Decoding the below Scala code

Question

While reading a book on Scala i stumbled upon the following code. Am unable to segregate the code into the function, parameters, variables.

 val feeds = Map("Andy Hunt"   -> "blog.toolshed.com",  
                "Dave Thomas" -> "pragdave.me",     
                "NFJS"        -> "nofluffjuststuff.com/blog")

val fiterName = feeds filter { element =>   
  val (key, value) = element    
  (key startsWith "D") && (value contains "pragprog")   
}

Can some one explain me the code step by step?

Yuval Itzchakov · Accepted Answer · 2017-01-25 12:41:35Z

6

feeds is a Map[String, String] which maps a person to a blog.

Then that map is iterates with a filter, which attempts to filter any author which starts with the capital letter D and his value contains the word "pragprog".

When you filter on a Map, you get a tuple which has the key as the first element and the value as the second element. Using round brackets, it looks like this:

val filterName = feeds.filter(element => {
   val (key, value) = element    
   key.startsWith("D") && value.contains("pragprog")
})

Note that filter by itself is a higher order function, it takes another function as input. This function takes an argument of type A, which in our case is a tuple (String, String), and produces a boolean which indicates if the element matches the condition or not. Inside the filter, they used tuple deconstruction:

val (key, value) = element

Which takes a Tuple2[String, String] and calls it's unapply method, allowing you to access the components of the tuple by name, and not via ._1 and _.2

edited Jan 25, 2017 at 12:41

answered Jan 25, 2017 at 12:33

Yuval Itzchakov

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4 Comments

Gurupraveen Over a year ago

Well i understood that looking at the code but what am wondering is? As per the spec filter takes a predicate (String,String)=>Boolean. With that in mind what does val (key,value) = element mean? then on what value the boolean expression (key startsWith "D") && (value contains "pragprog") is invoked?

Yuval Itzchakov Over a year ago

@Gurupraveen Updated the answer to reflect your question.

Gurupraveen Over a year ago

One more question, what if i don't want to use tuple deconstruction? how would i achieve same results?

Yuval Itzchakov Over a year ago

@Gurupraveen element._1.startsWith("D") && element._2.contains("pragprog")

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