[Numpy-discussion] loop through values in a array and find maximum as looping

[josef.pktd at gmail.com]

On Tue, Dec 6, 2011 at 9:36 PM, Olivier Delalleau <shish at keba.be> wrote:
> The "out=a" keyword will ensure your first array will keep being updated. So
> you can do something like:
>
> a = my_list_of_arrays[0]
> for b in my_list_of_arrays[1:]:
>   numpy.maximum(a, b, out=a)

I didn't think of the out argument which makes it more efficient, but
in my example I used Python's reduce which takes an iterable and not
one huge array.

Josef


>
> -=- Olivier
>
> 2011/12/6 questions anon <questions.anon at gmail.com>
>>
>> thanks for all of your help, that does look appropriate but I am not sure
>> how to loop it over thousands of files.
>> I need to keep the first array to compare with but replace any greater
>> values as I loop through each array comparing back to the same array. does
>> that make sense?
>>
>>
>> On Wed, Dec 7, 2011 at 1:12 PM, Olivier Delalleau <shish at keba.be> wrote:
>>>
>>> Thanks, I didn't know you could specify the out array :)
>>>
>>> (to the OP: my initial suggestion, although probably not very efficient,
>>> seems to work with 2D arrays too, so I have no idea why it didn't work for
>>> you -- but Nathaniel's one seems to be the ideal one anyway).
>>>
>>> -=- Olivier
>>>
>>>
>>> 2011/12/6 Nathaniel Smith <njs at pobox.com>
>>>>
>>>> I think you want
>>>>   np.maximum(a, b, out=a)
>>>>
>>>> - Nathaniel
>>>>
>>>> On Dec 6, 2011 9:04 PM, "questions anon" <questions.anon at gmail.com>
>>>> wrote:
>>>>>
>>>>> thanks for responding Josef but that is not really what I am looking
>>>>> for, I have a multidimensional array and if the next array has any values
>>>>> greater than what is in my first array I want to replace them. The data are
>>>>> contained in netcdf files.
>>>>> I can achieve what I want if I combine all of my arrays using numpy
>>>>> concatenate and then using the command numpy.max(myarray, axis=0) but
>>>>> because I have so many arrays I end up with a memory error so I need to find
>>>>> a way to get the maximum while looping.
>>>>>
>>>>>
>>>>>
>>>>> On Wed, Dec 7, 2011 at 12:36 PM, <josef.pktd at gmail.com> wrote:
>>>>>>
>>>>>> On Tue, Dec 6, 2011 at 7:55 PM, Olivier Delalleau <shish at keba.be>
>>>>>> wrote:
>>>>>> > It may not be the most efficient way to do this, but you can do:
>>>>>> > mask = b > a
>>>>>> > a[mask] = b[mask]
>>>>>> >
>>>>>> > -=- Olivier
>>>>>> >
>>>>>> > 2011/12/6 questions anon <questions.anon at gmail.com>
>>>>>> >>
>>>>>> >> I would like to produce an array with the maximum values out of
>>>>>> >> many
>>>>>> >> (10000s) of arrays.
>>>>>> >> I need to loop through many multidimentional arrays and if a value
>>>>>> >> is
>>>>>> >> larger (in the same place as the previous array) then I would like
>>>>>> >> that
>>>>>> >> value to replace it.
>>>>>> >>
>>>>>> >> e.g.
>>>>>> >> a=[1,1,2,2
>>>>>> >> 11,2,2
>>>>>> >> 1,1,2,2]
>>>>>> >> b=[1,1,3,2
>>>>>> >> 2,1,0,0
>>>>>> >> 1,1,2,0]
>>>>>> >>
>>>>>> >> where b>a replace with value in b, so the new a should be :
>>>>>> >>
>>>>>> >> a=[1,1,3,2]
>>>>>> >> 2,1,2,2
>>>>>> >> 1,1,2,2]
>>>>>> >>
>>>>>> >> and then keep looping through many arrays and replace whenever
>>>>>> >> value is
>>>>>> >> larger.
>>>>>> >>
>>>>>> >> I have tried numpy.putmask but that results in
>>>>>> >> TypeError: putmask() argument 1 must be numpy.ndarray, not list
>>>>>> >> Any other ideas? Thanks
>>>>>>
>>>>>> if I understand correctly it's a minimum.reduce
>>>>>>
>>>>>> numpy
>>>>>>
>>>>>> >>> a = np.concatenate((np.arange(5)[::-1],
>>>>>> >>> np.arange(5)))*np.ones((4,3,1))
>>>>>> >>> np.minimum.reduce(a, axis=2)
>>>>>> array([[ 0.,  0.,  0.],
>>>>>>       [ 0.,  0.,  0.],
>>>>>>       [ 0.,  0.,  0.],
>>>>>>       [ 0.,  0.,  0.]])
>>>>>> >>> a.T.shape
>>>>>> (10, 3, 4)
>>>>>>
>>>>>> python with iterable
>>>>>>
>>>>>> >>> reduce(np.maximum, a.T)
>>>>>> array([[ 4.,  4.,  4.,  4.],
>>>>>>       [ 4.,  4.,  4.,  4.],
>>>>>>       [ 4.,  4.,  4.,  4.]])
>>>>>> >>> reduce(np.minimum, a.T)
>>>>>> array([[ 0.,  0.,  0.,  0.],
>>>>>>       [ 0.,  0.,  0.,  0.],
>>>>>>       [ 0.,  0.,  0.,  0.]])
>>>>>>
>>>>>> Josef
>>>>>>
>>>>>> >>
>>>>>> >> _______________________________________________
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>>>>>> >>
>>>>>> >
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