Looking over this primelist issue... One of the first things I would probably do, would be to review the dictionary entry for p:
http://www.jsoftware.com/help/dictionary/dpco.htm And notice: 1 p: 1 2 3 4 5 0 1 1 0 1 So primelist might be written as: primelist=: 1&p: # ] Testing: primelist 1 2 3 4 5 2 3 5 Meanwhile, if you want the primes from a 10 by 10 array, first translate the array into a list (using comma): arr=: 1+i.10 10 primelist,arr 2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97 Or, for "change numbers in arr to 1 if the sum of the indices is prime, I might do something like this: arr^0 p:+/&i./$arr 1 2 1 1 5 1 7 1 9 10 11 1 1 14 1 16 1 18 19 20 1 1 23 1 25 1 27 28 29 1 1 32 1 34 1 36 37 38 1 40 41 1 43 1 45 46 47 1 49 1 1 52 1 54 55 56 1 58 1 60 61 1 63 64 65 1 67 1 69 70 1 72 73 74 1 76 1 78 79 80 81 82 83 1 85 1 87 88 89 1 91 92 1 94 1 96 97 98 1 100 Explanation: I can change the elements of arr to 1 by raising them to the zero-th power (changing to an arbitrary value would require slightly more work). I can get zeros corresponding to primes using 0 p: (from reading that dictionary entry). The expression +/&i./$arr works out like this: +/&i./$arr NB. the concise expression +/&i./10 10 NB. definition of $ 10 +/&i. 10 NB. definition of / (i.10) +/ (i.10) NB. definition of & using +/ and i. 0 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 10 2 3 4 5 6 7 8 9 10 11 3 4 5 6 7 8 9 10 11 12 4 5 6 7 8 9 10 11 12 13 5 6 7 8 9 10 11 12 13 14 6 7 8 9 10 11 12 13 14 15 7 8 9 10 11 12 13 14 15 16 8 9 10 11 12 13 14 15 16 17 9 10 11 12 13 14 15 16 17 18 Mostly, I guess, there are a few dozen vocabulary words here to master. Once you've gotten comfortable with them, you should have a rough idea of which ones might be relevant to a topic. Also, there's the matter of getting used to working on arrays "as a whole" and/or thinking about treating each item independently when possible. That's all I can think of that might be relevant for this kind of thing. Thanks, -- Raul On Sat, Oct 11, 2014 at 1:33 AM, Jon Hough <[email protected]> wrote: > Using (2 ! >:) is clearly better than doing my double for-loop. I'm > embarrassed I missed that. > > The real meat of my confusion with multidimensional arrays is in not just > finding the indices but doing something with the elements at the indices. > > e.g. For a single dimension array, there could be a function to fund the > primes less than 100. > > > primelist =: (I.@:(1&=)@:(1&p:)) { ] > > > and primelist >: i. 100 > > > should spit out > > > 2 3 5 ... ...97 > > > But what if instead of having >: i. 100 > > I had ( for some reason ) > > arr =: 10 10 $ >: i. 100 > > > So I have a 10 by 10 matrix of all positive ints up to 100. > > > primelist clearly will not work on arr. But if I want to return the list > of primes, as a single dimensional list I'm not sure how to do that. > > > Or, for example, if I want to change the elements of arr to 1 if and only > if the sum of the (i, j) indices are prime (just a random example). > > > In procedural python this could be quickly done with a double for-loop and > a prime test. In J this type of problem still escapes me. > > > > > Date: Fri, 10 Oct 2014 19:35:26 -0400 > > From: [email protected] > > To: [email protected] > > Subject: Re: [Jprogramming] Project Euler 85, Python and J > > > > countRects=: */@(2 ! >:) NB. How many pairs each of > vertical > > * horizontal lines > > getSizes=: ,@(>:/~) # [: ,/ ,"0/~ NB. All pairs of i. y > > idxClosest=: 4 : '(i. <./)@(x |@:- ])y'"(0 2) NB. Index of mat y to > > value x > > ({~ *2e6*&idxClosest@:(countRects"1)) getSizes >: i.200 NB. > Closest > > to 2e6 > > 77 36 > > ({~ *1e6*&idxClosest@:(countRects"1)) getSizes >: i.200 NB. > Closest > > to 1e6 > > 63 31 > > countRects"1 ] 63 31,:77 36 NB. How close is each? > > 999936 1999998 > > > > > > On Fri, Oct 10, 2014 at 1:14 PM, Linda Alvord <[email protected]> > > wrote: > > > > > What is the correct answerfor this problem? > > > > > > Linda > > > > > > -----Original Message----- > > > From: [email protected] > > > [mailto:[email protected]] On Behalf Of Stefano > > > Lanzavecchia > > > Sent: Friday, October 10, 2014 11:47 AM > > > To: [email protected] > > > Subject: Re: [Jprogramming] Project Euler 85, Python and J > > > > > > Actuary the use of ravel and antibase is common practice to solve > > > certain problems in APL and isn't considered cheating. So I wouldn't > > > say it's "not nice" but I would definitely go for antibase instead of > > > a combination of floored-divide and modulus. As a bonus, a solution > > > based on antibase would scale to problems of any rank and not just 2d > > > matrices. > > > > > > Have fun! > > > -- > > > Stefano > > > > > > > On 10/ott/2014, at 17:35, Sebastiano Tronto < > [email protected] > > > > > > > wrote: > > > > > > > > Hi, > > > > A dirty trick to get the job done would be to ravel the matrix ( , ), > > > solve > > > > the 1d version of the problem and then get the "true" indexes with > > > > something like (<.