We can express bitwise < (y and not x) as 20 b. . Is there a way to express
bitwise </\ using b. ? Or, in general, without actually having to explode
an integer into its component bits, and then reassemble them?
-Dan
----- Original Message ---------------
Subject: Re: [Jprogramming] math requests
From: Raul Miller <rauldmil...@gmail.com>
Date: Mon, 3 Feb 2014 09:44:15 -0500
To: Programming forum <programm...@jsoftware.com>
I am going over the messages in this thread slowly, because this
subject requires some thought. But that also means that some of my
responses in this thread will appear slowly.
Anyways:
On Thu, Jan 30, 2014 at 1:02 PM, Pascal Jasmin <godspiral2...@yahoo.ca>
wrote:
> http://repl.it/languages/Python is a useful resource for figuring out python
> code. But here is the only part I had trouble understanding:
>
> leftmostbit =: 2&#.@:({. , 0 $~ 2 -~ #)@:(2&#. inv) NB. for some reason
> divides msb by 2.
Here was the version I came up with:
leftmostBit=: </\&.#:
If I compare this to the python code:
def leftmost_bit( x ):
assert x > 0
result = 1
while result <= x: result = 2 * result
return result // 2
The python variable 'result' is indeed divided by 2 (// in python is
like <.@% in J), but notice that there's an extra 2 * result in the
loop when result is equal to x.
Example use:
leftmostBit"0 i.10
0 1 2 2 4 4 4 4 8 8
And here's the comparable python (leaving out 0 because of the assert):
[leftmost_bit(x) for x in range(1,10)]
[1, 2, 2, 4, 4, 4, 4, 8, 8]
Anyways, python is a bit quirky (but maybe that is a characteristic of
any computer system), but inspecting the data can work there just as
it can work in J.
Thanks,
--
Raul
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