Re: Extracting bits out of huge numbers
- From: hofer <blabla@xxxxxxxxxx>
- Date: Thu, 31 Jul 2008 02:40:10 -0700 (PDT)
Hi Bugbear,
I'm not that gifted in explaining, but I t'll try.
First if your question concerns the representation of binary numbers:
Then look at http://en.wikipedia.org/wiki/Binary_numeral_system
Second:
ust a small example however just with 8 bit numbers.
Please keep in mind, that I really want to work with really big
numbers.
well an unsigned 8 bit number can have values from
0 to 255
The number 26 for example would be represented as
00011010 in binary format
For small numbers I could just use
printf("%08b",26) to get the binary string
# result should be 00011010
going from binary string representation to decimal could be done with
the oct() function
I just have to prefix the bit string with "0b" and pass it to the
oct() function.
print oct("0b"."00011010");
# result shoud be 26
the bits of this number are number from right to left so the bt on the
right is bit 0 and the bit on the left is bit7
the number 26
would have
bit 0 = 0
bit 1 = 1
bit 2 = 0
bit 3 = 1
bit 4 = 1
bit 5 = 0
bit 6 = 0
bit 7 = 0
if I want to extract bits 4 to bits 2 from my number 26 it would mean
taking
bit 2 = 0
bit 3 = 1
bit 4 = 1
which is 110 or 0b110 (0b marks a binary number in perl)
and would be equivalent to number 6 in decimal.
so I could do for example
$num = 26; #
$size=8; # tha value has a max size of 8 bits
$bitfrom=4;
$bitto=2;
$bitstring = sprintf("%0%sizeb",$num); # -> "00011010"
# the first character in this string is bit 7, tha last one is bit 0
so to get bits 4 to 2 I could now do
$bits4to2 = substr($bitstring,-$from,$from-$to+1); # -> "110"
$num4to2 = oct("0b".$bits4to2); # -> 6
I hope that clarified a little.
bye H
On Jul 30, 10:32 am, bugbear <bugbear@xxxxxxxxxxxxxxxxxxxxxxxxx>
wrote:
hofer wrote:
Hi,
Now I would like to extract certain bits out of this huge number:
Example Bits 16 bis 12 should result in 0b00001 == 0x1 == 1
Bits 17 bis 12 should result in 0b100001 == 0x21 == 33
I don't understand your examples :-(
BugBear
.
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