1. A basic usage of for is similar to Java and can be used to enumerate collection elements

val filesHere = (new java.io.File(".")).listFiles
for(file <- filesHere)
    println(file)

Supports all types of collection types

for(i <- 1 to 4)
    println(i)

2. Filter

A for expression can add a filter - add one or more if statements inside the for parentheses.

For example, instead of enumerating every element in a collection, iterate over only certain elements that qualify.

val filesHere = (new java.io.File(".")).listFiles
for(file <- filesHere
    if file.isFile
    if file.getName.endsWith(".scala"))
        println(file)

3. Nested iteration

The for expression supports multiple iterations

4. Bind intermediate variables

5. Generate a new collection

Basic syntax: for clauses yield body , where clauses yield is the keyword

Fourth, use try expressions to handle exceptions

1. Throwing an exception

Exceptions are thrown in the same way as in Java

val half =
  if (n % 2 == 0)
    n/2
  else
    throw new RuntimeException("n must be even")

2. Catch exceptions

try{}catch{}

3.finally

If there is a return value in finally, the return value in try will be overwritten, so try to avoid using return in finally

4. Generate return value

5. Match expression

This expression supports selecting one of multiple choices for execution, similar to switch in Java

true/false boolean expression match {
  case "salt" => println("pepper")
  case "chips" => println("salsa")
  case "eggs" => println("bacon")
  case _ => println("huh?")
}

Pay attention to the following three points when using:

One is that any type of constant can be used in a case statement, not just an int enumeration type.
Second, each case statement does not need to be used break , Scala does not support " fall through".
The third is the default matching of Scala _ , its function is similar to that in java default.

6. Scala does not provide break and continue, but some methods can be used to achieve the functions of break and continue

var i = 0
var foundIt=false
while (i < args.length && !foundIt) {
    if (!args(i).startsWith("-")) {
    if(args(i).endsWith(".scala"))
        foundIt=true
    }
    i=i+1
}

Of course, Scala scala.util.control defines break control structures in packages. Its implementation is to call the function to the superior by throwing an exception.

import scale.util.control.Breaks._
import java.io._

val in = new BufferedReader(new InputStreamReader(System.in))

breakable {
  while(true) {
    println("? ")
    if(in.readLine()=="") break
  }
}

Scala Learning Essays - Control Statements

Guess you like