For example, let's say I have a list
'("The" " " "Brown" " " "Cow")
I want to turn this into
"The Brown Cow"
Is there a command in clojure that does this?
3 Answers
I would rather use apply for that:
(apply str '("The" " " "Brown" " " "Cow"))
Since it calls the function just once, it is much more efficient for large collections:
(defn join-reduce [strings] (reduce str strings))
(defn join-apply [strings] (apply str strings))
user> (time (do (join-reduce (repeat 50000 "hello"))
nil))
"Elapsed time: 4708.637673 msecs"
nil
user> (time (do (join-apply (repeat 50000 "hello"))
nil))
"Elapsed time: 2.281443 msecs"
nil
1 Comment
Leon Grapenthin
To be more specific, it does perform better because using the variadic overload of
str requires only one StringBuilder instance.As Chris mentioned you can just use clojure.string/join
another way without using a library (assuming you don't want any spaces.) is:
(reduce str '("The" " " "Brown" " " "Cow"))
will return
"The Brown Cow"
str takes a list of things and turns them into one string. You can even do this: (str "this is a message with numbers " 2 " inside")
4 Comments
leetwinski
isn't
(apply str coll) better? since it calls the str function just once, while reduce does it (dec (count coll)) times.
amalloy
@leetwinski Yes. There's never a reason to call
reduce str on a sequence.
JustGage
Didn't know that, sorry guys! I'm pretty new to Clojure.
Sam Estep
@JustGage Don't worry too much about it; this is just another way to learn new things. I'm guessing that the main reason your answer has been downvoted so much is that it happened to be accepted by the OP.
Please, people, just say 'No' to the lists! So simple with vectors:
(ns clj.demo
(:require [clojure.string :as str] ))
(def xx [ "The" " " "Brown" " " "Cow" ] )
(prn (str/join xx))
;=> "The Brown Cow"
Quoted lists like:
'( "The" " " "Brown" " " "Cow" )
are much harder to type and read, and also more error-prone than a simple vector literal:
[ "The" " " "Brown" " " "Cow" ]
Also cut/paste is much easier, as you don't need to worry about adding the quote or forgetting to add it.
Note that (str/join ... can also accept an optional separator as the first argument. You can either use the 1-char string like the first example or a "literal" as in the 2nd example:
(ns clj.demo
(:require
[clojure.string :as str]
[tupelo.core :as t] ))
(let [words ["The" "Brown" "Cow"]]
(t/spyx (str/join " " words))
(t/spyx (str/join \space words)))
with results
(str/join " " words) => "The Brown Cow"
(str/join \space words) => "The Brown Cow"
2 Comments
leetwinski
vectors? as far as i know,
clojure.string/join works for any seq. What did you want to say? that using vector is faster here? No, it's not. In fact join is even a bit slower than apply str in this particular case.Leon Grapenthin
He wants to say that the idiomatic default literal for sequentials in Clojure source code is the vector. This readability over performance tradeoff is made in every notable codebase.
str/join is a fine solution.
[ "The" " " "Brown" " " "Cow" ]