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Question on simple arithmetic using perl

by perlbaski (Sexton)
on Sep 28, 2011 at 21:27 UTC ( #928429=perlquestion: print w/replies, xml ) Need Help??
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";

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.

      Thank you Monk!! So does this mean its fine to use such arithmetic and not worry about whether perl can handle it correctly?
        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
Re: Question on simple arithmetic using perl
by eyepopslikeamosquito (Chancellor) 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";
    x=123456789012345678901234567891 y=1.23456789012346e+029
    Update: See also bigint and bignum.

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