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Re^2: Math::FixedPrecision and bignum clash

by mje (Curate)
on Oct 08, 2014 at 13:21 UTC ( #1103172=note: print w/replies, xml ) Need Help??

in reply to Re: Math::FixedPrecision and bignum clash
in thread Math::FixedPrecision and bignum clash

Thank you thanos1983 for the obvious time you've spent looking at this. I came to the same conclusion as you (and hence my title) that there is some sort of clash between Math::BigFloat/bignum and Math::FixedPrecision but Math::FixedPrecision is really only a Math::BigFloat wrapper so I don't understand that.

I can only think using bignum causes math inside Math::FixedPrecision (of which there is very little) to use Math::BigFloats and this breaks things or more likely something else (the missing mantissa is very strange). I'd really like to know what is going on and I'll have another go at looking in to it tonight.

Although your alternative appears to work it is not quite what Math::FixedPrecision does. ffround does bankers rounding and Math::FixedPrecision does no rounding. e.g.,

use strict; use warnings; use Math::FixedPrecision; my $x = 3.145; my $y = Math::FixedPrecision->new($x, 2); print "$x, $y\n"; # outputs 3.145, 3.14

If you change your 1.1415 to 1.145 you'll see the difference.

Basically the 2 modules in question were written by 2 different people. One module only needed a dozen or so lines of code using Math::FixedPrecision. The other module was massive and had loads of math so the author chose to simplify it and use bignum. This problem only occurred when someone tried to write a test which used both.

I have some possible solutions not that disimilar to yours but it is nagging at me as why you cannot mix the two and what is going on.

Thanks again for looking at this, it is much appreciated.

Replies are listed 'Best First'.
Re^3: Math::FixedPrecision and bignum clash
by McA (Priest) on Oct 08, 2014 at 14:41 UTC

    Hi mje,

    IMHO your assumption of rounding in Math::FixedPrecision is not true. When you have a look at the constructor of Math::FixedPrecision you'll see the the value 3.145 having 3 digits after the point will be rounded to two digits (what is given as second parameter) with ffround (line 80 of sources).

    The effect you see is caused by mathematical (odd) rounding (default of Math::BigFloat) with your example. Have a look at these two examples:

    #!/usr/bin/perl use strict; use warnings; use 5.010; use Math::FixedPrecision; my $b = Math::FixedPrecision->new(3.145, 2); say $b; # output: 3.14 my $c = Math::FixedPrecision->new(3.155, 2); say $c; # output 3.16

    UPDATE: Added output for the reader.


      Yes, thanks. I was wrong and it is using bankers rounding. Looks at the last digit being kept and if odd rounds down and even rounds up so long as first digit beyond the precision is >= 5. That is another bug in the code we're using here which does not want bankers rounding.

Re^3: Math::FixedPrecision and bignum clash
by thanos1983 (Parson) on Oct 08, 2014 at 14:29 UTC

    Hello again mje,

    Possibly there is a conflict between the two modules, but in any case I do not think so that Math::FixedPrecision is a bad module. It can handle big float numbers just like bignum.

    Sample of code:

    #!/usr/bin/perl use strict; use warnings; use Math::FixedPrecision; my $x = 3.1415; my $y = Math::FixedPrecision->new($x, 3); print "$x, $y\n"; __END__ 3.1415, 3.142

    It seems correct since anything above or equal to .5 should round up one unit. Which I am getting the same result with bignum.

    At this point the only difference that I could find in between these two modules is that bignum also can handle big integers with precision and accuracy. It just gives you more options to play around.

    Apart from that with a quick look that I took, I can not spot other differences between them.

    Out of curiosity what is your test that you are running that you want to use both modules? As far as I can see if you use bignum you get anything you need.

    Seeking for Perl wisdom...on the process of learning...not there...yet!

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