Kotlin learning three: set and lambda

List Created in traversal

Create a read only collection , Can not add 、 Modify or delete the contents of the collection ：

        val myList = listOf("A", "B", "C")
        for (i in 0 until myList.size) {
            Log.w(" list ", "" + myList.get(i))
        }

Create a set that can be modified ：

        val myList = mutableListOf("A", "B", "C")
        myList.add("D")
        for (i in 0 until myList.size) {
            Log.w(" list ", "" + myList.get(i))
        }

Map Use

establish , Save data and get data ：

        // establish 
        val myMap = HashMap<Int, String>()
        // Add data 
        myMap[1] = "A"
        myMap[2] = "B"
        myMap[3] = "C"
        // Take out the data 
        val value = myMap[2]
        Log.w(" Take out the data ", "" + value)

You can write like this ：

        // establish 
        val myMap = mutableMapOf(1 to "A", 2 to "B", 3 to "C")
        // Add data 
        myMap[8] = "X"
        // Take out the data 
        for ((myK, myV) in myMap) {
            Log.w(" Take out the data ", "key:" + myK + ",value:" + myV)
        }

There is no mutable Is only readable , Yes  mutable It can be modified .

Lambda expression

Lambda An expression is an expression with parameters .

Grammatical structure ：{ Parameter name 1： Parameter type , Parameter name 2： Parameter type ->  The body of the function }

It is simply understood as a method that can pass in a piece of code . Generally, I write it myself , You can only pass in parameters and then process them in the method body , and Lambda It can send a piece of code for processing .

Let's take a simple example , I want to take the longest data in a list ：

        val myList = listOf("aA", "BBBbb", "CCcc")
        val lambda = { name: String -> name.length }
        val maxLength = myList.maxBy(lambda)
        Log.w(" Take out the longest ", "" + maxLength)

It can be simplified as ：

        val myList = listOf("aA", "BBBbb", "CCcc")
        val maxLength = myList.maxBy({ name: String -> name.length })
        Log.w(" Take out the longest ", "" + maxLength)

kotlin Regulations , The last parameter is zero lambda when , Can be lambda Put it outside , Then omit () , So it's like this ：

        val myList = listOf("aA", "BBBbb", "CCcc")
        val maxLength = myList.maxBy{ name: String -> name.length }
        Log.w(" Take out the longest ", "" + maxLength)

because lambda There is only one parameter , So parameters can be omitted , use it Keyword substitution , So it's like this ：

        val myList = listOf("aA", "BBBbb", "CCcc")
        val maxLength = myList.maxBy { it.length }
        Log.w(" Take out the longest ", "" + maxLength)

Another example , Want to capitalize everything , be used map function ：

        val myList = listOf("aA", "BBBbb", "CCcc")
        val newList = myList.map { it.toUpperCase() }
        for (value in newList) {
            Log.w(" The value is ", value)
        }

above map Function can map each element in the list to other values .

Use filter The filter length of the function is 4 What's inside , And they are all capitalized ：

        val myList = listOf("aA", "BBBbb", "CCcc")
        val newList = myList.filter { it.length < 5 }.map { it.toUpperCase() }
        for (value in newList) {
            Log.w(" The value is ", value)
        }

filter You can filter the data in the set .

any function ： Judge whether at least one of the set satisfies the condition ;

all function ： Determine whether all elements in the set meet the conditions ;

        val myList = listOf("aA", "BBBbb", "CCcc")
        val mAny = myList.any { it.length > 3 }
        val mAll = myList.all { it.length > 3 }
        Log.w(" return ", "" + mAny + "," + mAll)