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Re: Irrational numbers

by tobyink (Abbot)
on Dec 17, 2012 at 21:05 UTC ( #1009231=note: print w/ replies, xml ) Need Help??


in reply to Irrational numbers

Certain irrational numbers have a compact but useful representation; for example the square-root of two can be represented as bless(\'2 ** 0.5', 'Surd'). A suitably smart implementation of the Surd class could figure out that root-2 times 3-root-2 equals 6 without resorting to floating point arithmetic.

The problem with π is that it's not just irrational; it's transcendental. There's no convenient, accurate representation of it. You've just got to store an approximation as a float or string or whatever, and it will only be accurate to a certain number of decimal places.

perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'


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Re^2: Irrational numbers
by grondilu (Pilgrim) on Dec 17, 2012 at 22:20 UTC

    I'm afraid you missed the point I was trying to make. Pi could be represented as a lazy yet infinite list of rationals converging towards pi. So could any irrational numbers, including those who are transcendental.

    Representing real numbers seems like a very natural use case of lazy lists.

      Let's say you want pi to a million digits. Using a lazy list populated by the Machin's formula will take you several hours of calculation. But a hard-coded decimal representation of a million digits will take less than a megabyte of memory/disk space, even if inefficiently stored as one digit per byte and can be retrieved virtually instantly.

      In practice, there's little reason to need that sort of precision. A 64-bit float has about 20 decimal digits of precision, which is enough to calculate the circumference of the Earth to less than a hair's breadth. Apparently 61 digits of pi is enough to calculate the circumference of the observable universe to within one Planck length.

      In practice, floats are simply good enough for most purposes.

      perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'

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