Extract specific elements from array to create ranges

Question

I want to extract very specific elements from an array to create various ranges.

For example,

ranges = np.arange(0, 525, 25)
#array([  0,  25,  50,  75, 100, 125, 150, 175, 200, 225, 250, 275, 300, 325, 350, 375, 400, 425, 450, 475, 500])

I want to create ranges that output as this-

I know if I want to extract every number into a range, I can write something like this-

for start, end in zip(ranges[:-1], ranges[1:]):
    print(start, end)

Which would give the result-

However, I'm not sure how to extract every other element from the array.

Your ranges are not uniform, some with length 50, one with length 25. What's the logics here? — Quang Hoang
– Quang Hoang, Commented Aug 11, 2020 at 19:49
Requirements keep changing arbitrarily, but np.stack((np.arange(0, 500, 75), np.arange(50, 550, 75)), axis=1) seems like what you want? — ggorlen
– ggorlen, Commented Aug 11, 2020 at 19:54
ranges = np.arange(0, 525, 25); ranges.reshape(-1,3)[:,[0,2]]? — Quang Hoang
– Quang Hoang, Commented Aug 11, 2020 at 19:56

Mad Physicist · Accepted Answer · 2020-08-11 21:31:42Z

1

There is no need to stack anything here, or create more buffers than the one you already have. A couple of reshapes and simple indices should do the trick.

Notice that you are taking the first and last element in a sequence of three. That means you can truncate your array to a multiple of three, reshape to have N rows of three elements, and simply extract the first and last one of each.

k = 3
ranges = np.arange(0, 525, 25)
ranges[:ranges.size - ranges.size % k].reshape(-1, k)[:, ::k - 1]

Here is a breakdown of the one-liner:

trimmed_to_multiple = ranges[:ranges.size - ranges.size % k]
reshaped_to_n_by_k = trimmed_to_multiple.reshape(-1, k)
first_and_last_column_only = reshaped_to_n_by_k[:, ::k - 1]

answered Aug 11, 2020 at 21:31

Mad Physicist

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ggorlen · Accepted Answer · 2020-08-11 19:57:53Z

0

You can create your two ranges, then stack them on the column axis:

np.stack((np.arange(0, 500, 75), np.arange(50, 550, 75)), axis=1)

Generalized:

>>> start = 50
>>> step = 75
>>> end = 550
>>> np.stack((np.arange(0, end - start, step), np.arange(start, end, step)), axis=1)
array([[  0,  50],
       [ 75, 125],
       [150, 200],
       [225, 275],
       [300, 350],
       [375, 425],
       [450, 500]])

answered Aug 11, 2020 at 19:57

ggorlen

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David Kaftan · Accepted Answer · 2020-08-11 20:09:27Z

0

lowestMidPoint, highestMidPoint = 25, 475
ranges = [[midpoint-25, midpoint+25] for midpoint in range(lowestMidPoint,highestMidPoint+1,75)]

will get you the results as an array of arrays. You can then just call np.array(ranges) to get it in numpy.

To answer your question "how do i get every other entry in the array?"... if you have a numpy array

you can slice out as follows

stride = 2
out[::stride,:]

to get every other row

answered Aug 11, 2020 at 20:09

David Kaftan

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