2011/2/3 Base <basselh...@gmail.com>: > Thats the one! > > Now I just have to understand it... > > The first apply is throwing me a little. > > > On Feb 3, 3:29 pm, Laurent PETIT <laurent.pe...@gmail.com> wrote: >> Hello, >> >> 2011/2/3 Base <basselh...@gmail.com>: >> >> > Hi All - >> >> > I recall seeing a beautiful method for doing the following on this >> > site, but cannot seem to find it.... >> > I would like to transform >> >> > (def a [[1 2 3] >> > [4 5 6] >> > [7 8 9]]) >> >> > to >> >> > [[1 4 7] >> > [2 5 8] >> > [3 6 9]] >> >> user=> (vec (apply map vector a)) >> [[1 4 7] [2 5 8] [3 6 9]]
Let's get rid of the call to vec, and focus on the core: (apply map vector a) : how do we get to this ? let's start inside out: we want to take advantage of the fact that the function map can take several colls. This way, it will first apply its function to all the first items of the colls, then the second items, etc. That's it! user=>(map (fn [c1 c2 c3] [c1 c2 c3]) [:c1.1 :c1.2 :c1.3] [:c2.1 :c2.2 :c2.3] [:c3.1 :c3.2 :c3.3]) ([:c1.1 :c2.1 :c3.1] [:c1.2 :c2.2 :c3.2] [:c1.3 :c2.3 :c3.3]) Then we realize that (fn [c1 ...] [...]) is just a special case of the function vector, so : user=>(map vector [:c1.1 :c1.2 :c1.3] [:c2.1 :c2.2 :c2.3] [:c3.1 :c3.2 :c3.3]) ([:c1.1 :c2.1 :c3.1] [:c1.2 :c2.2 :c3.2] [:c1.3 :c2.3 :c3.3]) And then, hmm, we would like to generalize the above so that it works with any number of collections. The function vector already allows this. And Clabamgo ! The apply function is really cool, because it accepts that its last argument be a seq, so we can rewrite the above: user=>(apply map vector (list [:c1.1 :c1.2 :c1.3] [:c2.1 :c2.2 :c2.3] [:c3.1 :c3.2 :c3.3])) ([:c1.1 :c2.1 :c3.1] [:c1.2 :c2.2 :c3.2] [:c1.3 :c2.3 :c3.3]) That's it, the list in the last position of the call to apply can now be of any size. HTH, -- Laurent >> >> Cheers, >> >> -- >> Laurent > > -- > You received this message because you are subscribed to the Google > Groups "Clojure" group. > To post to this group, send email to clojure@googlegroups.com > Note that posts from new members are moderated - please be patient with your > first post. > To unsubscribe from this group, send email to > clojure+unsubscr...@googlegroups.com > For more options, visit this group at > http://groups.google.com/group/clojure?hl=en -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en
