[Numpy-discussion] Numpy Array filling

Damian Eads eads@soe.ucsc....
Thu Jul 3 10:53:24 CDT 2008


> On Thu, Jul 3, 2008 at 6:57 AM, Brain Stormer <brnstrmrs@gmail.com> wrote:
>> I am using numpy to create an array then filling some of the values
>> using a
>> for loop, I was wondering if there is way to easily fill the values
>> without
>> iterating through sort of like "array.fill[start:stop,start
>> :stop]"?  The reason for my question is, in some cases, I might have to
>> fill
>> hundreds (within a 10,000x10,000 matrix) of values and I am not sure if
>> iteration is the right way to go.
>>
>> Code:
>> from numpy import *
>> x, y = 5, 5
>> matrix = zeros((y,x), int)

Brain,

I would avoid calling your variable matrix since it collides with
numpy.matrix class, which you've imported with *.

The topic to look up in the documentation is called "slicing". You'll find
good explanations here.

   http://www.scipy.org/NumPy_for_Matlab_Users
   http://www.scipy.org/Tentative_NumPy_Tutorial
   http://www.tramy.us/   (Guide to NumPy)

Try to vectorize (avoid for loops) your code whenever possible. When for
loops are needed, use xrange for large loops.

I hope this helps. :-)

Damian


>> print matrix
>> fillxstart, fillystart = 1,1
>> fillxstop,fillystop = 4, 4
>> for i in range(fillystart,fillystop,1):
>>     for j in range(fillxstart,fillxstop,1):
>>         matrix[i,j] = 1
>> print matrix
>>
>> Output before filling:
>> [[0 0 0 0 0]
>>  [0 0 0 0 0]
>>  [0 0 0 0 0]
>>  [0 0 0 0 0]
>>  [0 0 0 0 0]]
>> Output after filling:
>> [[0 0 0 0 0]
>>  [0 1 1 1 0]
>>  [0 1 1 1 0]
>>  [0 1 1 1 0]
>>  [0 0 0 0 0]]
>
>>> matrix[fillxstart:fillxstop, fillystart:fillystop] = 1
>>> matrix
>
> array([[0, 0, 0, 0, 0],
>        [0, 1, 1, 1, 0],
>        [0, 1, 1, 1, 0],
>        [0, 1, 1, 1, 0],
>        [0, 0, 0, 0, 0]])
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