http://www.perlmonks.org?node_id=928429

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

Hi Monks,

I ran below code with use integer line commented and uncommented. 

#!/usr/bin/perl
#use integer;
my $max_neg_num=((2**32)-1);
print "\nmax neg num is $max_neg_num\n";
$error=4294967263 -$max_neg_num-1;
print "error is $error \n";

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With use integer commented: max neg num is 4294967295 error is -33

With use integer uncommented. max neg num is -2 error is -32

When I looked at integer pragma documentation, it says numbers are wrapped around after largest +ve number i.e. 2^31-1. By that logic max neg number should be -1 represented by 2^32-1.

Also, I am surprised that without integer pragma, how come I am able to see the results correctly even though it is 32 bit machine? I think its a naive question, but I am afraid I will remain naive if I dont ask it. I would like to know how such arithmetic is handled on 32 bit machines. Is there a potability issue if I write the code as above with use integer commented?

Please enlighten me!!

Replies are listed 'Best First'.
Re: Question on simple arithmetic using perl
by Anonymous Monk on Sep 28, 2011 at 21:34 UTC

    When the number becomes too large for an integer it is automagically promoted to a float.

[reply]
      Thank you Monk!! So does this mean its fine to use such arithmetic and not worry about whether perl can handle it correctly?
[reply]
        No. A float can represent a wider range of values, but a lesser number of significant digits than an integer.

        I'm not quite sure what the question is: 

        #!/usr/bin/perl -w
use strict;

my $all_bits_are_on = -1;
printf ("all_bits => %X\n", $all_bits_are_one);

my $x = $all_bits_are_on + 1;
print "Adding -1 to +1 is: $x\n";

__END__
all_bits => FFFFFFFF
Adding -1 to +1 is: 0

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Re: Question on simple arithmetic using perl
by eyepopslikeamosquito (Archbishop) on Sep 29, 2011 at 10:58 UTC

    Also, be aware that Perl supports arbitrary size integers via the core Math::BigInt module. For example: 

    use Math::BigInt;
my $x = Math::BigInt->new('123456789012345678901234567890');
my $y = '123456789012345678901234567890';
$x += 1;
$y += 1;
print "x=$x\n";
print "y=$y\n";

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    prints: 
    x=123456789012345678901234567891
y=1.23456789012346e+029

    [download]
    Update: See also bigint and bignum.

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