Hi, On Sun, Oct 30, 2011 at 2:38 AM, Berthold Höllmann <berthold@xn--hllmanns-n4a.de> wrote:
Matthew Brett <matthew.brett@gmail.com> writes:
Hi,
Can anyone think of a good way to set a float128 value to an arbitrarily large number?
As in
v = int_to_float128(some_value)
?
I'm trying things like
v = np.float128(2**64+2)
but, because (in other threads) the float128 seems to be going through float64 on assignment, this loses precision, so although 2**64+2 is representable in float128, in fact I get:
In [35]: np.float128(2**64+2) Out[35]: 18446744073709551616.0
In [36]: 2**64+2 Out[36]: 18446744073709551618L
So - can anyone think of another way to assign values to float128 that will keep the precision?
Just use float128 all the was through, and avoid casting to float in between:
.>>> "%20.1f"%float(2**64+2) '18446744073709551616.0' .>>> np.float128(np.float128(2)**64+2) 18446744073709551618.0
Ah yes - sorry - that would work in this example where I know the component parts of the number, but I was thinking in the general case where I have been given any int. I think my code works for that, by casting to float64 to break up the number into parts: In [35]: def int_to_float128(val): ....: f64 = np.float64(val) ....: res = val - int(f64) ....: return np.float128(f64) + np.float128(res) ....: In [36]: int_to_float128(2**64) Out[36]: 18446744073709551616.0 In [37]: int_to_float128(2**64+2) Out[37]: 18446744073709551618.0 Thanks, Matthew