标签：返回 index val kotlin list element Returns 实例 操作符

val dropwhile=list.dropWhile {it > 1 }

/**

• dropLastWhile 返回根据特定的函数 从最后一项开始 直到不满足条件后返回 列表
• Returns a list containing all elements except last elements that satisfy the given [predicate].
• 返回[0, 1, 2, 3, 4, 5, 6, 5, 4, 3, 2, 1, 0]
  */
  val dropLastWhile= list.dropLastWhile { it>4 }

/**
*filter 返回所有符合给定函数条件的元素。

• Returns a list containing only elements matching the given [predicate].
• [5, 6, 5]
  */
  val filter=list.filter { it>4 }

/**

• filterNot 返回所有不符合给定函数条件的元素
• Returns a list containing all elements not matching the given [predicate].
• [0, 1, 2, 3, 4, 4, 3, 2, 1, 0]
  */
  val filterNot=list.filterNot { it>4 }

/**

• filterNotNull 返回非null元素
• Returns a list containing all elements that are not null.
• [0, 1, 2, 3, 4, 5, 6, 5, 4, 3, 2, 1, 0]
  */
  val filterNotNull= list.filterNotNull()

/**

• 返回满足该ranger的元素集合
• Returns a list containing elements at indices in the specified [indices] range.
• [0, 1, 2, 3, 4, 5, 6]
  */
  val slice= list.slice(0…6)

/**

• listOf(0,4,7)是集合list的坐标
• Returns a list containing elements at specified [indices].
• [0, 4, 5]
  */
  val slice2= list.slice(listOf(0,4,7))

/**
*返回前n项

• Returns a list containing first [n] elements.
• [0, 1, 2, 3]
  */
  val take= list.take(4)

/**

• 返回后n项
• Returns a list containing last [n] elements.
• [3, 2, 1, 0]
  */
  val takeLast= list.takeLast(4)

/**

• 从第一项开始判断，直到不符合就返回，返回符合的前几项数据
• Returns a list containing first elements satisfying the given [predicate].
• []
  */
  val takeWhile= list.takeWhile { it>4 }

• 映射操作符

private val list= listOf(0,1,2,3,4,5,4,3,2,1,0,-1)

/**

• 返回满足条件的集合
• Returns a list containing the results of applyin

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g the given [transform] function

• to each element in the original collection.
• [false, false, false, true, true, true, true, true, false, false, false, false]
  */
  val map=list.map { it>2 }

/**

• 返回特定函数后的集合，参数是Iterable类型，
• Returns a single list of all elements yielded from results of [transform]
• function being invoked on each element of original collection.
• [0, 1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6, 4, 5, 3, 4, 2, 3, 1, 2, 0, 1, -1, 0]
  */
  val flatMap=list.flatMap { listOf(it, it + 1) }

/**

• 根据函数将集合分组，返回map类型对象
• Groups elements of the original collection by the key returned by the given [keySelector] function
• applied to each element and returns a map where each group key is associated with a list of corresponding elements.
• The returned map preserves the entry iteration order of the keys produced from the original collection.
• {false=[0, 1, 2, 3, 3, 2, 1, 0, -1], true=[4, 5, 4]}
• @sample samples.collections.Collections.Transformations.groupBy
  */
  val groupBy=list.groupBy {value-> value>3 }

/**

• 返回一个集合，通过 角标和值 来生成
• Returns a list containing the results of applying the given [transform] function
• to each element and its index in the original collection.
• @param [transform] function that takes the index of an element and the element itself
• and returns the result of the transform applied to the element.
• [0, 1, 2, 3, 4, 5, 4, 3, 2, 1, 0, -1]
  */
  val mapIndexed=list.mapIndexed { index, value ->value}

/**

• 返回一个每一个非null元素根据给定的函数转换所组成的List
• Returns a list containing only the non-null results of applying the given [transform] function
• to each element in the original collection.
• [0, 2, 4, 6, 8, 10, 8, 6, 4, 2, 0, -2]
  /
  val mapNotNull=list.mapNotNull { it2 }

• 元素操作符

private val list= listOf(0,1,2,3,4,5,6,4,3,2,1,0,-1)

//如果指定元素可以在集合中找到，则返回true。
val contains=list.contains(2)

