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?

There's nothing non-functional about using the value of one Var inside the `def` form creating another Var. In fact, that's the natural thing to do if the second Var depends on the first. Of course if you want a general method of concatenating n copies of a seq, where n might or might not be known beforehand, then you do need a better solution (as found in the answers here). — Michał Marczyk, Apr 23 '10 at 18:50
Actually, that's what I meant (N copies). I agree that my two-times function is "functional" -- it is just not practical if I want 100 copies :-). — Ralph, Apr 23 '10 at 19:27

score 8 · Accepted Answer · answered Apr 23 '10 at 19:06

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)

I also came up with (take 8 (cycle suits)). Seems to work, but I had to figure the "8" part out myself, without the help of a computer :-). — Ralph, Apr 23 '10 at 20:33

Carl Smotricz · Answer 2 · 2015-09-17T19:40:14.053

2

(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.

edited Sep 17 '15 at 19:40

answered Apr 23 '10 at 18:37

Carl Smotricz

66,391
18
125
167

2

Fixed the function name (`assoc` -> `apply`). – Michał Marczyk Apr 23 '10 at 18:53
Oh, how stupid of me. I use `apply` all the time, you'd think I'd know its name by now... Thanks for the fix! – Carl Smotricz Apr 23 '10 at 19:15
`(doc replicate)` as of Clojure 1.7 says `DEPRECATED: Use 'repeat' instead.` – David J. Sep 16 '15 at 15:53

score 2 · Answer 3 · answered Apr 23 '10 at 18:46

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)

score 1 · Answer 4 · answered Apr 24 '10 at 18:02

1

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

answered Apr 24 '10 at 18:02

Vagif Verdi

4,816
1
26
31

Repeating vectors in Clojure

4 Answers4