How to do a for-loop in Clojure?

Question

I'm learning myself some Clojure and I'm using Quil. I would like to know how to translate a for-loop into Clojure:

This is how I would do it in Java or similar languages:

for ( int i = 0; i < numSides; i++ ) { float posX = cos( theta * i ); float posY = sin( theta * i ); ellipse( posX, posY, polySize, polySize ); }

My Clojure attempt:

 (let [theta (/ PI num-sides) angle (range 0 num-sides) pos-x (cos (* theta angle)) pos-y (sin (* theta angle))] (dorun (map #(ellipse % % % %) pos-x pos-y poly-size poly-size)))

found similar question here stackoverflow.com/questions/9981943/… — Asraful
– Asraful, Commented Mar 10, 2013 at 9:48

Ankur · Accepted Answer · 2013-03-10 10:06:45Z

All the ways that you have looked for are basically to work with sequences where as a loop is about executing things for a specific number of times. Clojure provide dotimes to do things for certain number of times:

(dotimes [i 10] (println i))

So your code becomes something like:

 (dotimes [i num-sides] (let [pos-x (cos (* theta i)) pos-y (sin (* theta i))] (ellipse pos-x pos-y poly-size poly-size)))

mikera · Accepted Answer · 2013-03-10 10:26:50Z

If you genuinely want an C-style for loop, then my clojure-utils libray has a handy for-loop macro that lets you do stuff like:

(for-loop [i 0 , (< i num-sides) , (inc i)] ... do stuff.....)

Normally however, I will find myself using one of the following:

(dotimes [i num-sides] ....) - do something a specific number of times
(doseq [x some-sequence] ....) - do something for every element in a sequence
(for [i (range n)] ...) - constructing a list with n elements

How about making the macro more C-ish, so that we can do: (for-loop [ [i 1 j 1] (< i 10) [i (inc i) j (* j i)] ] .....)
@Ankur interesting idea.... haven't needed that myself but can see the value for some use cases.

mudphone · Accepted Answer · 2013-05-25 07:10:41Z

2

Perhaps this is somewhat academic, but I like using Clojure's "for comprehensions" for this kind of thing. The code would look like this:

(dorun (for [i (range num-sides) :let [pos-x (Math/cos (* i theta)) pos-y (Math/sin (* i theta))]] (quil.core/ellipse pos-x pos-y poly-size poly-size)))

edited May 25, 2013 at 7:10

answered May 24, 2013 at 20:36

mudphone

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1 Comment

Mars Over a year ago

But for is constructing a lazy sequence, unnecessarily, in this case. If you don't need the output sequences, as in this case, you can do the same thing--with the same binding tricks--using doseq. Simply replace for by doseq in the above code. In many situations, the cost of producing the additional sequence is negligible, so that the only advantage of doseq is that it communicates to readers that you're not trying to create a sequence. In some situations the cost of creating an unnecessary sequence can make a difference in performance, however.

Dave Liepmann · Accepted Answer · 2014-08-23 07:49:23Z

Doseq with range is often appropriate for looping over a specific number of values in order to create side effects. I would implement your loop as follows:

(doseq [i (range 0 num-sides)] (ellipse (cos (* theta i)) (sin (* theta i)) poly-size poly-size))

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