Update On Learning Clojure

I am getting back to learning Clojure. I got Carin Meir’s Living Clojure.

The last section is a ten-week learning program. When I first read that, I got excited. But then I got to that part, and I was less excited. The first part is 4Clojure problems. I have tried starting it a few times, and I keep randomly scanning the Clojure cheat sheet and just throw functions together. I don’t feel like I am learning anything.

I got stuck on problem 30. Here is the answer I tried:

(reduce (fn [r x]
    (println "r is " r ", x is " x "; last element of r is " (last r))
    (if (= (last r) x) 
        r 
        (conj r x)))
        []
        [1 1 2 3 3 2 2 3] )

(The last vector is one of the problem inputs.)

I got it to work, but I kept getting vectors, but not lists, which is what 4Clojure wanted.

I did search for some answers. There are a few github repositories with 4Clojure answers. The “provided” answer uses the “map” function. I kept trying “reduce”. “map” returns a result that is the same size as the input, while “reduce” can change the size of the collection. I thought that was the way to get the answer, since the name of the problem is “Compress a Sequence”.

I suppose I could have added something to change the result to return a list, but that just seemed way too complicated. So I caved.

2016-05-04_01.10.02 postscript: I admit I used the provided solution to problem 30. A lot of the other users whose solutions I track had more complicated solutions than the publicly available one, and they looked more like the one I was trying.

But their solutions to 31 were like the “provided” solution to 30.

2016-05-04_03.52.13 postscript: I might be using some of the answer repos a lot. I got a solution for problem 33 that works in my repl fine for all of the problems in 33. But online it chokes on the third one for some reason.

I got a tip from Norm Richards to set my solution to “__”. This allows you to run the examples right in the repl. Here is my solution that works on my laptop, but not on 4Clojure.com:

(def __ #(apply interleave (take %2 (repeat %1))))

(= (__ [1 2 3] 2) '(1 1 2 2 3 3))

(= (__ [:a :b] 4) '(:a :a :a :a :b :b :b :b))

(= (__ [4 5 6] 1) '(4 5 6))

(= (__ [[1 2] [3 4]] 2) '([1 2] [1 2] [3 4] [3 4]))

(= (__ [44 33] 2) [44 44 33 33])

(def __ #(apply interleave (take %2 (repeat %1))))

(= (__ [1 2 3] 2) '(1 1 2 2 3 3))

(= (__ [:a :b] 4) '(:a :a :a :a :b :b :b :b))

(= (__ [4 5 6] 1) '(4 5 6))

(= (__ [[1 2] [3 4]] 2) '([1 2] [1 2] [3 4] [3 4]))

(= (__ [44 33] 2) [44 44 33 33])

I seem to get a lot of solutions that will work for all but one of the problems.

You’re welcome.

Image from Gospel Book from the Bamberg Cathedral, an early 11th-century manuscript, manuscript housed at Bavarian State Library, webpage information here, image from World Document Library, image assumed allowed under Fair Use.