Question

Suppose there are 3 strings:

protein, starch, drink

Concatenating those, we could say what is for dinner:

Example:

val protein = "fish"
val starch = "chips"
val drink = "wine"

val dinner = protein + ", " + starch + ", " + drink

But what if something was missing, for example the protein, because my wife couldn’t catch anything. Then, we will have: ,chips, drink for dinner.

There is a slick way to concatenate the strings to optionally add the commas – I just don’t know what it is 😉. Does anyone have a nice idea?

I’m looking for something like:

val dinner = protein +[add a comma if protein is not lenth of zero] + starch .....

It’s just a fun exercise I’m doing, so now sweat if it can’t be done in some cool way. The reason that I’m trying to do the conditional concatenation in a single assignment, is because I’m using this type of thing a lot in XML and a nice solution will make things….. nicer.

Answer 1

When you say “it may be absent”, this entity’s type should be Option[T]. Then,

def dinner(components: List[Option[String]]) = components.flatten mkString ", "

You would invoke it like this:

scala> dinner(None :: Some("chips") :: Some("wine") :: Nil)
res0: String = chips, wine

In case you absolutely want checking a string’s emptiness,

def dinner(strings: List[String]) = strings filter {_.nonEmpty} mkString ", "

scala> dinner("" :: "chips" :: "wine" :: Nil)
res1: String = chips, wine