blocks|key|3203632|text|使用括号：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3203633|data(2)|code-block|syntax|javascript|3203634|但是您并不想经常使用列表，因为链表需要时间来遍历。如果想要索引到一个集合中，可以使用Vector+(不可变)或ArrayBuffer+(可变)或者Array+(这只是一个Java数组，除了同样是用(i)而不是[i]来索引它)。|offset|length|style|CODE|3203635|entityMap^0|0|0|16|6|1J|B|21|5|2Q|3|2W|3|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@$I|R|J|S|K|L]|$I|T|J|U|K|L]|$I|V|J|W|K|L]|$I|X|J|Y|K|L]|$I|Z|J|10|K|L]]|9|@]|A|$]]|$1|M|3|-4|5|6|7|11|8|@]|9|@]|A|$]]]|N|$]]

Use parentheses:

<pre><code>data(2)
</code></pre>

But you don't really want to do that with lists very often, since linked lists take time to traverse. If you want to index into a collection, use <code>Vector</code> (immutable) or <code>ArrayBuffer</code> (mutable) or possibly <code>Array</code> (which is just a Java array, except again you index into it with <code>(i)</code> instead of <code>[i]</code>).

blocks|key|283791|text|更安全的方法是使用lift，这样您就可以提取存在的值，如果不存在则优雅地失败。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|283792|data.lift(2)|code-block|syntax|javascript|283793|如果列表不足以提供该元素，则返回None，如果足够长，则返回Some(value)。|283794|scala>+val+l+=+List("a",+"b",+"c")
scala>+l.lift(1)
Some("b")
scala>+l.lift(5)
None|283795|当您执行可能以这种方式失败的操作时，最好使用一个选项，并让类型系统帮助确保您正在处理元素不存在的情况。|283796|说明：|BOLD|283797|这是因为list的apply+(只加圆括号，例如l(index))就像一个partial函数，只要list有元素就会定义它。List.lift方法将部分apply函数(仅为某些输入定义的函数)转换为普通函数(为任何输入定义)，基本上是将结果包装在一个选项中。|283798|entityMap^0|9|4|0|0|0|0|0|0|3|0|9|5|O|8|1Q|9|24|5|0^^$0|@$1|2|3|4|5|6|7|X|8|@$9|Y|A|Z|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|10|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|11|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|12|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|13|8|@]|D|@]|E|$]]|$1|Q|3|R|5|6|7|14|8|@$9|15|A|16|B|S]]|D|@]|E|$]]|$1|T|3|U|5|6|7|17|8|@$9|18|A|19|B|C]|$9|1A|A|1B|B|C]|$9|1C|A|1D|B|C]|$9|1E|A|1F|B|C]]|D|@]|E|$]]|$1|V|3|-4|5|6|7|1G|8|@]|D|@]|E|$]]]|W|$]]

Safer is to use <code>lift</code> so you can extract the value if it exists and fail gracefully if it does not.

<pre><code>data.lift(2)
</code></pre>

This will return None if the list isn't long enough to provide that element, and Some(value) if it is.

<pre><code>scala&gt; val l = List("a", "b", "c")
scala&gt; l.lift(1)
Some("b")
scala&gt; l.lift(5)
None
</code></pre>

Whenever you're performing an operation that may fail in this way it's great to use an Option and get the type system to help make sure you are handling the case where the element doesn't exist.

Explanation:

This works because List's <code>apply</code> (which sugars to just parentheses, e.g. <code>l(index)</code>) is like a partial function that is defined wherever the list has an element. The <code>List.lift</code> method turns the partial <code>apply</code> function (a function that is only defined for some inputs) into a normal function (defined for any input) by basically wrapping the result in an Option.

