I am trying to do it as follows:
def contains(x: Int, l: List[Int]) = l match { // this is just l.contains(x)
case _ :: x :: _ => true
case _ => false
}
Unfortunately it does not work
scala> contains(0, List(1, 2, 3))
res21: Boolean = true
scala> contains(1, List(1, 2, 3))
res22: Boolean = true
scala> contains(3, List(1, 2, 3))
res23: Boolean = true
Could you please explain why ?
To match a number equal to x you can put it into backticks:
def contains(x: Int, l: List[Int]) = l match {
case _ :: `x` :: _ => true
case _ => false
}
Unfortunately :: matcher takes exactly one item from list – the first one, so this code would work only to find the second item in l:
scala> contains(1, List(1,2,3))
res2: Boolean = false
scala> contains(2, List(1,2,3))
res3: Boolean = true
scala> contains(3, List(1,2,3))
res4: Boolean = false
I believe, you can't match for an arbitrary item in a list without recursion:
def contains(x: Int, l: List[Int]): Boolean = l match { // this is just l.contains(x)
case `x` :: xs => true
case _ :: xs => contains(x, xs)
case _ => false
}
:: takes exactly one item :(The first case matches any item in a non empty list, note,
scala> contains(123, List(1, 2, 3))
res1: Boolean = true
scala> contains(123, List())
res2: Boolean = false
A recursive method that matches against the head item of the list may work.
First, x in case section is alias for local variable. It's not x you passed to method.
Second, _ :: x :: _ matches any list with two elements and more. So all your outputs is true.
This might work,
def contains(y: Int, l: List[Int]) = l match { // this is just l.contains(x)
case _ :: x :: _ if(x == y)=> true
case _ => false
}
Your approach does not work, because the x in the pattern match is bound to whatever value the second list element has. It's basically a fresh variable.
Alternative to S.K's answer
def contains(y: Int, l: List[Int]) = l match { // this is just l.contains(x)
case _ :: x :: _ => x == y
case _ => false
}
or you can also write
def contains[A](y: A, l: Seq[Int]) = (l.lift)(1).exists(_ == y)
