Get item in the list in Scala

Question

How in the world do you get just an element at index i from the List in scala  I tried get i   and   i  - 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  def buildTree data  List Data2D   Node      if data length    1         var point Data2D   data 0     Nope - does not work               return null    Looking at the List api does not help  as my eyes just cross

User · Accepted Answer

Use parentheses   data 2    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 Vector  immutable  or ArrayBuffer  mutable  or possibly Array  which is just a Java array  except again you index into it with  i  instead of  i

User · Answer

Why parentheses    Here is the quote from the book programming in scala       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    Here are a few examples how to pull certain element  first elem in this case  using functional programming style         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            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

User · Answer

Please use parenthesis    to access the list elements  list name index

User · Answer

Safer is to use lift so you can extract the value if it exists and fail gracefully if it does not   data lift 2    This will return None if the list isn t long enough to provide that element  and Some value  if it is   scala gt  val l   List  a    b    c   scala gt  l lift 1  Some  b   scala gt  l lift 5  None   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 apply  which sugars to just parentheses  e g  l index   is like a partial function that is defined wherever the list has an element  The List lift method turns the partial apply 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

[scala] Get item in the list in Scala?

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