[Numpy-discussion] strange behavior of variable

Sudheer Joseph sudheer.joseph@yahoo....
Sun Aug 18 03:50:45 CDT 2013


Thanks a lot Eric,
                    Here comes the smartness of python!!, I am just climbing the bottom steps...
with best regards,
Sudheer


 
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Sudheer Joseph 
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>________________________________
> From: Eric Firing <efiring@hawaii.edu>
>To: numpy-discussion@scipy.org 
>Sent: Sunday, 18 August 2013 1:57 PM
>Subject: Re: [Numpy-discussion] strange behavior of variable
> 
>
>On 2013/08/17 9:49 PM, Sudheer Joseph wrote:
>> Hi,
>>           I have defined a small function to find the n maximum values
>> of an array as below. With in it I assign the input array to a second
>> array and temporarily make the array location after first iteration as
>> nan. I expected this temporary change to be limited to the second
>> variable. However my initial variable gets modified. Can any one through
>> some light to what is happening here?. In case of matlab this logic works.
>>
>> ######
>> #FUNCTION maxn
>> ######
>> import numpy as np
>> def max_n(a,n):
>>       b=a
>
>This is not making "b" a copy of "a", it is simply making it an alias 
>for it.  To make it a copy you could use "b = a[:]", or "b = a.copy()"
>
>It sounds like you don't really need a function, however.  Try this:
>
># test data:
>a = np.random.randn(10)
>n = 2
>
># One-line solution:
>biggest_n = np.sort(a)[-n:]
>
>print a
>print biggest_n
>
>If you want them ordered from largest to smallest, just reverse the list:
>
>biggest_n = biggest_n[::-1]
>
>Eric
>
>
>>       result=[]
>>       for i in np.arange(1,n+1):
>>           mxidx=np.where(b==max(b))
>>           result.append(mxidx)
>>           b[mxidx]=np.nan
>>       result=np.ravel(result)
>>       return(result)
>>
>> ### TEST
>> In [8]: x=np.arange(float(0),10)
>>
>> In [9]: max
>> max    max_n
>>
>> In [9]: max_n(x,2)
>> Out[9]: array([9, 8])
>>
>> In [10]: x
>> Out[10]: array([  0.,   1.,   2.,   3.,   4.,   5.,   6.,   7.,  nan,  nan])
>> ***************************************************************
>> Sudheer Joseph
>> Indian National Centre for Ocean Information Services
>> Ministry of Earth Sciences, Govt. of India
>> POST BOX NO: 21, IDA Jeedeemetla P.O.
>> Via Pragathi Nagar,Kukatpally, Hyderabad; Pin:5000 55
>> Tel:+91-40-23886047(O),Fax:+91-40-23895011(O),
>> Tel:+91-40-23044600(R),Tel:+91-40-9440832534(Mobile)
>> E-mail:sjo.India@gmail.com;sudheer.joseph@yahoo.com
>> Web- http://oppamthadathil.tripod.com
>> ***************************************************************
>>
>>
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>>
>
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