Converting a matlab script into python

Question

I'm an undergrad at university particapting in a research credit with a professor, so this is pretty much an independent project for me.

I am converting a matlab script into a python (3.4) script for easier use on the rest of my project. The 'find' function is employed in the script, like so:

keyindx = find(summags>=cumthresh,1)

Keyindx would contain the location of the first value inside summag above cumthresh

So, as an example:

summags = [ 1 4 8 16 19]
cumthresh = 5

then keyindx would return with an index of 2, whose element corresponds to 8.

My question is, I am trying to find a similar function in python (I am also using numpy and can use whatever library I need) that will work the same way. I mean, coming from a background in C I know how to get everything I need, but I figure there's a better way to do this then just write some C style code.

So, any hints about where to look in the python docs and about finding useful functions in general?

• Liquidmetal: Also, if you downvote me then could you please offer a reason? I'm not sure what about my question wasn't good...
• Andy Hayden: Not the downvoter, but I suspect the reason is you don't show what you've tried nor give a small example array which demonstrates the behaviour. What does find do, what have you tried etc.
• Chris Taylor: @AndyHayden I think it's reasonable to expect that anyone answering a question tagged "matlab" will know what the find function does!
• Liquidmetal: Thanks Andy and Chris, I appreciate the feedback and have updated my question as well as fixed an error in what I thought find returned.
• knedlsepp: possible duplicate of MATLAB-style find() function in Python

Answer 1

A quick search led me to the argwhere function which you can combine with [0] to get the first index satisfying your condition. For example,

>> import numpy as np
>> x = np.array(range(1,10))
>> np.argwhere(x > 5)[0]
array([5])

This isn't quite the same as saying

find(x > 5, 1)

in MATLAB, since the Python code will throw an IndexError if none of the values satisfy your condition (whereas MATLAB returns an empty array). However, you can catch this and deal with it appropriately, for example

try:
 ind = np.argwhere(x > 5)[0]
except IndexError:
 ind = np.array([1])

Answer 2

np.nonzero(x) gives a tuple of the nonzero indices. That value can then be used to index any array of the matching size.

In [1262]: x=np.arange(6).reshape(2,3)
In [1263]: ind=np.nonzero(x>3)
In [1264]: x[ind]
Out[1264]: array([4, 5])
In [1265]: ind
Out[1265]: (array([1, 1], dtype=int32), array([1, 2], dtype=int32))

• Liquidmetal: Thank you for the suggestion! Still getting used to pythonic style, very new to it. :)