On Apr 9, 4:38 am, rockins <[EMAIL PROTECTED]> wrote: > I cannot understand it well, can anyone explain me why and how > loghelper() can compute any base logarithm? Or could anyone give me > some reference(such as, books or papers)?
loghelper is there so that log(n) can be computed for any positive integer n---it's nothing to do with computing logs to an arbitrary base. All of the other math functions convert an integer argument to a float first. That conversion fails if the integer is larger than the largest representable float (around 1.7e308 on most systems). For example: >>> from math import sqrt, log >>> sqrt(10**600) Traceback (most recent call last): File "<stdin>", line 1, in <module> OverflowError: long int too large to convert to float >>> log(10**600) 1381.5510557964274 The sqrt call first tries to convert 10**600 to a float, giving an OverflowError (even though the actual square root *is* representable as a float). The log call goes through loghelper instead, which doesn't try to convert 10**600 to a float, but instead computes the log based on the top few bits of 10**600 (in its internal binary representation) and on the number of bits required to represent 10**600. You're not going to learn much about math function implementations from mathmodule.c: all it does it wrap the platform libm functions. Mark -- http://mail.python.org/mailman/listinfo/python-list
