kotlin 集合：filter/groupBy

過濾與分組

filter

private fun filter() {
    val mList = mutableListOf(1, 2, 3, 4, 5, 6, 7, 8, 9, 0)
    val resultList = mList.filter {
        it > 5
    }
    println(resultList)
}
//[6, 7, 8, 9]

groupBy :

分組。即根據條件把集合拆分爲爲一個Map<K,List>類型的集合

private fun groupBy() {
    val mList = arrayListOf(0, 1, 2, 3, 4, 5, 6)
    val groupByList = mList.groupBy {
        //定義key
        it % 2 == 0
    }
    println(groupByList)
    //這裏返回兩個元素的map
    //{true=[0, 2, 4, 6], false=[1, 3, 5]}
    groupByList.entries.forEach {
        println("${it.key}==========${it.value}")
    }
    //true==========[0, 2, 4, 6]
    //false==========[1, 3, 5]

}

private fun groupBy2() {
    val mList = arrayListOf(0, 1, 2, 3, 4, 5, 6)
    val groupByList = mList.groupBy {
        //定義key
        if (it % 2 == 0) {
            "偶數"
        } else {
            "奇數"
        }
    }
    println(groupByList)
    //這裏返回兩個元素的map
    //{偶數=[0, 2, 4, 6], 奇數=[1, 3, 5]}
    groupByList.entries.forEach {
        println("${it.key}==========${it.value}")
    }
    //偶數==========[0, 2, 4, 6]
    //奇數==========[1, 3, 5]

}