Scala basic tutorial -- 15 -- recursion

Scala Basic course –15– recursive

Chapter goal

1. Understand the overview of recursion
2. Master factorial cases
3. Master the Fibonacci series case
4. Master the case of printing catalog files

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1. recursive

Recursion refers to Method calls its own situation . When it comes to complex operations , We will often use it . When using recursion , Pay attention to the following three points :

1. Recursion must have an exit , Otherwise, it is easy to cause Dead recursion .
2. Recursion must be regular .
3. Construction methods cannot be recursive .
4. Recursive methods must have The data type of the return value .

for example : The following code is written recursively .

def show() = {
    
  show()
}

2. Case a : Find the factorial

2.1 summary

The so-called factorial actually refers to Numbers 1 The cumulative multiplication result to this number , for example 5 The factorial of is equivalent to 5 * 4 * 3 * 2 * 1 , 4 The factorial of is equivalent to
4 * 3 * 2 * 1 , According to the above description , We can draw two conclusions :

1. The factorial formula is ( for example : Ask for numbers n The factorial ): n! = n * (n - 1)!
2. 1 The factorial of is equal to 1, namely : 1! = 1

2.2 demand

Calculation 5 The factorial .

2.3 Reference code

//  Case study :  seek 5 The factorial .
object ClassDemo01 {
    
 //1.  Define methods ,  Used to find numbers n The factorial .
 def factorial(n:Int):Int = if(n == 1) 1 else n * factorial(n - 1)
 def main(args: Array[String]): Unit = {
    
  //2.  call factorial Method ,  Used to obtain 5 The factorial .
  val num = factorial(5)
  //3.  Print the results .
  println(num)
}
}

2.4 Memory diagram

summary

stay Scala in , Memory is divided into five parts , Respectively Stack , Pile up , Method area , Local method area , register , The characteristics are as follows :

• Stack
    function :
     Execution of all code .
     Store local variables .
    characteristic : according to First in, then out Sequential execution , The method is recycled immediately after execution .
• Pile up :
    function : Store all new What's coming out ( namely : object ).
    characteristic : Be GC Recycling .
• Method area :
    function : Store bytecode files , Methods and other data .
    characteristic : After the program is executed , Recycling resources by the operating system .
• Local method area :
    and Local method relevant , Understanding can .
• register
    and CPU relevant , Understanding can .

Factorial diagram

3. Case 2 : Fibonacci sequence

3.1 summary

It is said that there was an Italian youth named Fibonacci , One day he raised a very interesting question , hypothesis :

1. A pair of little rabbits will grow into a pair of big rabbits in a month .
2. Every pair of big rabbits gives birth to a pair of little rabbits every month .
3. Assuming that all rabbits do not die , ask : 1 To the little rabbit , 1 How many pairs of rabbits will it become after years ?

3.2 Thought analysis

namely : Known sequence 1, 1, 2, 3, 5, 8, 13…, ask : The first 12 What's the number ?

3.3 Reference code

//  Case study :  Fibonacci sequence 
object ClassDemo02 {
    
 //1.  Define methods ,  Used to obtain the logarithm of rabbits .
 def rabbit(month:Int):Int = {
    
  if(month == 1 || month == 2) 1
  else rabbit(month -1) + rabbit(month - 2)
}
 def main(args: Array[String]): Unit = {
    
  //2.  Calling method ,  For the first 12 A couple of months .
  val num = rabbit(12)
  //3.  Print the results .
  println(num)
}
}

4. Case three : Print catalog file

4.1 demand

1. Definition printFile(dir:File) Method , This method receives a file directory , Used to print all the file paths in this directory .
2. stay main Test in method printFile() Method .

4.2 Purpose

Investigate recursive , Java Of File class Related content .

Be careful : because Scala It's dependence JVM Of , therefore Java Class library in , Scala It can also be called seamlessly ,

4.3 Reference code

import java.io.File
// Case study :  Get all the files in the specified directory .
object ClassDemo03 {
    
 //1.  Definition printFile() Method ,  Used to print all file information under the specified directory .
 def printFile(dir: File): Unit = {
    
  if (!dir.exists()) {
    
   println(" The path you entered does not exist ")
 } else {
    
   val listFiles:Array[File] = dir.listFiles()
   for(listFile <- listFiles) {
    
    if(listFile.isFile) println(listFile)
    else printFile(listFile)
  }
 }
 //2.main Method ,  As the main entrance to the program .
 def main(args: Array[String]): Unit = {
    
  //3.  Calling method show()
  printFile(new File("d:\\abc"))
}
}