Glen PittPladdy :: BlogPerl: dealing with extreme numbers using Math::BigFloat  
I've been working on a statistical analysis project which involves processing of large numbers of data points (currently 15 million and counting) and combining them into an overall ratiolike representation. While optimising code I discovered that I sometimes get very different results depending on the order that the data is processed. That should not happen  it's rather like finding $a * $b != $b * $a Further investigation leads to the discovery that this is due to floating point numbers getting so small they zero out and thus the earlier in the data a zero is reached and subsequent data is ignored. Preserving integrityFirst up I decided I had to be able to get the most out of the standard Perl floating point. In my case I am multiplying a load of numbers between 0 and 1 resulting in the overall numbers gradually reducing until it rounds to zero. As this is a ratio, integrity can be preserved far longer simply by normalising the ratio each time:
my @ratio = ( 1, 1 ); That can give a considerable amount of additional scope for the use of the basic Perl float. Trouble is eventually that isn't enough... Enter Math::BigFloatMath::BigFloat provides arbitrary length floating point arithmetic, but at a cost: performance In my tests I found it had a performance hit in the order of 250x which when processing large data sets is a nonstarter. That doesn't mean that it can't be used, just that it needs care. Hanging on to the bitter endMy approach involves using basic Perl floating point for a temporary ratio until the point that there is a risk of rounding to zero, at which point we use Math::BigFloat to transfer the temporary ratio into the master ratio:
my @ratio = ( Math::BigFloat>bone(), Math::BigFloat>bone() ); Too longThere is still one catch I ran into with Math::BigFloat  with the number of calculations, the number of significant (non zero) digits can grow really large. When it comes to divide out the ratio everything comes to a halt. The problem is that the number of significant digits can get extremely big, and to divide these extremely long numbers takes a long time. If you are dealing with these extremes then you may want to round the numbers to something sensible before processing further: @ratio = map { $_>fround ( 20 ) } @ratio; 

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