;; List Comprehension vs Map and Filter ;; The consequences of map taking many possible arguments were not clear to me until I realized that ;; it's 'side by side' iteration. ;; I asked this question: ;; http://stackoverflow.com/questions/14737391/howdoicombinetwovectorsofvectorselementwiseinclojure ;; on Stack Overflow, and the answer made me feel like a complete idiot. ;;  ;; I can write: (for [i [1 2 3]] (* 2 i)) ; (2 4 6) ;; Or I can write (map (fn[i] (* 2 i)) [1 2 3]) ; (2 4 6) ;; And which is easier to read is a bit of a matter of taste ;; I can write (for [i [1 2 3]] (for [j [4 5 6]] (* i j))) ; ((4 5 6) (8 10 12) (12 15 18)) ;; Or I can write (map (fn[j] (map (fn[i] (* i j)) [4 5 6])) [1 2 3]) ; ((4 5 6) (8 10 12) (12 15 18)) ;; And I think most people will find the first one easier to read. Certainly I found it easier to write. ;; I can write (for [i [1 2 3], j [4 5 6]] (* i j)) ; (4 5 6 8 10 12 12 15 18) ;; Or I can write (mapcat (fn[j] (map (fn[i] (* i j)) [4 5 6])) [1 2 3]) ; (4 5 6 8 10 12 12 15 18) ;; And so I had a sort of feeling that list comprehension and map/filter were equivalent but that ;; list comprehensions were somewhat easier to read and write. ;; This certainly seems to be the case in python, where there have been occasional attempts to take ;; map, filter and reduce out of the language, in favour of list comprehensions ;; But occasionally, I'd like to iterate over two structures at once. ;; So [1 2 3], [4 5 6], * > [4 10 18], say ;; And I am usually writing some horror like: (for [[i,j] (partition 2 (interleave [1 2 3] [4 5 6]))] (* i j)) ; (4 10 18) ;; But in fact: (map * [1 2 3] [4 5 6]) ; (4 10 18) ;; In which case I am thinking that the version with map is way clearer, and I am also thinking that ;; perhaps list comprehensions cannot do everything that map and filter can. ;; Or maybe I just don't know the syntax for side by side iteration. Or even the proper name. The ;; earlier combination is called 'cartesian product', and I am tempted to call the latter one ;; 'elementwise combination'. ;; Can anyone enlighten me on this subject? ;; Here's a nice application of multiargument map, closely related to a real problem I'm trying to solve: (def a '[[c c c] [y y y] [m m m]]) (def b '[[r g b] [r g b] [r g b]]) (def c '[[x y z] [y z x] [z x y]]) (mapv (partial mapv vector) a b c) ; [[[c r x] [c g y] [c b z]] ; [[y r y] [y g z] [y b x]] ; [[m r z] [m g x] [m b y]]] ;; Thank you to Alex Stoddard for that, and indeed a certain amount of associated enlightenment!
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Thursday, February 7, 2013
FOR vs MAP
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If you think the following Python cases:
ReplyDeleteIn [1]: [i*j for (i, j) in zip([1,2,3], [4,5,6])]
Out[1]: [4, 10, 18]
In [2]: [i*j for i in [1,2,3] for j in [4,5,6]]
Out[2]: [4, 5, 6, 8, 10, 12, 12, 15, 18]
The real trick is zip() in Python. So you can easily implement the Clojure version of (zip) like mentioned here: http://stackoverflow.com/questions/3382688/zippingcollections
How we can generalize the last clever example (map (partial map function) a b c) to arbitrary dimension vector ?
ReplyDeleteYou can simplify usage of map
ReplyDelete(map #(* 2 %) [1 2 3])
however in nested map case you have to revert back to fn version.