CODEWITHSUNDEEP
scalalistscala-collections
Monika
Monika

Reputation: 143

List concatenation not working in scala

I am trying to concatenate scala list in loop using below code.

var names: List[String] = Nil
val cluster_id = List("149095311_0", "149095311_1")
for (id <- cluster_id) {
  val influencers_name = searchIndex(s"id : $id", "id", "influencers", searcher)
  println("In Loop " + influencers_name)
  names :::= influencers_name
}
for(n <- names) println("List element -> " + n)

But when I iterate over final list it give me individual list's instead of individual elements of concatenated List.

Below is the O/P of above code:

In Loop List(kroger 10TV DispatchAlerts)
In Loop List(kroger seanhannity SenTedCruz)
List element -> kroger seanhannity SenTedCruz 
List element -> kroger 10TV DispatchAlerts

Upvotes: 2

Views: 732

Answers (3)

sheunis
sheunis

Reputation: 1544

Your code isn't very functional in that you are mutating variables. The following is more elegant:

def searchIndex(s: String): List[String] = {
  if (s == "149095311_0") List("kroger 10TV DispatchAlerts")
  else List("kroger seanhannity SenTedCruz")
}

val cluster_id = List("149095311_0", "149095311_1")

val names = cluster_id.foldLeft(List[String]()) {
  (acc, id) => acc ++ searchIndex(id)
}

for(n <- names) println("List element -> " + n)

Where '++' is used to concatenate the elements of two lists.

Upvotes: 1

Mikel San Vicente
Mikel San Vicente

Reputation: 3863

It looks like the problem is in searchIndex method that is retreiving a List[String] with a single String that contain all the values separated by a space, fix that method to make sure that it retrieves a List with one elemente per value.

To check if this is right try this, this is just a workaround, you should fix searchIndex

var names: List[String] = Nil
val cluster_id = List("149095311_0", "149095311_1")
for (id <- cluster_id) {
  val influencers_name = searchIndex(s"id : $id", "id", "influencers", searcher).flatMap(_.split(' '))
  ("In Loop " + influencers_name)
  names = influencers_name ::: names
}
for(n <- names) println("List element -> " + n)

Upvotes: 0

Mahesh Chand
Mahesh Chand

Reputation: 3250

The reason is, When you do names ::: List("anything") -> it does not add anything to names. Instead, it creates a new collection. For example,

scala> var names: List[String] = Nil
names: List[String] = List()

scala> names ::: List("mahesh")
res0: List[String] = List(mahesh)

You can achive that

scala> names ::: List("chand")
res1: List[String] = List(chand)

scala> res0 ::: List("chand")
res2: List[String] = List(mahesh, chand)

When I added "Mahesh" to it, it has created new collection naming res0. When I added again different string, here "chand" it has created another collection. But when I added "chand" to the created collection, it has concatenated to correct collection,

You can achive What you want to do,

scala> for(i <- List("a" ,"b" )){
     | names = i :: names } 

scala> names
res11: List[String] = List(b, a)

Upvotes: 0

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