@%&200 , 200&|). > > > > For example, if you needed to just find the max: > > > > (<.@%&200 , 200&|) (i. >./) , m > > > > where m is your matrix. > > > > > > > > I know this isn't a nice way to solve the problem, but it should > work. > > > > > > > > Sebastiano > > > > > > > > 2014-10-07 6:37 GMT+02:00 Jon Hough <[email protected]>: > > > > > > > >> Project Euler 85: https://projecteuler.net/problem=85 > > > >> This problem is not really conceptually hard, but I am struggling > with a > > > J > > > >> solution.I have solved it in Python: > > > >> ============================================= > > > >> def pe85(larg, rarg): count = 0 llist = range(1, larg+1) > > > >> rlist = range(1, rarg+1) > > > >> for l in llist: for r in rlist: > count += > > > >> l*r > > > >> return count > > > >> > > > >> if __name__ == "__main__": # test for 2x3 grid, as in question. > > > k > > > >> = pe85(2,3) print "Test value: "+str(k) l1 = > range(1,200) > > > # > > > >> 200 lucky guess l2 = range(1,200) bestfit = 10000 # just > a big > > > >> number area = 0 for i in l1: for j in l2: > > > >> diff = abs(2000000 - pe85(i,j)) if > diff > > > < > > > >> bestfit: area = i*j > > > >> bestfit = diff > > > >> print "AREA is "+str(area) > > > >> > > > >> > > > >> ================================================The above script > will > > > give > > > >> the final area of the closest fit to 2 million. (The python code > may not > > > be > > > >> the best). Also I tested all possibilities up to 200x200, which was > > > chosen > > > >> arbitrarily(~ish). > > > >> Next my J. I go the inner calculation ok (i.e. see the function pe85 > > > >> above). In J I have: > > > >> pe85 =: +/@:+/@:((>:@:i.@:[) *"(0 _) (>:@:i.@:])) > > > >> NB. I know, too brackety. Any tips for improvement appreciated. > > > >> > > > >> > > > >> But from here things get tricky. If I do the calculation over > 200x200 > > > >> possibilities I end up with a big matrix, of which I have to find > the > > > >> closest value to 2 million, of which then I have to somehow get the > > > (x,y) > > > >> values of and then find the area by x*y. > > > >> > > > >> The main issue is getting the (x,y) from the best fit value of the > > > array. > > > >> > > > >> i.e. If I do pe85"(0)/~ 200, I get a big array, and I know I can > get the > > > >> closest absolute value to 2 million but then I need to get the > original > > > >> values to multiply together to give the best fit area. Actually I > have > > > >> bumped into this issue many times. It is easy enough in a 1-d > array,just > > > do: > > > >> (I. somefunc ) { ]) > > > >> > > > >> or similar to get the index. But for two indices the problem is > beyond > > > me > > > >> at the moment. Any help appreciated.Regards,Jon > > > >> > > > >> > > > >> > > > >> > ---------------------------------------------------------------------- > > > >> For information about J forums see > http://www.jsoftware.com/forums.htm > > > > > ---------------------------------------------------------------------- > > > > For information about J forums see > http://www.jsoftware.com/forums.htm > > > ---------------------------------------------------------------------- > > > For information about J forums see http://www.jsoftware.com/forums.htm > > > > > > ---------------------------------------------------------------------- > > > For information about J forums see http://www.jsoftware.com/forums.htm > > > > > > > > > > > -- > > Devon McCormick, CFA > > ---------------------------------------------------------------------- > > For information about J forums see http://www.jsoftware.com/forums.htm > > > ---------------------------------------------------------------------- > For information about J forums see http://www.jsoftware.com/forums.htm > ---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm
