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This queue is for tickets about the Math-BigInt CPAN distribution.

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Id: 110725
Status: new
Priority: 0/
Queue: Math-BigInt

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Owner: Nobody in particular
Requestors: gdg [...] zplane.com
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Subject: BigFloat new() on a BigRat object yields unexpected result
Date: Mon, 28 Dec 2015 15:06:31 -0700
To: bug-math-bigint [...] rt.cpan.org
From: Glenn Golden <gdg [...] zplane.com>
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The attached annotated example shows some unexpected (to me) behavior of Math::BigFloat->new(). On my setup (version info below), the output is: perl: v5.22.0 BigFloat: 1.999710 BigRat: 0.2608 ref(x): Math::BigRat ref(y): Math::BigRat x: 42/43 y: 42/43 The underlying question that lead to this example is: Is there a clean way to go about creating a BigFloat object that has the same mathematical value as a given BigRat object? The only way I was able to make it work was to create two BigFloats, one from the numerator and one from the denominator of the BigRat, and then divide them. Works fine, just wondering if there's a cleaner approach. Show quoted text
--------------------------- cut here -------------------------------- use warnings; use strict; use Math::BigFloat; use Math::BigRat; print "perl: $^V\n"; print "BigFloat: $Math::BigFloat::VERSION\n"; print "BigRat: $Math::BigRat::VERSION\n"; print "\n"; my ($x, $y); # # Create $x as a BigRat. # $x = Math::BigRat->new('42/43'); if ($x->is_nan()) { die "x is nan"; } # No die(), as expected # # Now we want to create $y as a BigFloat having the same mathematical value # as $x. Naively, we try this: # $y = Math::BigFloat->new($x); # # The above statement behaves as follows, both unexpected (to me): # # 1. The new($x) does not fail. I expected it to fail since, according to # the BigFloat man page ("Input" subsection) the input to new() is # neither "...a BigFloat object nor a string of any of the following # forms." # # 2. The resulting $y winds up as a BigRat. # if ($y->is_nan()) { die "y is nan"; } # Does not die(). [unexpected] print "ref(x): " . ref($x) . "\n"; # Reports $x as BigRat as expected print "ref(y): " . ref($y) . "\n"; # Reports $y as BigRat: [unexpected] print "x: $x\n"; # Prints "42/43" print "y: $y\n"; # Prints "42/43" ---------------------------------------------------------------------
Subject: Re: [rt.cpan.org #110725] BigFloat new() on a BigRat object yields unexpected result
Date: Mon, 28 Dec 2015 15:57:31 -0700
To: Bugs in Math-BigInt via RT <bug-Math-BigInt [...] rt.cpan.org>
From: Glenn Golden <gdg [...] zplane.com>
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Just to clarify a point in my prior message which was sloppily worded: I said I was expecting BigFloat->new() to produce a BigFloat with the "same mathematical value" as a given BigRat, which was perhaps overstated, making it sound as though I was expecting lossless conversion (which is of course impossible in general). I simply meant "the same value" in the usual approximate sense when assigning any value to a float, i.e. the numerical value that most closely approximates the given value within the available mantissa precision. In other words, my expectation, in view of the documentation, is that BigFloat->new() _would_ perform the lossy conversion of BigRat to BigFloat rather than preserving the lossless BigRat representation and returning a BigRat object (with losslessness as a motivation for doing so). And in any case, the example behaves identically even if the BigRat $x is a power of 2, hence exactly representable as a BigFloat.


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