如何从具有特定属性的列表中获取“集合集”的计数

Question

我想从具有特定属性的列表中获取集合的计数值 提前致谢

class User(val age = 0, val group = "school")

fun main(
    val list = listof(User(1, "school"), User(2, "school"), User(3, "company"))

    fun getCount() {
        //how to get 2 (the count of set such as ("school", "company").size)
    }
}

Answer 1

如果我理解你的问题是正确的，你想知道你有多少组（在你的例子 2 中：“学校”和“公司”

您可以创建一个Grouping，例如：

data class User(val age: Int = 0, val group: String = "school")

fun main() {
    val list = listOf(User(1, "school"), User(2, "school"), User(3, "company"), User(16, "banana"))
    println(getCount(list))
    println(getCountOf(list,"banana"))
}

// returns the number of groups
fun getCount(list: List<User>): Int {
    val grouping = list.groupingBy { it.group }.eachCount()
    return grouping.keys.size
}

//returns the size of a given group (0 if group not found)
fun getCountOf(list: List<User>, group: String): Int {
    val grouping = list.groupingBy { it.group }.eachCount()
    return grouping[group] ?: 0
}

Answer 2

您可以将列表过滤到“学校”组中的人，然后计算，也许？

list .filter { p -> group ==  "school"  }