[Python-Dev] Expert floats (original) (raw)

Andrew Koenig ark-mlist at att.net
Tue Mar 30 15:12:29 EST 2004

But you can't get away from that via any decimal rounding rule. One of the objections the 754 committee had to the Scheme rule is that moving rounded shortest-possible decimal output to a platform with greater precision could cause the latter platform to read in an unnecessarily poor approximation to the actual number written on the source platform. It's simply a fact that decimal 1.1000000000000001 is a closer approximation to the number stored in an IEEE double (given input "1.1" perfectly rounded to IEEE double format) than decimal 1.1, and that has consequences too when moving to a wider precision.

But if you're moving to a wider precision, surely there is an even better decimal approximation to the IEEE-rounded "1.1" than 1.1000000000000001 (with even more digits), so isn't the preceding paragraph a justification for using that approximation instead?

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