CODEWITHSUNDEEP
clojure
Matt Briggs
Matt Briggs

Reputation: 42248

How to repeat string n times in idiomatic clojure way?

In Ruby, "str" * 3 will give you "strstrstr". In Clojure, the closest I can think of is (map (fn [n] "str") (range 3)) Is there a more idiomatic way of doing it?

Upvotes: 43

Views: 13007

Answers (7)

dalanicolai
dalanicolai

Reputation: 347

Just use java interop:

(.repeat "str" 3)

Upvotes: 0

Matias Bjarland
Matias Bjarland

Reputation: 4482

I wouldn't claim this is idiomatic, but it is also possible to repeat a string using the clojure cl-format function which clojure inherited from common lisp. Common lisp in turn transplanted it from FORTRAN which came up with this piece of work in the 50s.

And here we are in 2018...

Example: 

user=> (cl-format nil "~v@{~a~:*~}" 5 "Bob")
"BobBobBobBobBob"

The format string works as follows:

  • the first format specifier is ~5@{ where the five is pulled from the argument list since there is a v in front of the @. the ~5@{ in turn means iterate 5 times using the entire argument list (the string "Bob") as the iterable.
  • inside the curlies (the curly expression is ended by the ~}), we print the string "Bob" with ~a and then "move the argument pointer" back one position using the ~:* construct so that we can "consume" the argument "Bob" again.

Voila, 5 repetitions of Bob.

It should be noted that cl-format can either return the produced string (if the second argument is nil), print it to the current *out* (if the second argument is true), or print it to a writer (if the second argument is a writer).

For the all too gory details on the format string syntax you can refer to:

Formatted Output to Character Streams

in the common lisp the language reference and possibly to:

Eliminating format from lisp

for why it could be argued that cl-format is a bad idea.

Upvotes: 3

Jacek Laskowski
Jacek Laskowski

Reputation: 74779

Just to throw some more awesome and hopefully thought-provoking solutions.

user=> (clojure.string/join (repeat 3 "str"))
"strstrstr"

user=> (format "%1$s%1$s%1$s" "str")
"strstrstr"

user=> (reduce str (take 3 (cycle ["str"])))
"strstrstr"

user=> (reduce str (repeat 3 "str"))
"strstrstr"

user=> (reduce #(.concat %1 %2) (repeat 3 "str"))
"strstrstr"

Upvotes: 15

Maurits Rijk
Maurits Rijk

Reputation: 9985

And one more fun alternative using protocols:

(defprotocol Multiply (* [this n]))

Next the String class is extended:

(extend String Multiply {:* (fn [this n] (apply str (repeat n this)))})

So you can now 'conveniently' use:

(* "foo" 3)

Upvotes: 41

colinf
colinf

Reputation: 1866

You could also use the repeat function from clojure.contrib.string. If you add this to your namespace using require such as

(ns myns.core (:require [clojure.contrib.string :as str]))

then

(str/repeat 3 "hello")

will give you

"hellohellohello"

Upvotes: 10

Maurits Rijk
Maurits Rijk

Reputation: 9985

Or use the repeat function that comes with clojure-contrib' string package. In that case you can use (clojure.contrib.string/repeat 3 "str") which results in "strstrstr".

Upvotes: 4

Joost Diepenmaat
Joost Diepenmaat

Reputation: 17761

How about this?

(apply str (repeat 3 "str"))

Or just

(repeat 3 "str")

if you want a sequence instead of a string.

Upvotes: 73

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