Hi, I tried replying already but somehow it didn't seem to get through.
I read Jon's main question as: how do I get the indices of the 1-s in a
multi-dimensional boolean array, like I get them using I. in 1-d:
I. 0 0 1 0 0 1 1 0 0
2 5 6
A usual there are multiple options, such as:
Idot =. $ #: I.@:,
]m2 =: 0=?4 6$5
0 1 0 0 0 1
0 0 0 0 0 1
0 1 1 0 0 0
0 1 0 0 0 0
Idot m2
0 1
0 5
1 5
2 1
2 2
3 1
Idot m3 =: 2 3 4$,m2
0 0 1
0 1 1
0 2 3
1 0 1
1 0 2
1 1 3
Also nice is:
4$.$.m2
0 1
0 5
1 5
2 1
2 2
3 1
, which I think I saw in a Rosetta contribution of Marshall Lochbaum. I.e.
Idot2 =: 4&$.@:$.
I. duplicates indices of items with integer values > 1 :
I.i.4
1 2 2 3 3 3
If you want the same behaviour in multiple dimensions, you go:
Idot3 =: (5&$. # 4&$.) @: $.
Idot3 i.2 3
0 1
0 2
0 2
1 0
1 0
1 0
1 1
1 1
1 1
1 1
1 2
1 2
1 2
1 2
1 2
Ben
________________________________________
From: [email protected]
[[email protected]] on behalf of Jon Hough
[[email protected]]
Sent: Tuesday, October 07, 2014 06:37
To: [email protected]
Subject: [Jprogramming] Project Euler 85, Python and J
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
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