Implement reversing a sequence in a way that is idiomatic for Clojure

Question

The task is to write a function that reverses a sequence. I am trying to do this as efficiently and clojure-ish as possible, but I can't figure out the best way. It doesn't feel right to create a variable and start appending items to that variable. What would seem idiomatic to me was if there was a way to do the following:

From the Clojure docs

(map-indexed (fn [idx itm] [idx itm]) "foobar")
--> ([0 \f] [1 \o] [2 \o] [3 \b] [4 \a] [5 \r])

What I have done is this:

(defn testing [x]
(map-indexed (fn [idx itm] [(- (count x) idx) itm]) x))

Which yields:

(testing "foobar")
--> ([6 \f] [5 \o] [4 \o] [3 \b] [2 \a] [1 \r])

What I can't figure out is how to essentially pack that back up into a new list with the correct indexes. Honestly, even the concept of declaring a variable is escaping me in Clojure at the moment. All my experience is with Python and Ruby.

You can find the source code of reverse at clojuredocs.org/clojure_core/clojure.core/reverse#source. — Thumbnail
– Thumbnail, Commented Apr 14, 2014 at 20:29
Can I suggest that you re-title the question "Implement reversing a sequence in a way that is idiomatic for Clojure". The idiomatic way to return a sequence in general is usually lazily, but that isn't the case for this problem. — Thumbnail
– Thumbnail, Commented Apr 15, 2014 at 12:36

Kyle · Accepted Answer · 2014-04-14 20:08:12Z

4

Strings are sequable so we can use clojure.core/reduce

(defn rev [x]
  (reduce #(str %2 %1) x))

(rev "foobar") ;; "raboof"

The idea is that for each iteration, we create a new string that is the next character in the sequence prepended to the current string.

All my experience is with Python and Ruby

Here is the same in Ruby:

def rev(x)
  x.chars.reduce { |s, c| c + s }
end

For non-strings

(defn rev [x]
  (reduce conj () x))

(rev [1 2 3 4 5]) ;; (5 4 3 2 1)

edited Apr 14, 2014 at 20:08

answered Apr 14, 2014 at 19:32

Kyle

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4 Comments

Borromakot Over a year ago

Excellent. Thanks much! I knew this was one of those things that would be simple even though I was over here banging my head against the wall.

Borromakot Over a year ago

Actually @Kyle , for some reason I'm having an equal amount of trouble making this work with a seq that isn't a string. Any thoughts?

Kyle Over a year ago

@Borromakot I added a solution for non-strings. If you were to use this with a string you may want to (apply str (rev "foobar")) to get a string as output.

Borromakot Over a year ago

Thanks! Great stuff. I was looking in the entirely wrong place. I had assumed that I was just trying to connect the two, using concat or conj or something along those lines. Didn't even know about cons. Thanks again!

Thumbnail · Accepted Answer · 2014-04-15 08:44:08Z

1

You can reverse a sequence using indexes, as your question proposes, as follows:

(defn reverse [s]
  (let [v (vec s)] (map v (range (dec (count v)) -1 -1))))

... or, perversely,

(def reverse (comp rseq vec))

edited Apr 15, 2014 at 8:44

answered Apr 14, 2014 at 20:43

Thumbnail

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