Filter a 2D numpy array

Question

I want to have a sub array (between min and max) of a numpy 2D ndarray

    xy_dat = get_xydata()
    x_displayed = xy_dat[((xy_dat > min) & (xy_dat < max))]

min and max are float in order to be compare with the first value of the array xy_dat

xy_dat is a 2D numpy array :

[[ 735964.            1020.        ]
 [ 735964.04166667    1020.        ]
 [ 735964.08333333    1020.        ]
 ..., 
 [ 736613.39722222    1095.        ]
 [ 736613.40416667    1100.        ]
 [ 736613.41111111    1105.        ]]

x_displayed is correctly filtered but I have lost the second value (it is now a 1D array) :

[ 735964.04166667  735964.08333333  735964.125      
 ...,  
736613.39027778  736613.39722222  736613.40416667]

How make the filter on the first value and keep the other ?

That's because your comparison is not 2D. For example what does it mean is you have two numbers in one row and one is inside your range and one is not? — piman314
– piman314, Commented Dec 19, 2017 at 11:23

willeM_ Van Onsem · Accepted Answer · 2017-12-19 11:26:33Z

15

You should perform the condition only over the first column:

x_displayed = xy_dat[((xy_dat[:,0] > min) & (xy_dat[:,0] < max))]

What we do here is constructing a view where we only take into account the first column with xy_dat[:,0]. By now checking if this 1d is between bounds, we construct a 1D boolean array of the rows we should retain, and now we make a selection of these rows by using it as item in the xy_dat[..] parameter.

edited Dec 19, 2017 at 11:26

answered Dec 19, 2017 at 11:24

willeM_ Van Onsem

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5 Comments

IMCoins Over a year ago

[:,0] means through all the first array, with only the first element of the second one ? Didn't know, that's neat !

willeM_ Van Onsem Over a year ago

@IMCoins: it means the first column of every row yes :)

IMCoins Over a year ago

Oh right, it only works with numpy.arrays. I tried it on some python arrays, and some incredible sadness came into my eyes. :'(

cs95 Over a year ago

@IMCoins You can always subclass lists and overload __getitem__ yourself :)

willeM_ Van Onsem Over a year ago

@IMCoins: well we can construct a subclass of the Python vanilla lists to let it work with lists as well. A possible problem however is that in Python items can be hybrid and the sublists can have different size. So how would you work with [1, [], [14], 'ab']? Simply ignore non-sublists and empty lists?

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