• 本篇文章會討論到下列 MutableList 方法

  • sort

  • forEach

  • filter

sort

1. sort

基礎的排序

數字會依照升序排列

 fun main (){

val numbers = mutableListOf(4,1,3,2)
numbers.sort()
println("Sort into ascending: $numbers")

 // result
// Sort into ascending: [1, 2, 3, 4]

}

單字會依照第一個字母在字母表的順序做升序排列

 fun main (){

val numbers = mutableListOf("one", "two", "three", "four")
numbers.sort()
println("Sort into ascending: $numbers")

 // result
// Sort into ascending: [four, one, three, two]

}

2. sortDescending

sort 是升序 而 sortDescending 是降序

數字會依照降序排列

fun main (){

val numbers = mutableListOf(4,1,3,2)
numbers.sortDescending()
println("Sort into descending: $numbers")

// result
//Sort into descending: [4, 3, 2, 1]

}

單字會依照第一個字母在字母表的順序做降序排列

 fun main (){

val numbers = mutableListOf("one", "two", "three", "four")
numbers.sortDescending()
println("Sort into descending: $numbers")

// result
// Sort into descending: [two, three, one, four]

}

3. sortBy

1. 可以自訂排序條件
2. 做升序排列

下面依照 String 字數 做升序排列

 
fun main (){

val numbers = mutableListOf("one", "two", "three", "four")
numbers.sortBy { it.length }
println("Sort into ascending by length: $numbers")
 
}

// result:
// Sort into ascending by length: [one, two, four, three]

4. sortByDescending

1. 可以自訂排序條件
2. 做降序排列

下面依照單字最後一個字母在字母表的順序做降序排列

 fun main (){

val numbers = mutableListOf("one", "two", "three", "four")
numbers.sortByDescending { it.last() }
println("Sort into descending by the last letter: $numbers")

}

// result:
// Sort into descending by the last letter: [four, two, one, three]

forEach

forEach 用於進行遍歷每個元素 。

fun main (){

val numbers = mutableListOf<Int>(1,2,3,4,5)

numbers.forEach{ println(it * 2) }

// result:
// 2
// 4
// 6
// 8
// 10

}

filter

過濾所有符合條件的元素,形成一個新的集合
it 代表 List 中的單一元素, it 可以換作 其它 的名稱

fun main (){

val numbers = mutableListOf<Int>(1,2,3,4,5)

val filterList = numbers.filter{ it > 2}

println(filterList)
 
}

// result
//[3, 4, 5]