blocks|key|3203671|text|为什么用括号？|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3203672|这是programming+in+scala一书中的一句话。|offset|length|3203673|这个例子说明了|3203674|的另一个重要思想，它将让您深入了解为什么在Scala中使用括号访问数组。Scala的特殊情况比Java少。数组只是类的实例，就像Scala中的任何其他类一样。当您将一个或多个值的括号应用于一个变量时，Scala会将代码转换为对该变量调用一个名为apply的方法。因此，greetStrings(i)被转换为greetStrings.apply(i)。因此，在Scala中访问数组的元素只是一个简单的方法调用。这一原则并不局限于数组:对象对括号中某些参数的任何应用都将转换为apply方法调用。当然，只有当该类型的对象实际定义了apply方法时，才会编译。所以这不是特例；这是一个普遍的规则。|blockquote|3203675|3203676|3203677|下面是几个使用函数式编程风格拉取特定元素(在本例中是第一个元素)的示例。|3203678|++//+Create+a+multdimension+Array+
++scala>+val+a+=+Array.ofDim[String](2,+3)
++a:+Array[Array[String]]+=+Array(Array(null,+null,+null),+Array(null,+null,+null))
++scala>+a(0)+=+Array("1","2","3")
++scala>+a(1)+=+Array("4",+"5",+"6")
++scala>+a
++Array[Array[String]]+=+Array(Array(1,+2,+3),+Array(4,+5,+6))|code-block|syntax|javascript|3203679|3203680|++//+1.+paratheses
++scala>+a.map(_(0))
++Array[String]+=+Array(1,+4)
++//+2.+apply
++scala>+a.map(_.apply(0))
++Array[String]+=+Array(1,+4)
++//+3.+function+literal
++scala>+a.map(a+=>+a(0))
++Array[String]+=+Array(1,+4)
++//+4.+lift
++scala>+a.map(_.lift(0))
++Array[Option[String]]+=+Array(Some(1),+Some(4))
++//+5.+head+or+last+
++scala>+a.map(_.head)
++Array[String]+=+Array(1,+4)|3203681|entityMap|0|LINK|mutability|MUTABLE|url|https://www.artima.com/pins1ed/next-steps-in-scala.html^0|0|2|K|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|14|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|15|8|@]|9|@$D|16|E|17|1|18]]|A|$]]|$1|F|3|G|5|6|7|19|8|@]|9|@]|A|$]]|$1|H|3|I|5|J|7|1A|8|@]|9|@]|A|$]]|$1|K|3|-4|5|6|7|1B|8|@]|9|@]|A|$]]|$1|L|3|-4|5|6|7|1C|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|1D|8|@]|9|@]|A|$]]|$1|O|3|P|5|Q|7|1E|8|@]|9|@]|A|$R|S]]|$1|T|3|-4|5|6|7|1F|8|@]|9|@]|A|$]]|$1|U|3|V|5|Q|7|1G|8|@]|9|@]|A|$R|S]]|$1|W|3|-4|5|6|7|1H|8|@]|9|@]|A|$]]]|X|$Y|$5|Z|10|11|A|$12|13]]]]

Why parentheses? 

Here is the quote from the book <a href="https://www.artima.com/pins1ed/next-steps-in-scala.html">programming in scala</a>. 

<blockquote>
 Another important idea illustrated by this example will give you insight into why arrays are accessed with parentheses in Scala. Scala has fewer special cases than Java. Arrays are simply instances of classes like any other class in Scala. When you apply parentheses surrounding one or more values to a variable, Scala will transform the code into an invocation of a method named apply on that variable. So greetStrings(i) gets transformed into greetStrings.apply(i). Thus accessing an element of an array in Scala is simply a method call like any other. This principle is not restricted to arrays: any application of an object to some arguments in parentheses will be transformed to an apply method call. Of course this will compile only if that type of object actually defines an apply method. So it's not a special case; it's a general rule.
</blockquote>

Here are a few examples how to pull certain element (first elem in this case) using functional programming style. 

<pre><code> // Create a multdimension Array 
 scala&gt; val a = Array.ofDim[String](2, 3)
 a: Array[Array[String]] = Array(Array(null, null, null), Array(null, null, null))
 scala&gt; a(0) = Array("1","2","3")
 scala&gt; a(1) = Array("4", "5", "6")
 scala&gt; a
 Array[Array[String]] = Array(Array(1, 2, 3), Array(4, 5, 6))
</code></pre>

<hr>

<pre><code> // 1. paratheses
 scala&gt; a.map(_(0))
 Array[String] = Array(1, 4)
 // 2. apply
 scala&gt; a.map(_.apply(0))
 Array[String] = Array(1, 4)
 // 3. function literal
 scala&gt; a.map(a =&gt; a(0))
 Array[String] = Array(1, 4)
 // 4. lift
 scala&gt; a.map(_.lift(0))
 Array[Option[String]] = Array(Some(1), Some(4))
 // 5. head or last 
 scala&gt; a.map(_.head)
 Array[String] = Array(1, 4)
</code></pre>

blocks|key|2989985|text|请使用括号()访问列表元素索引(List_name)|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2989986|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

Please use parenthesis () to access the list elements 
list_name(index)

How in the world do you get just an element at index i from the List in scala?
I tried <code>get(i)</code>, and <code>[i]</code> - nothing works. Googling only returns how to &quot;find&quot; an element in the list. But I already know the index of the element!
Here is the code that does not compile:
<pre><code>def buildTree(data: List[Data2D]):Node ={
 if(data.length == 1){
 var point:Data2D = data[0] //Nope - does not work
 
 }
 return null
}
</code></pre>
Looking at the <a href="https://www.scala-lang.org/api/current/scala/collection/immutable/List.html" rel="noreferrer">List api</a> does not help, as my eyes just cross.

Get item in the list in Scala?

Java

到底如何从scala中的列表中获取索引i处的元素呢？我试过get(i)，但[i] --什么都不起作用。Googling只返回如何在列表中“查找”元素。但是我已经知道元素的索引了！下面是不能编译的代码：def buildTree(data: List[Data2D]):Node ={  if(data.length ==...

问在Scala中获取列表中的项目？
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