kotlin 集合 ： map/mapNotNull/mapIndexed/mapIndexedNotNull/flatMap

map :

// map{...} : 把每個元素按照特定的方法進行轉換，並返回一個新的集合
private fun map() {
    val mList = arrayListOf(null, 0, 1, 2, 3, 4, 5, 6, null)
    val mapList = mList.map {
        it?.let { it * 2 }
    }
    println(mapList)
}
//[null, 0, 2, 4, 6, 8, 10, 12, null]

mapNotNull :

// mapNotNull{...} : 同map{}相同，過濾掉轉換之後爲null的元素
private fun mapNotNull() {
    val mList = arrayListOf(null, 0, 1, 2, 3, 4, 5, 6, null)
    val mapNotNullList = mList.mapNotNull {
        it?.let { it * 2 }
    }
    println(mapNotNullList)
}
//[0, 2, 4, 6, 8, 10, 12]

mapIndexed :

// mapIndexed{index,result} : 在map{} 函數上多了一個下標功能
private fun mapIndexed() {
    val mList = arrayListOf(null, 0, 1, 2, 3, 4, 5, 6, null)
    val mapIndexedList = mList.mapIndexed { index, value ->
        value?.let {
            if (index % 2 == 0) {
                value * 2
            } else {
                value * 3
            }
        }
    }
    println(mapIndexedList)
}
//[null, 0, 2, 6, 6, 12, 10, 18, null]

mapIndexedNotNull :

// mapIndexedNotNull{index,result} : 在mapNotNull{}函數上多了一個下標功能
private fun mapIndexedNotNull() {
    val mList = arrayListOf(null, 0, 1, 2, 3, 4, 5, 6, null)
    val mapIndexedNotNullList = mList.mapIndexedNotNull { index, value ->
        value?.let {
            if (index % 2 == 0) {
                value * 2
            } else {
                value * 3
            }
        }
    }
    println(mapIndexedNotNullList)
}
//[0, 2, 6, 6, 12, 10, 18]

flatMap :

private fun flatMap() {
    val mList = arrayListOf(null, 0, 1, 2, 3, 4, 5, 6, null)
    val flatMapList = mList.flatMap {
        arrayListOf(it?.let { it * 2 })
    }
    println(flatMapList)
}
//[null, 0, 2, 4, 6, 8, 10, 12, null]