Hi all:
I recently discovered that the numerical_approx() function (aliased as n()) does not perform as i expected. Is the following behavior intentional?
sage: x = pi.n(digits=3)
sage: print x, x.n(), QQ(x), QQ(x).n()
3.14 3.14160156250000 333/106 3.14150943396226
Where are the extra digits of x coming from to produce the last three results in the output above?
When you write pi.n(digits=3), you do not get the rational number 314/100. What you get is a floating point number $x$ with a guarantee that $|\pi - x| \le 0.01$ (I believe this number also "knows" how many digits you care about; that's why print shows exactly 3 digits). Because floating point numbers are internally stored in base 2, it is impossible to represent 314/100 exactly.
I think that Jason's point is that with y=3.14, you get something with a lot of (base 2) precision, which can be nicely turned into a fraction, while when you limit it to 3 digits, this is the best you can get. At least, that's how I understand Jason's answer, and I think he's right.
They don't come from anywhere special.
sage: x = pi.n(digits=3)
sage: y = QQ(x)
sage: type(y)
<type 'sage.rings.rational.Rational'>
sage: y.n()
3.14150943396226
sage: y.n(digits=100)
3.141509433962264150943396226415094339622641509433962264150943396226415094339622641509433962264150943
sage: n(333/106)
3.14150943396226
Once you turn x into a rational, it behaves as any other rational would. In particular, it has an eventually repeating decimal expansion, etc. The point is that you tried to turn a low-precision number into a rational, and this is what happens. Sorry that the rationals don't 'hold precision', but I don't think this would be intended.
Asked: 2011-08-09 22:51:39 +0100
Seen: 606 times
Last updated: Aug 11 '11
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Let me elaborate. I expected that `x` = 3.14, so that `x.n()` = 3.14000000000000 and `QQ(x)` = 157/50. But `x == 3.14` returns `False`, `x.n()` = 3.14160156250000 and `QQ(x)`= 333/106. Huh? Why are there more than 3 digits stored in x?
Thanks for the clarification!