----- Original Message -----
From: "Joe Schell" <[EMAIL PROTECTED]>
> > Ok - so it seems that there are 3 ingredients that must be present for
the
> > problem to arise:
> > 1) Big numbers - beyond 2^31-1.
> > 2) The '%' operator.
> > 3) A 'bare number' (for want of a better term).
> >
> > If any one of those ingredients is missing, then there will be no
> > problem -
> > though I note that we haven't yet tested the '-' (subtraction) operator.
> >
> > Zat how you see it ?
>
> Your (1) above is more complex. The error occurs in the following range
>
> 2^31-1 to 2^32-1
>
> Where 2^32 works correctly.
>
Could I prevail upon you one more time to check that ? It's not what I get.
I'm finding that, when multiplying by a numeric literal constant, instead of
getting 2^32 returned to '$result', I'm getting zero.
And I find that I'm getting the error if the value being returned to
'$result' is greater than 2^31-1. That is, the stipulation that the return
value has to be less than 2^32 - 1 does not apply in my case.
In general, I'm finding that the 'wrong' result is equal to the 32 low-order
bits of the 'correct' result, (with bit 33 being treated as a sign bit).
That doesn't correlate all that well with what's happening internally, but
it seems to be one way of deriving the 'wrong' value from the 'correct'
value.( There's a bit of elaboration on this below.)
Here's a script you could run for comparison purposes. It doesn't contain
the conversion to binary - but that's not so important - so long as you are
getting the same decimal values as I am. ( Best to try it with both of the
values that I've supplied for '$num1'.)
#####################################
my $base = 167772160; # no longer a power of 2
my $result;
my $num1 = 1919;
#my $num1 = 1258;
$result = ($num1 % 1000) * 167772160;
print "wrong = $result\n";
$result = ($num1 % 1000) * $base;
print "right = $result\n";
#################################
It prints (for $num1 = 1919)
wrong = -436207616
right = 154182615040
The respective binary representations being
11100110000000000000000000000000
10001111100110000000000000000000000000
You'll find the first binary number is the binary version of 436207616, and
the minus sign comes about because bit 33 of the 'correct' binary number is
'1'.
It prints (for $num1 = 1258)
wrong = 335544320
right = 43285217280
The respective binary representations being
00010100000000000000000000000000
101000010100000000000000000000000000
Here again, the first binary number is the binary representation of
335544320, and it stays positive because bit 33 of the 'correct' value is
'0'.
And to test the case for '2^32' versus '0', try:
my $t = 1256;
my $x = ($t % 1000) * 16777216; # ( = 2^32)
print $x, "\n";
# prints 0 for me (ignoring the sign bit).
To check what happens when 2^31 is returned:
my $t = 1002;
my $x = ($t % 1000) * 1073741824; # ( = 2^31)
print $x, "\n";
# prints -2147483648 for me, and I have no explanation
# for the presence of the minus sign. Since bit 33 of the #correct value is
'0', I would have expected a positive #return value (and hence a correct
return value).
2^31 - 1 is prime and will never be returned, and 2^31 - 2 is returned
correctly, as, presumably are all other values less than 2^31 - 2.
The main thing that I'd like to verify is that you are getting the same
return values as I did for those pieces of code.
Any additional comments (from anyone) are welcome - but it occurs to me
that, while *I* find this a little intriguing, others (including Joe,
perhaps) might be finding it downright tedious and bloody boring.
So long as you're finding the same return values as me, then I'm ready to
post to the porters and see what they have to say about it. I did a search
of their archive and couldn't find anything on this particular matter.
Note the nonchalant employment of the term 'numeric literal constant' above.
Now ....... if I can just refrain from referring to an 'array' as a
'thingy with a what's-it-from-an-email-address stuck in front' ........ :-)
Cheers,
Rob
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