Scala: higher order function to concatenate Strings

Question

I'm starting with Scala and now I am talking about higher order functions but I am having a hard time dealing with this way of programming using functions as inputs.

I must code a higher order function using only fold, scan and/or reduce that concatenates a string like so:

concatenate(List("S", "T", "R", " example!") , f) //> res1: List[String] = List(STR example!, TR example!, R example!, " example!", "")

Does anyone know how can I approach this problem?

akuiper · Accepted Answer · 2017-04-14 21:26:50Z

Use scan only:

List("S", "T", "R", " example!").reverse.scan("")((x, y) => y + x).reverse // res72: List[String] = List(STR example!, TR example!, R example!, " example!", "")

Use scanRight:

List("S", "T", "R", " example!").scanRight("")(_+_) // res73: List[String] = List(STR example!, TR example!, R example!, " example!", "")

Thank you very much! Can you just explain to me what does this do: "(x, y) => y + x" ?
It's used to guarantee the paste order of left + right, notice the list is reversed initially, if using x + y, then you will end up with example! RTS instead of the order you wanted.

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