CODEWITHSUNDEEP
listscalaconcatenation
John Redyns
John Redyns

Reputation: 5903

String concatenation from within a single list

Scala is new to me so I'm not sure the best way to go about this.

I need to simply take the strings within a single list and join them. So, concat(List("a","b","c")) returns abc.

Should I first see how many strings there are in the list, that way I can just loop through and join them all? I feel like that needs to be done first, that way you can use the lists just like an array and do list[1] append list[2] append list[3], etc..

Edit:

Here's my idea, of course with compile errors..

def concat(l: List[String]): String = {
var len = l.length
var i = 0
    while (i < len) {
        val result = result :: l(i) + " "
    }
result
}

Upvotes: 15

Views: 27684

Answers (6)

Grigory Kislin
Grigory Kislin

Reputation: 17980

A bit longer that mkString but more efficient:

s.foldLeft(new StringBuilder())(_ append _).toString()

Upvotes: 0

user178841
user178841

Reputation:

How about this, on REPL

List("a","b","c") mkString("")

or in script file

List("a","b","c").mkString("")

Upvotes: 37

maasg
maasg

Reputation: 37435

Some options to explore for you:

  1. imperative: for-loop; use methods from the List object to determine loop length or use for-each List item
  2. classical functional: recursive function, one element at the time using
  3. higher-order functions: look at fold.

Given the basic level of the problem, I think you're looking at learning some fundamentals in programming. If the language of choice is Scala, probably the focus is on functional programming, so I'd put effort on solving #2, then solve #1. #3 for extra credits.

Upvotes: 2

agilesteel
agilesteel

Reputation: 16859

I'm just assuming here that you are not only new to Scala, but also new to programming in general. I'm not saying SO is not made for newbies, but I'm sure there are many other places, which are better suited for your needs. For example books...

I'm also assuming that your problem doesn't have to be solved in a functional, imperative or some other way. It just has to be solved as a homework assignment.

So here are the list of things you should consider / ask yourself:

  • If you want to concat all elements of the list do you really need to know how many there are?
  • If you think you do, fine, but after having solved this problem using this approach try to fiddle around with your solution a little bit to find out if there is another way.
  • Appending the elements to a resulting list is a thought in right direction, but think about this: in addition to being object-oriented Scala is also a full-blown functional language. You might not know what this means, but all you need to know for now is this: it is pretty darn good with things like lists (LISP is the most known functional language and it stands for LISt Processing, which has to be an indication of some kind, don't you think? ;)). So maybe there is some magical (maybe even Scala idiomatic) way to accomplish such a concatination without defining the resulting list yourself.

Upvotes: -1

Landei
Landei

Reputation: 54574

In functional programming, fold ... is a family of higher-order functions that iterate an arbitrary function over a data structure in some order and build up a return value.

http://en.wikipedia.org/wiki/Fold_%28higher-order_function%29

That sounds like something you could use.

As string concatenation is associative (to be exact, it forms a monoid having the empty String as neutral element), the "direction" of the fold doesn't matter (at least if you're not bothered by performance).

Speaking of performance: In real life, it would be a good idea to use a StringBuilder for the intermediate steps, but it's up to you if you want to use it.

Upvotes: 0

asthasr
asthasr

Reputation: 9397

This exercise is designed to encourage you to think about the problem from a functional perspective. You have a set of data over which you wish to move, performing a set of identical operations. You've already identified the imperative, looping construct (for). Simple enough. Now, how would you build that into a functional construct, not relying on "stateful" looping?

Upvotes: 1

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