/**

• 返回给定index对应的元素，如果index数组越界则会 抛出IndexOutOfBoundsException
• Returns an element at the given [index] or throws an [IndexOutOfBoundsException] if the [index] is out of bounds of this collection.
• 2
  */
  val elementAt=list.elementAt(2)

/**

• 返回给定index对应的元素，如果index数组越界则会根据给定函数返回默认值,第二个参数default，lamdba表达式
• Returns an element at the given [index] or the result of calling the [defaultValue]
• function if the [index] is out of bounds of this collection.
• 2
  */
  val elementAtOrElse=list.elementAtOrElse(2){“error”}

/**

• 返回给定index对应的元素，如果index数组越界则会 返回null
• Returns an element at the given [index] or null if the [index] is out of bounds of this list.
• null
  */
  val elementAtOrNull=list.elementAtOrNull(19)

/**

• Returns first element.
• @throws [NoSuchElementException] if the list is empty.
• 0
  */
  val first=list.first()

/**

• 返回符合给定函数条件的第一个元素,没有回抛异常
• Returns the first element matching the given [predicate].
• @throws [NoSuchElementException] if no such element is found.
• 4
  */
  val first2=list.first { it>3 }

/**

• 返回符合给定函数条件的第一个元素，如果没有符合则返回null
• Returns the first element matching the given [predicate], or null if element was not found.
• null
  */
  val firstOrNull=list.firstOrNull { it>9 }

/**

• 返回指定元素的第一个index，如果不存在，则返回-1
• Returns the index of the first occurrence of the specified element in the list, or -1 if the specified
• element is not contained in the list.
• 3
  */
  val indexOf=list.indexOf(3)

/**

• 返回第一个符合给定函数条件的元素的index，如果没有符合则返回-1
• Returns index of the first element matching the given [predicate], or -1 if the list does not contain such element.
• 0
  */
  val indexOfFirst=list.indexOfFirst { it%3==0 }

/**

• 返回最后一个符合给定函数条件的元素的index，如果没有符合则返回-1
• Returns index of the last element matching the given [predicate], or -1 if the list does not contain such element.
• 11
  */
  val indexOfLast=list.indexOfLast { it%3==0 }

/**

• 返回符合给定函数条件的最后一个元素,没有抛异常
• Returns the last element matching the given [predicate].
• @throws [NoSuchElementException] if no such element is found.
• 6
  */
  val last=list.last { it>4 }

/**

• 返回指定元素的最后一个index，如果不存在，则返回-1
• Returns the index of the last occurrence of the specified element in the list, or -1 if the specified
• element is not contained in the list.
• 8
  */
  val lastIndexOf=list.lastIndexOf(3)

/**

• 返回符合给定函数条件的最后一个元素，如果没有符合则返回null
• Returns the last element matching the given [predicate], or null if no such element was found.
• null
  */
  val lastOrNull=list.lastOrNull { it>8 }

/**

• 返回符合给定函数的单个元素，如果没有符合或者超过一个，则抛出异常
• Returns the single element matching the given [predicate], or throws exception if there is no or more than one matching element.
• 6
  */
  val single=list.single { it>5 }

/**

• 返回符合给定函数的单个元素，如果没有符合或者超过一个，则返回null
• Returns the single element matching the given [predicate], or null if element was not found or more than one element was found.
• null
  */
  val singleOrNull=list.singleOrNull { it>8 }

• 生产操作符

private val list1= listOf(0,1,2,3,4,5)
private val list2= listOf(4,5,2,1,5)

/**

• Returns a list of pairs built from elements of both collections with same indexes. List has length of shortest collection.
• [(0, 4), (1, 5), (2, 2), (3, 1), (4, 5)]
  */
  val zip= list1.zip(list2)

/**

• Returns a list of values built from elements of both collections with same indexes using provided [transform]. List has length of shortest collection.
• [4, 6, 4, 4, 9]
  */
  val zip2= list1.zip(list2){it1,it2->it1+it2}

//[(0, 0), (1, 1), (2, 2), (3, 3), (4, 4), (5, 5)]
val zip4= list1.zip(list1+list2)

//[0, 2, 4, 6, 8, 10]
val zip3= list1.zip(list1+list2){it1,it2->it1+it2}

/**

• Returns a pair of lists, where
• first list is built from the first values of each pair from this collection,

标签：返回,index,val,kotlin,list,element,Returns,实例,操作符
来源： https://blog.csdn.net/m0_65512599/article/details/122219827

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