[Numpy-discussion] How to set array values based on a condition?
Mon Mar 24 02:09:42 CDT 2008
I am eager to implement the C version of the set_where function but
would like to do so in a numpy-esque way. Having implemented several
internal and released Python/C packages, I am familiar with the PyArray
object and the PyArrayIterObject and the like. After looking through the
code I noticed the @typ@ notation in the .src files, which looks like
some kind of pseudo-template to allow ndarray methods to work over
arbitrary data types. I surmise these files are fed through a shell or
Python script to generate the actual code? In my own code, I've been
using C++ very carefully to handle such generality but I understand that
numpy is strictly written in C.
The set_where function is very similar to ndarray.__add__ in that its
arguments can either be scalars or arrays (all arrays must be of the
same shape, though). To get a sense of how a numpy operator is
implemented can someone point me to the templatized array __add__
function? I tried searching through the files below (numpy/core/src) but
I can't quite pin down where the add functionality is implemented.
Seeing PyArray_ArgMax, I thought PyArray_XXX might have been the naming
convention for numpy methods but the fact that PyArray_Add (or
PyArray_Plus) is not defined makes me unsure.
arraymethods.c multiarraymodule.c _sortmodule.c.src
arrayobject.c scalarmathmodule.c.src ucsnarrow.c
arraytypes.inc.src scalartypes.inc.src ufuncobject.c
_isnan.c _signbit.c umathmodule.c.src
Please advise. Thank you.
Damian Eads Ph.D. Student
Jack Baskin School of Engineering, UCSC E2-381
1156 High Street
Santa Cruz, CA 95064 http://www.soe.ucsc.edu/~eads
Damian Eads wrote:
> Damian Eads wrote:
>> Anne Archibald wrote:
>>> On 23/03/2008, Damian Eads <email@example.com> wrote:
>>>> I am working on a memory-intensive experiment with very large arrays so
>>>> I must be careful when allocating memory. Numpy already supports a
>>>> number of in-place operations (+=, *=) making the task much more
>>>> manageable. However, it is not obvious to me out I set values based on a
>>>> very simple condition.
>>>> The expression
>>>> generates a binary index mask y>=0 of the same size as the array y,
>>>> which is problematic when y is quite large.
>>>> I was wondering if there was anything like a set_where(A, cmp, B,
>>>> setval, [optional elseval]) function where cmp would be a comparison
>>>> operator expressed as a string.
>>>> The code below illustrates what I want to do. Admittedly, it needs to be
>>>> cleaned up but it's a proof of concept. Does numpy provide any functions
>>>> that support the functionality of the code below?
>>> That's a good question, but I'm pretty sure it doesn't, apart from
>>> numpy.clip(). The way I'd try to solve that problem would be with the
>>> dreaded for loop. Don't iterate over single elements, but if you have
>>> a gargantuan array, working in chunks of ten thousand (or whatever)
>>> won't have too much overhead:
>>> block = 100000
>>> for n in arange(0,len(y),block):
>>> yc = y[n:n+block]
>>> yc[yc<0] = -1
>>> It's a bit of a pain, but working with arrays that nearly fill RAM
>>> *is* a pain, as I'm sure you are all too aware by now.
>>> You might look into numexpr, this is the sort of thing it does (though
>>> I've never used it and can't say whether it can do this).
>>> Numpy-discussion mailing list
>> Hi Anne,
>> Since the thing I want to do is a common case, I figured that if I were
>> to take the blocked-based approach, I'd write a helper function to do
>> the blocking for me. Here it is:
>> import numpy
>> import types
>> def block_cond(*args):
>> block_cond(X1, ..., XN, cond_fun, val_fun, [else_fun])
>> Breaks the 1-D arrays X1 to XN into properly aligned chunks. The
>> cond_fun is a function that takes in the chunks of each array
>> returns an index or mask array. For each chunk c
>> C=cond_fun(X1[c], ..., XN[c])
>> The val_fun takes the masked or indexed chunks, and returns the
>> values each element should be set to
>> V=cond_fun(X1[c][C], ..., XN[c][C])
>> Finally, the first array's elements
>> blksize = 100000
>> if len(args) < 3:
>> raise ValueError("Nothing to do.")
>> if type(args[-3]) == types.FunctionType:
>> elsefn = args[-1]
>> valfn = args[-2]
>> condfn = args[-3]
>> qargs = args[:-3]
>> elsefn = None
>> valfn = args[-1]
>> condfn = args[-2]
>> qargs = args[:-2]
>> # Grab the length of the first array.
>> num = qargs.size
>> shp = qargs.shape
>> # Check that rest of the arguments are all arrays of the same size.
>> for i in xrange(0, len(qargs)):
>> if type(qargs[i]) != _array_type:
>> raise TypeError("Argument %i must be an array." % i)
>> if qargs[i].size != num:
>> raise TypeError("Array argument %i differs in size from the
>> previous arrays." % i)
>> if qargs[i].shape != shp:
>> raise TypeError("Array argument %i differs in shape from
>> the previous arrays." % i)
>> for a in xrange(0, num, blksize):
>> b = min(a + blksize, num)
>> fargs = [qarg[a:b] for qarg in qargs]
>> c = apply(condfn, fargs)
>> #print c
>> v = apply(valfn, [farg[c] for farg in fargs])
>> #print v
>> slc = qargs[a:b]
>> slc[c] = v
>> if elsefn is not None:
>> ev = apply(elsefn, [numpy.array(arg[a:b])[~c] for arg in
>> slc[~c] = ev
>> Let's try running it,
>> In : y=numpy.random.rand(10000000)
>> In : x=y.copy()
>> In : %time x[:] = x<=0.5
>> CPU times: user 0.39 s, sys: 0.01 s, total: 0.40 s
>> Wall time: 0.66 s
>> In : %time setwhere.block_cond(y, lambda y: y <= 0.5, lambda y: 1,
>> lambda y: 0)
>> CPU times: user 1.70 s, sys: 0.10 s, total: 1.80 s
>> Wall time: 2.28 s
>> The inefficient copying approach is almost 4 times faster than the
>> blocking approach. Ideas about what I'm doing wrong?
>> Would others find a proper C-based numpy implementation of the set_where
>> function useful? I'd offer to implement it.
> If I try it with the scipy.weave implementation I showed in my first
> posting of this thread, I get a factor of 3 speed up over the
> memory-inefficient copy approach and a factor of 10 speed up over the
> block-based approach.
> In : y=numpy.random.rand(10000000)
> In : %time setwhere.set_where(y, "<=", 0.5, 1, 0)
> CPU times: user 0.15 s, sys: 0.00 s, total: 0.15 s
> Wall time: 0.21 s
> This suggests a C implementation might be worth it.
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