What is the difference between map{} and map() [duplicate] - scala

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What is the formal difference in Scala between braces and parentheses, and when should they be used?
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Closed 6 years ago.
From this link showing some Scala examples: http://spark.apache.org/docs/latest/mllib-collaborative-filtering.html
what is the difference between map{} and map()?
val ratings = data.map(_.split(',') match { case Array(user, item, rate) =>
Rating(user.toInt, item.toInt, rate.toDouble)
})
val usersProducts = ratings.map { case Rating(user, product, rate) =>
(user, product)
}

Answer 1

map is a method which accepts a function as an argument. So map customarily is called just like methods get called: map(aFunction). However, Scala provides a lot of flexibility / shorthands in its syntax:
val list = List((1,2),(3,4))
//An ordinary method call. The braces just evaluate to an anonymous function
val a = list.map({ case (a, b) => a + b }) // List(3,7)
// Now lets define a non anonymous function of the same type as above:
def func(x: (Int,Int)) = x match {case (a,b) => a + b}
// Scala allows this readable "infix notation" for single parameter methods
val a = list map func // List(3,7)
//Now to the main point you asked, this syntax uses the same readable form as above
//but uses an anonymous function.
val a = list map {case (a,b) => a + b} // List(3,7)

Answer 2

val a = list.map(_.split(','))
// the above is a shorthand for
val a = list.map({ case i => i.split(',') })
// Now lets consider this
val a = list.map { case (a, b) => a + b }
// or this
val a = list map { case (a, b) => a + b }
// some people like the above ways of writing
// because they consider of less brackets as cleaner code.
// but the above two are another way to write
val a = list.map({ case (a, b) => a + b })
The thing to understand here is that in Scala you can use spaces instead of . to access instance methods.
So basically,
// lets say you have a class A
case class A(i: Int) {
def merged[B](b: B): String = i.toString + " :: " + b.toString
}
//Now, lets say you have an instance of A
val a = A(5)
// and an instance of String
val s: String = "abcd"
// Now, if you write
val merged = a merged s
// it is syntactic sugar for
val merged = a.merged(s)
Similarly, List[A] has a method map[B](f: A => B): List[B].
val list = List[Int](1, 2, 3)
// so when you write
val list2 = list map { case i => i + 1}
// its syntactic sugar for,
val list2 = list.map({ case i => i + 1 })
// There is a lot going on here
// if we were to do everything step by step
// first you are creating a partial function
val pf: PartialFunction[Int, Int] = { case i => i + 1 }
// Now, PartialFunction[Int, Int] is a subtype of Int => Int
// so we can refer to it as a Int => Int
val f: Int => Int = pf
// Now we pass it as an argument to map
val list2 = list.map(f)