[Numpy-discussion] Question about 64-bit integers being cast to double precision

Travis Oliphant oliphant at ee.byu.edu
Sun Oct 9 21:28:33 CDT 2005


What is the opinion of people here regarding the casting of 64-bit 
integers to double precision.

In scipy (as in Numeric), there is the concept of "Casting safely" to a 
type.  This concept is used when choosing a ufunc, for example. 

My understanding is that a 64-bit integer cannot be cast safely to a 
double-precision floating point number, because precision is lost in the 
conversion.   However, at least a signed 64-bit integer can usually be 
cast safely to a long double precision floating point number.

This is not too big a deal on 32-bit systems where people rarely request 
64-bit integers.

However, on some 64-bit systems (where the C long is 64-bit), Python's 
default integer is 64-bit.  Therefore, simple expressions like sqrt(2) 
which require conversion to floating point will look for the first 
floating point number that it can convert a 64-bit integer to safely.    
This can only be a long double.

The result is that on 64-bit systems, the long double type gets used a 
lot more.   Is this acceptable? expected?   What do those of you on 
64-bit systems think?

-Travis






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