Why does :+ appending to a Seq have no effect?

Question

i wanna have a result-Sequence with a Triple of (String, Int, Int) like this:

var all_info: Seq[(String, Int, Int)] = null

now that i try adding elements to my Seq as followed:

  if (all_info == null) {
    all_info = Seq((name, id, count))
  } else {
    all_info :+ (name, id, count)
  }

and print them

    Console.println(all_info.mkString)

Unfortunately, the printed result is only the first triple that is added by the if-clause and basically intializes a new Seq, since it's been just "null" before. All following triples which are supposed to be added to the Seq in the else-clause are not. I also tried different methods like "++" which won't work either ("too many arguments")

Can't really figure out what I'm doing wrong here.

Thanks for any help in advance! Greetings.

Answer 1

First of all instead of using nulls you would be better using an empty collection. Next use :+= so the result of :+ would not be thrown away — :+ produces a new collection as the result instead of modifying the existing one. The final code would look like

var all_info: Seq[(String, Int, Int)] = Seq.empty
all_info :+= (name, id, count)

As you can see, now you don't need ifs and code should work fine.

Answer 2

Method :+ creates new collection and leaves your original collection untouched.

You should use method +=. If there is no method += in all_info compiler will treat all_info += (name, id, count) as all_info = all_info + (name, id, count).

On the contrary, if you'll change the type of all_info to some mutable collection, you'll get method += in it, so your code will work as expected: method += on mutable collections changes target collection.

Note that there is no method :+= in mutable collections, so you'll get all_info = all_info :+ (name, id, count) even for mutable collection.