[SciPy-user] probably a stupid question: MatLab equivalent of "diff" ?

Stef Mientki s.mientki at ru.nl
Fri Dec 29 10:37:32 CST 2006


thanks Tom, Otto,Lev,
for the fast respons,
but the problem still exists (btw I'm fairly newbie, for those didn't 
discovered that ;-).

- I import SciPy

- I do some calculations to find specific events, and the last 
calculation is
      BP_detect = ( dBP_dt > 0 )   &   ( IOO > 40000 )

- now this is really great (compared to MatLab), because the resulting 
array is a real boolean array,
so it only consists of True and False and presumably only takes 8 bits 
per value ???

- now if I run some standard diff formule, I'll detect both the edges,
I expect that is due to the True/False elements of the array,
probably they are internally translated to +1 and -1 ???

- even this gives both edges
     BP_peaks = numpy.diff ( BP_detect)  > 0

- in the meanwhile I found a algorithm that works, but I find it very ugly
( I process the data in chunks, therefor I use and save the last sample 
"BP_peak_prev")

    BP_peak = logical_and(  concatenate((   ([BP_peak_prev]),  
BP_detect[:-1])),   logical_not(BP_detect)   )
    BP_peak_prev = BP_detect[-1]

Any Ideas ?
thanks,
Stef Mientki


Tom Denniston wrote:
> Ignore, mine.  Otto's answer is better.  I didn't see it.
>
> On 12/29/06, Tom Denniston <tom.denniston at alum.dartmouth.org> wrote:
>   
>> I may be misunderstanding you but isn't what you want just:
>>
>> arr[1:]-arr[:-1]
>>
>> If you only want positive differences it's:
>>
>> (arr[1:]-arr[:-1]) > 0
>>
>> or negative:
>>
>> (arr[1:]-arr[:-1]) < 0
>>
>> These should be very fast.
>>
>> --Tom
>>
>>
>> On 12/29/06, Stef Mientki <s.mientki at ru.nl> wrote:
>>     
>>> Does anyone know the equivalent of the MatLab "diff" function.
>>> The "diff" functions calculates the difference between 2 succeeding
>>> elements of an array.
>>> I need to detect (fast) the falling edge of a binary signal.
>>>
>>> There's derivate function in Python diff,
>>> but when you use an binary (true/false) input,
>>> it also detects rising edges.
>>>
>>> Probably a stupid question,
>>> but I still have troubles,
>>> digging to huge amount of information about Python.
>>>
>>> thanks,
>>> Stef Mientki
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>>> SciPy-user mailing list
>>> SciPy-user at scipy.org
>>> http://projects.scipy.org/mailman/listinfo/scipy-user
>>>
>>>       
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