How to skip appending some condition in map scala? - scala

Question

I have a situation in which I need to create a map from a collection by applying some filter inside like given in the code below:
//Say I have a list
//I don't have to apply filter function ...
val myList = List(2,3,4,5)
val evenList = myList.map(x=>{
if ( x is even) x
else 0
}
//And the output is : List(2,0,4,0)
//The output actually needed was List(2,4) without applying filter on top like - ```myList.filter```
//I have objects instead of numbers of a case class so the output becomes :List(object1, None, object2, None)
But actual output needed was : List(object1,object2)
//The updated scenario
val basket = List(2,4,5,6)
case class Apple(name:Option[String],size:Option[Int])
val listApples: List[Apple] = basket.map(x=>{
val r = new scala.util.Random
val size = r.nextInt(10)
if(x%2!=0){
Apple(None,None)
}
else Apple(Some("my-apple"),Some(size))
})
Current Output :
Apple(Some(my-apple),Some(2))
Apple(Some(my-apple),Some(0))
Apple(None,None)
Apple(Some(my-apple),Some(4))
Expected was :
Apple(Some(my-apple),Some(2))
Apple(Some(my-apple),Some(0))
Apple(Some(my-apple),Some(4))

Answer 1

I believe collect best suits your case. It takes a partial function as an argument and only if that function matches then the element is transformed and added to result:
val myList = List(2,3,4,5)
case class Wrapper(i: Int)
val evenList = myList.collect{
case x if x % 2 == 0 => Wrapper(x)
}
In this case only 2 and 4 will be wrapped inside Wrapper:
List(Wrapper(2), Wrapper(4))

Answer 2

I'm not sure if I understand you correctly, but why not just use a filter directly:
val myList = List(2,3,4,5)
myList.filter(_ % 2 == 0)
If you want to have the Filter as a function:
def even(n:Int) = n % 2 == 0
myList.filter(even)
After question update, here the difference between filter and collect:
Filter:
myList
.filter(even)
.map(s => Apple(Some("my-apple"),Some(s)))
Collect:
myList
.collect{ case s if(even(s)) => Apple(Some("my-apple"),Some(s))}
Both return List(Apple(Some(my-apple),Some(2)), Apple(Some(my-apple),Some(4)))
So the only difference is that you can do both steps at once with collect.
However for me to separate these 2 steps is mostly more readable.