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Getting started with Scala_ Immutable list and variable list

2022-06-27 02:22:00 【Big data interview classic】

list

The list is scala The most important 、 It's also the most common data structure .List It has the following properties ：

You can save duplicate values
In order

stay scala in , There are also two kinds of lists , One is immutable lists 、 The other is the variable list

Definition

Immutable lists are elements of lists 、 The length is immutable .

grammar

Use List( Elements 1, Elements 2, Elements 3, ...) To create an immutable list , Grammar format ：

       
        val/var  Variable name  = List( Elements 1,  Elements 2,  Elements 3...)
       
       
        1.

Use Nil Create an immutable empty list

       
        val/var  Variable name  = Nil
       
       
        1.

Use :: Method to create an immutable list

       
        val/var  Variable name  =  Elements 1 ::  Elements 2 :: Nil
       
       
        1.

[!TIP]
Use **:: Create a list by splicing , You have to add one at the end Nil**

Example 1

Create an immutable list , Store the following elements （1,2,3,4）

Reference code

       
        scala> val a = List(1,2,3,4)
        
a: List[Int] = List(1, 2, 3, 4)
       
       
        1.
        2.

Example 2

Use Nil Create an immutable empty list

Reference code

       
        scala> val a = Nil
        
a: scala.collection.immutable.Nil.type = List()
       
       
        1.
        2.

Example 3

Use :: Method to create a list , contain -2、-1 Two elements

Reference code

       
        scala> val a = -2 :: -1 :: Nil
        
a: List[Int] = List(-2, -1)
       
       
        1.
        2.

Variable list

Variable lists are elements of lists 、 The length is variable .

To use variable lists , First import import scala.collection.mutable.ListBuffer

[!NOTE]

Variable sets are all in mutable In bag
Immutable sets are all in immutable In bag （ The default import ）

Definition

Use ListBuffer[ Element type ]() Create an empty mutable list , Grammatical structure ：

       
        val/var  Variable name  = ListBuffer[Int]()
       
       
        1.

Use ListBuffer( Elements 1, Elements 2, Elements 3…) Create variable lists , Grammatical structure

Example 1

Create an empty variable list

Reference code

       
        scala> val a = ListBuffer[Int]()
        
  a: scala.collection.mutable.ListBuffer[Int] = ListBuffer()
       
       
        1.
        2.

Example 2

Create a mutable list , Contains the following elements ：1,2,3,4

Reference code

       
        scala> val a = ListBuffer(1,2,3,4)
        
a: scala.collection.mutable.ListBuffer[Int] = ListBuffer(1, 2, 3, 4)
       
       
        1.
        2.

Variable list operations

Get elements （ Use parentheses to access ( Index value )）
Additive elements （+=）
Add a list （++=）
Change the element （ Use parentheses to get elements , And then assign a value ）
Remove elements （-=）
Convert to List（toList）
Convert to Array（toArray）

Example

Define a variable list that contains the following elements ：1,2,3
Get first element
Add a new element ：4
Add a list , The list contains the following elements ：5,6,7
Remove elements 7
Convert variable list to immutable list
Convert variable list to array

Reference code

       
        //  Import immutable list 
        
scala> import scala.collection.mutable.ListBuffer
        
import scala.collection.mutable.ListBuffer
        

        
//  Create immutable lists 
        
scala> val a = ListBuffer(1,2,3)
        
a: scala.collection.mutable.ListBuffer[Int] = ListBuffer(1, 2, 3)
        

        
//  Get first element 
        
scala> a(0)
        
res19: Int = 1
        

        
//  Add an element 
        
scala> a += 4
        
res20: a.type = ListBuffer(1, 2, 3, 4)
        

        
//  Add a list 
        
scala> a ++= List(5,6,7)
        
res21: a.type = ListBuffer(1, 2, 3, 4, 5, 6, 7)
        

        
//  Remove elements 
        
scala> a -= 7
        
res22: a.type = ListBuffer(1, 2, 3, 4, 5, 6)
        

        
//  Convert to immutable list 
        
scala> a.toList
        
res23: List[Int] = List(1, 2, 3, 4, 5, 6)
        

        
//  Convert to array 
        
scala> a.toArray
        
res24: Array[Int] = Array(1, 2, 3, 4, 5, 6)
       
       
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