Map function in Scala

About map

The scala map function converts one collection A to another B by applying a function to every element in A . As per the Scala documentation, the definition of the map method is as follows:

def map[B](f: (A) ⇒ B): Traversable[B]

Important points about map method:

• every collection object has the map() method;
• map() takes some function as a parameter;
• map() applies the function to every element of the source collection;
• map() returns a new collection of the same type as the source collection, each element of which is the result of applying the input function to the corresponding element of the input array.

Simply put, you can call the map function on your collection, pass it a function, or an anonymous function, and transform each element of the collection.

Basic examples

Let's create a two lists, one list of integers and one of strings:

val nums = List(1, 2, 3)

If we call map function like this:

val result = nums.map((i: Int) => i * 2)
println(s"Resulting list of numbers = $result")

Output would be: Resulting list of numbers = List(2, 4, 6)

We see that this operation returns another list with the doubled elements.

Let's create another list, this time list of strings

val str = List("one", "two", "three")

We can call scala built in functions, something like this:

println(s"Original list: $str")
val caps = str.map(_.capitalize)
println(s"Capital first letter: $caps")
val lengths = str.map(_.length)
println(s"Lengths of strings: $lengths")

Output of this code is:

Original list: List(one, two, three)
Capital first letter: List(One, Two, Three)
Lengths of strings: List(3, 3, 5)

P-rocket-chip examples

Lets look some examples of map method used in P-rocket-chip:

• in the first example map function was called on indexed sequence id_raddr.This map method apply function rf_read on every element of that indexed sequence(reading from every address from sequence) and we get variable id_rs consisting of elements which are second value from read tuples.

• in the second example map function was used to iterate through all elements of vector inst, which is IBuf port, and get every parameter-inst of class Instruction through decoupled interface(_.bits.inst), thus generating vector of instuctions id_expanded_inst.

• in the third example map function was called on indexed sequence-bypass_sources of tuples with three elements. On this way we would get indexed sequence bypass_mux consisting of third elements of tuples.

Additional things

Notice that .capitalize (returns a new string with the first letter only converted to upper case) and .length (which gives the length of the string in characters), is scala string applicable methods. Also good to mention is that _ is used as placeholder syntax which means apply method on every element of list.

We need to distinguish between map() and Map(which is a collection of key/value pairs).