[Numpy-discussion] Add/multiply reduction confusion
Andrew Friedley
afriedle at indiana.edu
Mon Jun 29 10:42:35 EDT 2009
Hi,
I'm trying to understand how integer types are upcast for add/multiply
operations for my GSoC project (Implementing Ufuncs using CorePy).
The documentation says that for reduction with add/multiply operations,
integer types are 'upcast' to the int_ type (int64 on my system). What
exactly does this mean, internally? Where/when does the upcasting
occur? Is it a C-style cast, or a memory copy to a new temporary array?
I'm a confused as to which low-level ufunc loop type is used (and why).
This is what I see:
>>> a = numpy.arange(131072, dtype=numpy.int32)
>>> r = numpy.add.reduce(a)
>>> print type(r)
<type 'numpy.int64'>
>>> print hex(r)
0x1ffff0000L
Okay, fine. But I have my own ufunc, which defines only the following
types right now (I stripped it down for debugging):
>>> print corefunc.add.types
['ii->i', 'll->l']
NumPy has this, for comparison:
>>> print numpy.add.types
['??->?', 'bb->b', 'BB->B', 'hh->h', 'HH->H', 'ii->i', 'II->I', 'll->l',
'LL->L', 'qq->q', 'QQ->Q', 'ff->f', 'dd->d', 'gg->g', 'FF->F', 'DD->D',
'GG->G', 'OO->O']
Also just to verify I did this:
>>> print numpy.typeDict['i']
<type 'numpy.int32'>
>>> print numpy.typeDict['l']
<type 'numpy.int64'>
Yet when I call my own ufunc, this happens:
>>> a = numpy.arange(131072, dtype=numpy.int32)
>>> r = corefunc.add.reduce(a)
>>> print type(r)
<type 'numpy.int32'>
>>> print hex(r)
-0x10000
It looks like no upcasting is occurring here? My ii->i loop is being
used, not the ll->l loop.. why? I'm guessing this is something I am
doing wrong, any ideas what it is?
Andrew
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