Remove sequential NumPy array index in a loop over it's content

Question

I'm iterating over a NumPy (nd) array of OpenCV lines. I want to remove all lines which are outwith 8 degrees of vertical. I realise the numpy array is immutable and what I'm doing in the code is not right but it demonstrates the idea of what I'm trying to do;

index = 0
for line in self.lines[0]:
    if (line[1]*180)/np.pi > 8:                                      
        self.lines[0] = np.delete(self.lines[0], index, axis=0)
    index+=1

How can I go about removing these NumPy array indexes?

Thanks!

A better explanation of problem is required if you want someone to help you... — Irshad Bhat
– Irshad Bhat, Commented Dec 22, 2014 at 14:48
What actually this code line if (line[1]*180)/np.pi > 8 does, or what are you trying to achieve with it? — Irshad Bhat
– Irshad Bhat, Commented Dec 22, 2014 at 15:02
it checks whether the line is greater than 8 degrees and if so removes that item from the NumPy array. The question is mainly about removing from a NumPy array or creating a new one without these in it — Mark Corrigan
– Mark Corrigan, Commented Dec 22, 2014 at 15:09

Irshad Bhat · Accepted Answer · 2014-12-22 15:35:44Z

4

You cannot delete array indexes while iterating over that array. It will give wrong results. Say you are at iteration 5 and this index satisfies if condition and needs to be deleted, but if we delete this it will cause array element at index 6 to come at index 5 and next element selected will be 7 which comes to index 6 after deletion and thus condition is never checked for element initially at index 6 i.e., before deletion.

Basic idea to deal with this issue is to append these indices to a list and delete them outside lope. So your code is:

index = 0
idx = []
for line in self.lines[0]:
    if (line[1]*180)/np.pi > 8:     
       idx.append(index)         
    index+=1

self.lines[0] = np.delete(self.lines[0], idx, axis=0)

answered Dec 22, 2014 at 15:35

Irshad Bhat

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1 Comment

Mark Corrigan Over a year ago

I get an error with your solution. ValueError: invalid entry

sebix · Accepted Answer · 2015-01-02 14:20:52Z

1

I guess you are missing indentation in your code, so I interpret it like this:

index = 0
for line in self.lines[0]:
    if (line[1]*180)/np.pi > 8:                                      
        self.lines[0] = np.delete(self.lines[0], index, axis=0)
    index+=1

You can do that of course without a loop. First we do the check to build the boolean indexing array (True/False values for every item in the array):

index = self.lines[0,:,1]*180/np.pi > 8

Then we select the lines, where the condition is false:

a[~a%2==0]

Where np.nonzero converts from a boolean array to an array, where all indices with True-values are listed.

For example:

>>> a  = np.arange(10)
>>> a[~a%2==0]
array([1, 3, 5, 7, 9])

edited Jan 2, 2015 at 14:20

answered Dec 22, 2014 at 15:11

sebix

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3 Comments

Mark Corrigan Over a year ago

what does your second last line do? not familiar with the syntax

sebix Over a year ago

@MarkCorrigan I added now more explanations, an example and links to the docs. This would be now the way how to achieve your problem in a 'Numpythonic' way.

hpaulj Over a year ago

That last delete could be replaced with boolean indexing: a[~a%2==0]. delete is a complex function that always returns a new array. It does not directly modify its input.

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