[Tutor] finding a maximum between the absolute difference of several columns

[Elaina Ann Hyde]

On Fri, Feb 17, 2012 at 2:27 PM, Mark Lawrence <breamoreboy at yahoo.co.uk>wrote:

> On 17/02/2012 02:53, Elaina Ann Hyde wrote:
>
>> On Fri, Feb 17, 2012 at 1:41 PM, Rohan Sachdeva<rsachdev at usc.edu>  wrote:
>>
>>  The way I would do this is to put all of the differences in a list then
>>> take the maximum of the list. So..
>>>
>>> a = Vmatch3_1 - Vmatch3_2
>>> b = Vmatch3_1 - Vmatch3_3
>>> c = Vmatch3_3 - Vmatch3_2
>>>
>>> Vhel_fdiff3=max([a,b,c])
>>>
>>> That should work. a,b,c are pretty bad variable names...
>>>
>>> Rohan
>>>
>>> On Thu, Feb 16, 2012 at 5:04 PM, Elaina Ann Hyde<elainahyde at gmail.com>**
>>> wrote:
>>>
>>>  Hello all,
>>>>    I am still scripting away and have reached my next quandry, this one
>>>> is much simpler than the last, basically I read in a file with several
>>>> columns.
>>>> Vmatch3_1=dat[col1]
>>>> Vmatch3_2=dat[col2]
>>>> Vmatch3_3=dat[col3]
>>>> Vdav=5.0
>>>> Vhel_fdiff3=max(abs(Vmatch3_1 - Vmatch3_2),abs(Vmatch3_1 -
>>>> Vmatch3_3),abs(Vmatch3_3 - Vmatch3_2))
>>>> ----------------
>>>> What I would like this to return is the maximum difference in each case,
>>>> so I end up with one column which contains only the largest differences.
>>>> now I use this to write the condition:
>>>>
>>>> with_v1_3=(Vhel_fdiff3>= Vdav)
>>>>
>>>> I know the condition works and plots if
>>>>
>>>>  Vhel_fdiff3=(Vmatch3_1 - Vmatch3_2)
>>>>
>>>> for example, and I know this syntax would work if it was numbers instead
>>>> of columns.
>>>>
>>>>> max(abs(1-2),abs(3-7),abs(2-4)**)
>>>>>> 4
>>>>>>
>>>>>
>>>> The error is:
>>>> -----------------------
>>>> Traceback (most recent call last):
>>>>   File "double_plot.py", line 109, in<module>
>>>>     Vhel_fdiff3=max(abs(Vmatch3_1 - Vmatch3_2),abs(Vmatch3_1 -
>>>> Vmatch3_3),abs(Vmatch3_3 - Vmatch3_2))
>>>> ValueError: The truth value of an array with more than one element is
>>>> ambiguous. Use a.any() or a.all()
>>>> -----------------
>>>> So it can't handle the fact that it's columns of numbers and not single
>>>> numbers, so my question is, can I solve this without doing a loop around
>>>> it... use numpy, or some other function instead?
>>>> I've been searching around and haven't found any good ways forward so I
>>>> thought one of you might know.  Thanks
>>>> ~Elaina
>>>>
>>>> --
>>>> PhD Candidate
>>>> Department of Physics and Astronomy
>>>> Faculty of Science
>>>> Macquarie University
>>>> North Ryde, NSW 2109, Australia
>>>>
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>>>>
>>>>
>>>>
>>>
>> ------------------
>> Thanks for the replies so far.  I don't think that Rohan's idea solves the
>> numbers versus columns issue.  If I run it I get
>> -------------------
>> Traceback (most recent call last):
>>   File "double_plot.py", line 112, in<module>
>>     Vhel_fdiff3=max(a,b,c)
>> ValueError: The truth value of an array with more than one element is
>> ambiguous. Use a.any() or a.all()
>> ---------------------
>> which is just the same error, I looked at the forums and the link
>> suggested
>> and I guess maybe my problem is trickier than I first thought!
>> ~Elaina
>>
>> ______________________________**_________________
>> Tutor maillist  -  Tutor at python.org
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>>
>
> I've found this http://comments.gmane.org/**gmane.comp.python.tutor/72882<http://comments.gmane.org/gmane.comp.python.tutor/72882>
> .
>
> HTH.
>
> --
> Cheers.
>
> Mark Lawrence.
>
>
> ______________________________**_________________
> Tutor maillist  -  Tutor at python.org
> To unsubscribe or change subscription options:
> http://mail.python.org/**mailman/listinfo/tutor<http://mail.python.org/mailman/listinfo/tutor>
>




Interesting, thanks for all the ideas everyone.... I did finally get it to
work in a loop form.  Anyone wanting to save themselves lots of confusion,
firstly use .append, and secondly don't forget to convert created lists
back to arrays for numpy, here is the working code to look through columns,
match them, return and make an array to operate on with numpy:
-------------------------------------
Vhel_fdiff3=[]
for i in xrange(len(Vmatch3_1)):
    Vhel_fdiff3.append(max([abs(Vmatch3_1[i] -
Vmatch3_2[i]),abs(Vmatch3_1[i] - Vmatch3_3[i]),abs(Vmatch3_3[i] -
Vmatch3_2[i])]))

#print Vhel_fdiff3
#appending writes a list, but numpy which makes the with_ work needs an
array
Vhel_fdiff3=array(Vhel_fdiff3)
-------------------------------------
~Elaina
-- 
PhD Candidate
Department of Physics and Astronomy
Faculty of Science
Macquarie University
North Ryde, NSW 2109, Australia
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