Bit operation equivalent of pop function in PERL(remove MSB)

Question

In Perl, is there a bitwise operator that acts like >>, but removes the most significant bit? Sort of like how the >> operator is somewhat like the shift() function, I'm looking for a bit operator that's like pop().

110110 would return 10110

101 would return 01

Ultimately I'm trying to see whether a number in binary form is palindromic (i.e. 11011, 111, or 1010101), so ideally the operator would have a way of returning the bit it removes. It's okay if the operator doesn't, as I could do so mathematically, but in the interest of clean code, it would be awesome if it returned the MSB automatically. For the LSB, I do

$LSB=$mynum-2*($mynum>>1);
$mynum>>=1;

score 2 · Answer 1 · edited May 23 '17 at 12:11

2

see How to check if the binary representation of an integer is a palindrome?

edited May 23 '17 at 12:11

Community

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answered Dec 22 '10 at 05:28

You know, I spent way too long looking for something just like that to not have found it. What does WORDSIZE mean in that example? – Jake Stevens-Haas Dec 22 '10 at 05:44
@Ranting_Raven, I posted it in case you missed it. WORDSIZE is the size of a word on your machine (I assume) – Dec 22 '10 at 21:21

ysth · Accepted Answer · 2010-12-22T05:41:55.317

2

I can't think of a simpler way than just storing it as a string:

my $bits = sprintf '%b', $num;
while ( $bits =~ s/(.)// ) {
    print "removed $1\n";
}

though then your palindrome check is just

$bits eq reverse $bits

edited Dec 22 '10 at 05:41

answered Dec 22 '10 at 05:29

ysth

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score 1 · Answer 3 · answered Dec 22 '10 at 05:45

Since your values have variable numbers of bits you need a bit string or bit vector. Check out Bit::Vector on CPAN -- it seems to still be active.

But as the others have suggested for your problem it's probably easier for you just to deal with a plain old string.

score 0 · Answer 4 · answered Dec 22 '10 at 05:32

I don't know about Perl, but in C/C++ you'd do this like so:

unsigned flp2(unsigned x) {
   x = x | (x >> 1);
   x = x | (x >> 2);
   x = x | (x >> 4);
   x = x | (x >> 8);
   x = x | (x >>16);
   return x - (x >> 1);
}

unsigned most_significant_bit = flp2(my_number);
my_number &= most_significant_bit;

Courtesy of Hacker's Delight.

Note that to find the most significant bit in assembler you could use BSR, in MSVC _BitScanReverse, in GCC _builtin_clz. There are however no simple high-level (and portable) operators that I know of. Languages evolve much slower than CPUs and compilers.

Bit operation equivalent of pop function in PERL(remove MSB)

4 Answers4