[Jprogramming] Translating Mask Generation to J Code

[Yuvaraj Athur Raghuvir]

Henry,

1) Can you explain what you have done and how is it equivalent to what I
have asked?  I am having trouble understanding particularly after what Dan
has written as the characteristic of the sequence , viz., number of 1's in
the binary representation.

2) OEIS tells me that "3 msks 64" is a sequence "Numbers with binary weight
<= 3. " [3]. I am not able to see how


Thanks,
Yuva


[3]
http://www.research.att.com/~njas/sequences/index.html?q=0%2C+1%2C+2%2C+3%2C+4%2C+5%2C+6%2C+7%2C+8%2C+9%2C+10%2C11%2C12%2C13%2C14%2C16%2C17%2C18%2C19%2C20%2C21%2C22%2C24%2C25%2C26%2C28&language=english&go=Search


On Fri, Jan 15, 2010 at 12:04 PM, Henry Rich <HenryHRich at nc.rr.com> wrote:

> With d wired to 2, you can use
>
>    msks =: /:~@(> # ]) ~.@,@(+/~)@(0&,)@(i.@:>:@<.&.(2&^.))
>    msks 64
> 0 1 2 3 4 5 6 8 9 10 12 16 17 18 20 24 32 33 34 36 40 48
>
> If you need to change d, you can use
>
>    msks =: (] #~ (>: +/"1@#:)) i.
>    2 msks 64
> 0 1 2 3 4 5 6 8 9 10 12 16 17 18 20 24 32 33 34 36 40 48
>    3 msks 32
> 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 16 17 18 19 20 21 22 24 25 26 28
>
>
> Henry Rich
>
>
> Yuvaraj Athur Raghuvir wrote:
> > Hello,
> >
> > I want to rewrite Java Code[1] to J Code[2] . I want to check with the
> forum
> > members if there is a better way to achieve this.
> >
> > The issue is that in the setBits loop, the values are set using the
> > previously set values. In the process of loop elimination, I found a J
> form
> > and I hand coded the extension to the desired limit. The questions I have
> > are:
> > 1) For the J pattern I have found, is there a way to use ^: or $: forms?
> > 2) Based on the math behind the Java Code, is there a different way to
> > achieve this result? I am not able to formulate the problem so I did it
> the
> > debugging way.
> >
> > Thanks,
> > Yuva
> >
> > Java Code:
> >     byte[] setBits = new byte[256];
> >         int d  = 2;
> >     for (int i = 0; i < 256; ++i) setBits[i] = (byte) ((i & 1) +
> setBits[i
> >>> 1]);
> >     //println(setBits);
> >     // for every possible key mask...
> >     masks = new int[0];
> >     for (int i = 0; i < s; ++i)
> >       // if d or fewer bits are set...
> >       if (setBits[(i >> 24) & 0xFF] +
> >           setBits[(i >> 16) & 0xFF] +
> >           setBits[(i >>  8) & 0xFF] +
> >           setBits[ i        & 0xFF] <= d)
> >         // add it to the list of masks.
> >         masks = append (masks, i);
> >
> > My J Attempt:
> > ls   =: (33 b.) NB. bit left shift
> > and  =: (17 b.) NB. bit and
> > or   =: (23 b.) NB. bit or
> > exor =: (22 b.) NB. bitwise exclusive or
> >
> >
> >  v0 =: 13 : ' (2&| i.@# y) + (2&| i.@#y)  {~ (_1&ls i.@#y) '
> >  v1 =: 13 : ' (2&| i.@#y) + ( (-#y) {. (_1&ls i.+:@# y)) { y '
> >  t2 =. (v0,v1 at v0)  NB. t2 i.2 gives a string of length 4 on i.2
> >  t3 =. (t2,v1 at t2)  NB. 2^3
> >  t4 =. (t3,v1 at t3)  NB. 2^4
> >  t5 =. (t4,v1 at t4)  NB. 2^5
> >  t6 =. (t5,v1 at t5)  NB. 2^6
> >  t7 =. (t6,v1 at t6)  NB. 2^7
> >  t8 =. (t7,v1 at t7)  NB. 2^8
> >  sb =. t8 i.2       NB. setBits
> >  m0 =. (16bff&and)@:(0&ls + _8&ls + _16&ls + _24&ls) sb
> >  m1 =. <:&2 m0
> >  msk =: I. m1           NB. masks
> >
> >    t2 i.2
> > 0 1 1 2
> >    t3 i.2
> > 0 1 1 2 1 2 2 3
> >    t4 i.2
> > 0 1 1 2 1 2 2 3 1 2 2 3 2 3 3 4
> > ----------------------------------------------------------------------
> > For information about J forums see http://www.jsoftware.com/forums.htm
> >
> ----------------------------------------------------------------------
> For information about J forums see http://www.jsoftware.com/forums.htm
>