[Numpy-discussion] problems with multiple outputs with numpy.nditer

George Nurser gnurser@gmail....
Wed Aug 10 11:55:59 CDT 2011


Works fine with the [...]s.
Thanks very much.

--George

On 10 August 2011 17:15, Mark Wiebe <mwwiebe@gmail.com> wrote:
> On Wed, Aug 10, 2011 at 3:45 AM, George Nurser <gnurser@gmail.com> wrote:
>>
>> Hi,
>> I'm running numpy 1.6.1rc2 + python 2.7.1 64-bit from python.org on OSX
>> 10.6.8.
>>
>> I have a f2py'd fortran routine that inputs and outputs fortran real*8
>> scalars, and I normally call it like
>>
>> tu,tv,E,El,IF,HF,HFI = LW.rotate2u(u,v,NN,ff,0)
>>
>> I now want to call it over 2D arrays UT,VT,N,f
>>
>> Using steam-age indexing works fine:
>>
>> mflux_east,mflux_north,IWE,IWE_lin,InvFr,HFroude =
>> np.empty([6,ny-1,nx],dtype=np.float64)
>> for j in range(ny-1):
>>   for i in range(nx):
>>       u,v,NN,ff = [x[j,i] for x in UT,VT,N,f]
>>
>> mflux_east[j,i],mflux_north[j,i],IWE[j,i],IWE_lin[j,i],InvFr[j,i],HFroude[j,i],HFI
>> = LW.rotate2u(u,v,NN,ff,0)
>>
>>
>>
>> I decided to try the new nditer option, with
>>
>> it = np.nditer([UT,VT,N,f,None,None,None,None,None,None,None]
>>              ,op_flags=4*[['readonly']]+7*[['writeonly','allocate']]
>>              ,op_dtypes=np.float64)
>> for (u,v,NN,ff,tu,tv,E,El,IF,HF,HFI) in it:
>>   tu,tv,E,El,IF,HF,HFI = LW.rotate2u(u,v,NN,ff,0)
>>
>>
>> Unfortunately this doesn't seem to work. Writing
>> aa,bb,cc,dd,ee,ff,gg = it.operands[4:]
>
> One problem here is that the assignment needs to assign into the view the
> iterator gives, something a direct assignment doesn't actually do. Instead
> of
> a, b = f(c,d)
> you need to write it like
> a[...], b[...] = f(c,d)
> so that the actual values being iterated get modified. Here's what I get:
> In [7]: a = np.arange(5.)
> In [8]: b, c, d = a + 1, a + 2, a + 3
> In [9]: it = np.nditer([a,b,c,d] + [None]*7,
>    ...:         op_flags=4*[['readonly']]+7*[['writeonly','allocate']],
>    ...:         op_dtypes=np.float64)
> In [10]: for (x,y,z,w,A,B,C,D,E,F,G) in it:
>    ....:     A[...], B[...], C[...], D[...], E[...], F[...], G[...] = x, y,
> z, w, x+y, y+z, z+w
>    ....:
> In [11]: it.operands[4]
> Out[11]: array([ 0.,  1.,  2.,  3.,  4.])
> In [12]: it.operands[5]
> Out[12]: array([ 1.,  2.,  3.,  4.,  5.])
> In [13]: it.operands[6]
> Out[13]: array([ 2.,  3.,  4.,  5.,  6.])
> In [14]: it.operands[7]
> Out[14]: array([ 3.,  4.,  5.,  6.,  7.])
> In [15]: it.operands[8]
> Out[15]: array([ 1.,  3.,  5.,  7.,  9.])
> In [16]: it.operands[9]
> Out[16]: array([  3.,   5.,   7.,   9.,  11.])
> In [17]: it.operands[10]
> Out[17]: array([  5.,   7.,   9.,  11.,  13.])
>
> -Mark
>
>>
>> aa seems to contain the contents of UT (bizarrely rescaled to lie
>> between 0 and 1), while bb,cc etc are all zero.
>>
>>
>> I'm not sure whether I've just called it incorrectly, or whether
>> perhaps it's only supposed to work with one output array.
>>
>>
>> --George Nurser.
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>
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