Converting negative number to binary with specific width

pjkang7 has asked for the wisdom of the Perl Monks concerning the following question:

Hi, this is my first question post on PerlMonk. Just fyi, I have searched and tried for some good amount of time (>40min) and decided I could not find out what's wrong with my code. It's a simple one.

Basically, I am trying to convert negative number into specific length (10) binary. And it's simply refusing to do that. Here's the code:

my $neg = -1;
my $negBin = sprintf ("010.10b", $neg);
print $negBin; #returns a lot of 1s instead of 11_1111_1111. (without 
+underscores)
[download]

Another problem i had was if I am trying to convert 1024 into binary number:

my $num = 1024;
my $bin = sprintf ("010.10b", $num);
print $bin; #returns 100_0000_0000 instead of 00_0000_0000
[download]

Apparently, there's something wrong with my sprintf formating....and I can't figure it out. I have used substr to simply cut the string to my desired length as a way around it but I am still interested to find out what I did wrong... I turn to the wisdom of PerlMonks out there. Thank you.

Comment on Converting negative number to binary with specific width Select or Download Code

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Re: Converting negative number to binary with specific width by BrowserUk (Patriarch) on Nov 03, 2015 at 01:03 UTC
YOu're using the wring tool. pack & unpack are designed for this: `print unpack 'b10', pack 's', -1;; 1111111111 print unpack 'b10', pack 's', -21;; 1101011111 print unpack 'b10', pack 's', 1024;; 0000000000` [download] With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday' Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". I knew I was on the right track :) In the absence of evidence, opinion is indistinguishable from prejudice. div class=	[reply] [d/l]
Re^2: Converting negative number to binary with specific width by AnomalousMonk (Archbishop) on Nov 03, 2015 at 01:34 UTC
I was thinking something more along the lines of `c:\@Work\Perl\monks>perl -wMstrict -le "for my $n (-2, -1, 0, 1, 2, 1023, 1024, 1025) { my $bin = reverse unpack 'b10', pack 'v', $n; print qq{'$bin' <= $n}; } " '1111111110' <= -2 '1111111111' <= -1 '0000000000' <= 0 '0000000001' <= 1 '0000000010' <= 2 '1111111111' <= 1023 '0000000000' <= 1024 '0000000001' <= 1025` [download] to get closer to the `'b'` formatting of sprintf (although I'm not sure this is any better than substr-inging it). (Update: Also see perlpacktut.) Give a man a fish: `<%-{-{-{-<`	[reply] [d/l] [select]
Re^3: Converting negative number to binary with specific width by Anonymous Monk on Nov 03, 2015 at 06:31 UTC
what os the is use of pack 'v' for?	[reply]
Re^4: Converting negative number to binary with specific width by Anonymous Monk on Nov 03, 2015 at 07:35 UTC
Re^5: Converting negative number to binary with specific width by Anonymous Monk on Nov 03, 2015 at 07:44 UTC
Some notes below your chosen depth have not been shown here
Re^2: Converting negative number to binary with specific width by pjkang7 (Novice) on Nov 05, 2015 at 16:58 UTC
Yup I will definitely use pack unpack next time. I saw that before but I thought I should not have any problem with sprintf. I did manage to work my way through using substr but I do think pack unpack would work nicer. Thanks	[reply]
Re^3: Converting negative number to binary with specific width by BrowserUk (Patriarch) on Nov 05, 2015 at 18:18 UTC
Make sure you look at AnomalousMonk's version of the code. With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday' Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". I knew I was on the right track :) In the absence of evidence, opinion is indistinguishable from prejudice.	[reply]
Re: Converting negative number to binary with specific width by RichardK (Parson) on Nov 03, 2015 at 00:45 UTC
The problem is that the b format is for unsigned integers only, so your -1 gets turned into an unsigned int and printing that overflows your field width, giving you lots of ones.The help for sprintf says :- `%b an unsigned integer, in binary` [download] BTW sprintf always prints the entire number even if it's bigger than the field width, so this is never going to work for 10 bit binary.	[reply] [d/l]
Re^2: Converting negative number to binary with specific width by pjkang7 (Novice) on Nov 05, 2015 at 16:52 UTC
Thanks, I simply did want -1 to show up as 1111111111... of width 10. which, in unsigned int, it would mean infinite (hence lots of ones...) but I just wanted to cut it at ten 1s. I did not know sprintf always print the entire number. Thanks.	[reply]
Re: Converting negative number to binary with specific width by Anonymous Monk on Nov 03, 2015 at 14:58 UTC
I prefer bit-bashing to string-bashing. Whatever floats your boat. `printf '%0*b', $bits, $num & ((1<<$bits)-1)`	[reply] [d/l]
Re^2: Converting negative number to binary with specific width by Anonymous Monk on Nov 03, 2015 at 15:28 UTC
If you insist on strings, substr is still probably easier than pack. `substr(sprintf("%0*b", $bits, $num), -$bits)`	[reply] [d/l]