Repeating vectors in Clojure

Question

I am a Clojure newbie. I am trying to get two copies of a vector of card suits. The non-DRY way that I can come up with is

(def suits [:clubs :diamonds :hearts :spades])
(def two-times (concat suits suits))

There must be a more functional way (even if it takes more characters :-)). What if i want N times? Any suggestions?

All of the things I try, like

(replicate 2 suits)

results in two separate vectors:

([:clubs :diamonds :hearts :spades] [:clubs :diamonds :hearts :spades])

How do I "flatten" the structure?

Answer 1

concat gives you a lazy seq. If you want to end up with a (non-lazy) vector instead:

user> (into suits suits)
[:clubs :diamonds :hearts :spades :clubs :diamonds :hearts :spades]
user> (reduce into (replicate 2 suits))
[:clubs :diamonds :hearts :spades :clubs :diamonds :hearts :spades]

Depending whether you're accessing this by index a lot or iterating over it, either a vector or a seq might be more appropriate.

There's always cycle too, if you want an endless (lazy) stream of repeated elements:

user> (take 9 (cycle suits))
(:clubs :diamonds :hearts :spades :clubs :diamonds :hearts :spades :clubs)

Answer 2

A little experimentation with the REPL lead me to this solution:

user=> (def suits [:clubs :diamonds :hearts :spades])
#'user/suits
user=> suits
[:clubs :diamonds :hearts :spades]    
user=> (reduce concat (replicate 2 suits))
(:clubs :diamonds :hearts :spades :clubs :diamonds :hearts :spades)

Answer 3

(untested!)

(apply concat (repeat 2 suits))

will hopefully do the trick.

concat will of course concatenate 2 lists; apply can be used to smuggle a given function into the head position of an existing list for evaluation.

Answer 4

(take (* 2 (count suits)) (cycle suits))