How to let mkString skip null in scala?

Question

scala> Seq("abc", null).mkString(" ")
res0: String = abc null

but I want to get "abc" only

Is there a scala way to skip nulls?

As the old joke goes, "Doctor, doctor, it hurts when I do this." "So, don't do that." Use Option[String] as your type, and None instead of null. — Eric Jablow
– Eric Jablow, Commented Jan 28, 2014 at 1:44
I'd love to do so but these nulls come from Java code. Is this better: Seq(Option("abc"), Option(null)).flatMap(s => s).mkString(" ")? — NSF
– NSF, Commented Jan 28, 2014 at 1:55
Try it that way, then by filtering, then by changing the Java code. See which is faster while remaining correct. — Eric Jablow
– Eric Jablow, Commented Jan 28, 2014 at 1:57
@NSF, flatMap(s=>s) can be written more succinctly as flatten. — dhg
– dhg, Commented Jan 28, 2014 at 2:25

John Landahl · Accepted Answer · 2014-01-28 03:34:57Z

19

scala> val seq = Seq("abc", null, "def")
seq: Seq[String] = List(abc, null, def)

scala> seq.flatMap(Option[String]).mkString(" ")
res0: String = abc def

answered Jan 28, 2014 at 3:34

John Landahl

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1 Comment

KChaloux Over a year ago

+1 for the flatMap approach. This is what I would expect out of idiomatic Scala. It's also good to know that Option(null) reliably returns "None"

Rex Kerr · Accepted Answer · 2014-01-28 01:43:48Z

15

There's always Seq("abc", null).filter(_ != null).mkString(" ")

answered Jan 28, 2014 at 1:43

Rex Kerr

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2 Comments

Kevin Wright Over a year ago

Seq("abc", null).iterator.filter(_ != null).mkString(" ") to avoid building the intermediate collection.

KChaloux Over a year ago

@KevinWright Current versions of Scala have withFilter for that, don't they? Update: Just tried it - apparently mkString doesn't work directly with withFilter

Kristian Domagala · Accepted Answer · 2014-01-28 02:17:12Z

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Combination of Rex's answer and Eric's first comment:

Seq("abc", null).map(Option(_)).collect{case Some(x) => x}.mkString(" ")

The first map wraps the values resulting in Seq[Option[String]]. collect then essentially does a filter and map, discarding the None values and leaving only the unwrapped Some values.

answered Jan 28, 2014 at 2:17

Kristian Domagala

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1 Comment

dhg Over a year ago

Seq("abc", null).flatMap(Option(_)).mkString(" "). Flatten does what you want it to.

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