kotlin集合包属性函数（待分组）(4)

val list = listOf(1, 2, 3, 4, 5, 6)assertEquals(listOf(1, 3, 5), list.filterNot { it % 2 == 0 }) [原创文章：www.11jj.com]

[转载出处：www.11jj.com]

filterNotNull过滤非null元素

val list = listOf(1, 2, 3, 4, 5, 6)assertEquals(listOf(1, 2, 3, 4), listWithNull.filterNotNull())

filterTo过滤符合函数的元素再赋值给目标

val list = listOf(1, 2, 3, 4, 5, 6)val dest = mutableListOf<Int>()assertEquals(listOf(4, 5, 6, 7), list.filterTo(dest, { it > 3 }))

slice过滤列表中指定索引的元素

val list = listOf(1, 2, 3, 4, 5, 6)assertEquals(listOf(2, 4, 5), list.slice(listOf(1, 3, 4)))

take返回从前开始n个元素

val list = listOf(1, 2, 3, 4, 5, 6)assertEquals(listOf(1, 2), list.take(2))

takeLast返回从后开始n个元素

val list = listOf(1, 2, 3, 4, 5, 6)assertEquals(listOf(5, 6), list.takeLast(2))

takeWhile返回从前开始连续符合函数的元素

val list = listOf(1, 2, 3, 4, 5, 6)assertEquals(listOf(1, 2), list.takeWhile { it < 3 })

takeLastWhile返回从后开始连续符合函数的元素

val list = listOf(1, 2, 3, 4, 5, 6)assertEquals(listOf<Int>(), list.takeLastWhile { it < 3 })

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分组操作符

groupBy返回根据函数分组后的map，结果可根据第二函数转换

val list = listOf(1, 2, 3, 4, 5, 6)assertEquals(mapOf("odd" to listOf(1, 3, 5), "even" to listOf(2,4, 6)), list.groupBy { if (it % 2 == 0) "even" else "odd" })

val words = listOf("a", "abc", "ab", "def", "abcd")println(words.groupBy({ it.length }))println(words.groupBy({ it.length }, { it.contains("b") }))val programmer = listOf("K&R" to "C", "Bjar" to "C++", "Linus" to "C", "James" to "Java")println(programmer.groupBy({ it.second }))println(programmer.groupBy({ it.second }, { it.first }))println(programmer.groupBy({ it.second }, { it.second }))

{1=[a], 3=[abc, def], 2=[ab], 4=[abcd]}
 {1=[false], 3=[true, false], 2=[true], 4=[true]}
 {C=[(K&R, C), (Linus, C)], C++=[(Bjar, C++)], Java=[(James, Java)]}
 {C=[K&R, Linus], C++=[Bjar], Java=[James]}
 {C=[C, C], C++=[C++], Java=[Java]}

groupingBy根据函数分组，结合eachCount统计