Re: [Numpy-discussion] nditer: possible to manually handle dimensions with different lengths?

3 Oct 2011

      Some observations and questions about nested_iters. Nested_iters seems to
require that all input arrays have the same number of dimensions (so you
will have to pad some input shapes with 1s). Is there a way to specify how
the axes line are matched together like for nditer?

When I try to run the following program,

@cython.boundscheck(False)
def vars(vals, group, axis ):
cdef np.ndarray[double, ndim = 2] values
cdef np.ndarray[long long, ndim = 2] groups
cdef np.ndarray[double, ndim = 2] outs
cdef int size
cdef double value
 cdef int i, j
 cdef long long cgroup
cdef double min
cdef double max
cdef double open
 oshape = list(vals.shape)
bins = len(np.unique(group))
oshape = oshape+[bins]
oshape[axis] = 1
 out = np.empty(tuple(oshape))
 axes = range(vals.ndim)
axes.remove(axis)
 gshape = [1] * len(oshape)
gshape[axis] = len(group)
group.shape = gshape
vals = vals[...,np.newaxis]
 it0, it1 = np.nested_iters([vals,group, out],
 [axes, [axis,len(oshape) -1]],
 op_dtypes=['float64', 'int64', 'float64'],
 flags = ['multi_index', 'buffered'])
size = vals.shape[axis]
for x in it0:
values, groups, outs = it1.itviews

j = -1
for i in range(size):
if cgroup != groups[i,0]:
if j != -1:
outs[0,j] = garmanklass(open, values[i,0], min, max)
 cgroup = groups[i,0]
min = inf
max = -inf
open = values[i,0]
j += 1

 min = fmin(min, values[i,0])
max = fmax(max, values[i,0])

outs[0,j+1] = garmanklass(open, values[size -1], min, max)
return out

I get an error

  File "comp.pyx", line 58, in varscale.comp.vars (varscale\comp.c:1565)
    values, groups, outs = it1.itviews
ValueError: cannot provide an iterator view when buffering is enabled

Which I am not sure how to deal with. Any advice?

What I am trying to do here is to do a "grouped" calculation (the group
specified by the group argument) on the values along the given axis. I try
to use nested_iter to iterate over the specified axis and a new axis (the
length of the number of groups) separately so I can do my calculation.

On Mon, Oct 3, 2011 at 9:03 AM, John Salvatier wrote:
...
Thanks mark! I think that's exactly what I'm looking for. We even had a
previous discussion about this (oops!) (
http://mail.scipy.org/pipermail/numpy-discussion/2011-January/054421.html
).
I didn't find any documentation, I will try to add some once I understand
how it works better.
John
On Sat, Oct 1, 2011 at 2:53 PM, Mark Wiebe  wrote:
...
On Sat, Oct 1, 2011 at 1:45 PM, John Salvatier 
...
wrote:
...
I apologize, I picked a poor example of what I want to do. Your
suggestion would work for the example I provided, but not for a more complex
example. My actual task is something like a "group by" operation along a
particular axis (with a known number of groups).
Let me try again: What I would like to be able to do is to specify some
of the iterator dimensions to be handled manually by me. For example lets
say I have some kind of a 2d smoothing algorithm. If I start with an array
of shape [a,b,c,d] and I'd like to do the 2d smoothing over the 2nd and 3rd
dimensions, I'd like to be able to tell nditer to do normal broadcasting and
iteration over the 1st and 4th dimensions but leave iteration over the 2nd
and 3rd dimensions to me and my algorithm. Each iteration of nditer would
give me a 2d array to which I apply my algorithm. This way I could write
more arbitrary functions that operate on arrays and support broadcasting.
Is clearer?
Maybe this will work for you:
In [15]: a = np.arange(2*3*4*5).reshape(2,3,4,5)
In [16]: it0, it1 = np.nested_iters(a, [[0,3], [1,2]],
flags=['multi_index'])
In [17]: for x in it0:
   ....:     print it1.itviews[0]
   ....:
[[ 0  5 10 15]
 [20 25 30 35]
 [40 45 50 55]]
[[ 1  6 11 16]
 [21 26 31 36]
 [41 46 51 56]]
[[ 2  7 12 17]
 [22 27 32 37]
 [42 47 52 57]]
[[ 3  8 13 18]
 [23 28 33 38]
 [43 48 53 58]]
[[ 4  9 14 19]
 [24 29 34 39]
 [44 49 54 59]]
[[ 60  65  70  75]
 [ 80  85  90  95]
 [100 105 110 115]]
[[ 61  66  71  76]
 [ 81  86  91  96]
 [101 106 111 116]]
[[ 62  67  72  77]
 [ 82  87  92  97]
 [102 107 112 117]]
[[ 63  68  73  78]
 [ 83  88  93  98]
 [103 108 113 118]]
[[ 64  69  74  79]
 [ 84  89  94  99]
 [104 109 114 119]]
Cheers,
Mark
...
On Fri, Sep 30, 2011 at 5:04 PM, Mark Wiebe  wrote:
...
On Fri, Sep 30, 2011 at 8:03 AM, John Salvatier <
jsalvati@u.washington.edu> wrote:
...
Using nditer, is it possible to manually handle dimensions  with
different lengths?
For example, lets say I had an array A[5, 100] and I wanted to sample
every 10 along the second axis so I would end up with an array B[5,10]. Is
it possible to do this with nditer, handling the iteration over the second
axis manually of course (probably in cython)?
I want something like this (modified from
http://docs.scipy.org/doc/numpy/reference/arrays.nditer.html#putting-the-inn...
)
@cython.boundscheck(False)
def sum_squares_cy(arr):
    cdef np.ndarray[double] x
    cdef np.ndarray[double] y
    cdef int size
    cdef double value
    cdef int j
axeslist = list(arr.shape)
    axeslist[1] = -1
out = zeros((arr.shape[0], 10))
    it = np.nditer([arr, out], flags=['reduce_ok', 'external_loop',
                                      'buffered', 'delay_bufalloc'],
                op_flags=[['readonly'], ['readwrite', 'no_broadcast']],
                op_axes=[None, axeslist],
                op_dtypes=['float64', 'float64'])
    it.operands[1][...] = 0
    it.reset()
    for xarr, yarr in it:
        x = xarr
        y = yarr
        size = x.shape[0]
        j = 0
        for i in range(size):
           #some magic here involving indexing into x[i] and y[j]
    return it.operands[1]
Does this make sense? Is it possible to do?
I'm not sure I understand precisely what you're asking. Maybe you could
reshape A to have shape [5, 10, 10], so that one of those 10's can match up
with the 10 in B, perhaps with the op_axes?
-Mark
...
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