merging multiple numpy arrays

Question

i have 3 numpy arrays which store image data of shape (4,100,100).

arr1= np.load(r'C:\Users\x\Desktop\py\output\a1.npy')
arr2= np.load(r'C:\Users\x\Desktop\py\output\a2.npy')
arr3= np.load(r'C:\Users\x\Desktop\py\output\a3.npy')

I want to merge all 3 arrays into 1 array. I have tried in this way:

merg_arr = np.zeros((len(arr1)+len(arr2)+len(arr3), 4,100,100), dtype=input_img.dtype)

now this make an array of the required length but I don't know how to copy all the data in this array. may be using a loop?

What you man by merge into one array? do you want to concatenate them? do you want to sum up the pixel values? What would be the final shape of your array? — Kasravnd
– Kasravnd, Commented May 8, 2017 at 12:33
Oh.. i missed the (len(arr1)+len(arr2)+len(arr3), 4, ... part. Can you explaian why you want a shape of [12, 4, 100, 100]? Either [3, 4, 100, 100] or [12, 100, 100] would seem to make more sense. — MB-F
– MB-F, Commented May 8, 2017 at 12:38
i have 3 arrays of shape (60, 4,100,100), (14, 4,100,100), (6, 4,100,100).. I want them in a single array of shape (80,4,100,100) — FJ_Abbasi
– FJ_Abbasi, Commented May 8, 2017 at 12:42
I'm taking off the duplicate flag. That SO question dealt with 1d arrays, which need vstack. Here the arrays have the right shape to concatenate on the 1st dimension. It's evident from the comments that the problem isn't with concatenate. Something else is going on. — hpaulj
– hpaulj, Commented May 8, 2017 at 16:40

MB-F · Accepted Answer · 2017-05-08 12:40:52Z

7

This will do the trick:

merge_arr = np.concatenate([arr1, arr2, arr3], axis=0)

np.stack arranges arrays along a new dimension. Their dimensions (except for the first) need to match.

Demo:

arr1 = np.empty((60, 4, 10, 10))
arr2 = np.empty((14, 4, 10, 10))
arr3 = np.empty((6, 4, 10, 10))
merge_arr = np.concatenate([arr1, arr2, arr3], axis=0)
print(merge_arr.shape)  # (80, 4, 10, 10)

edited May 8, 2017 at 12:40

answered May 8, 2017 at 12:32

MB-F

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6 Comments

FJ_Abbasi Over a year ago

all of them are of different size, like (60,4,100,100), (14,4,100,100),(6,4,100,100)

MB-F Over a year ago

@FahadJahangir you did not say so in the question ;)

FJ_Abbasi Over a year ago

so here is the problem! the resultant array has all zeros in it. like this: [[[[0 0 0 ..., 0 0 0] [0 0 0 ..., 0 0 0] [0 0 0 ..., 0 0 0] ...,

MB-F Over a year ago

@FahadJahangir time to check if your input arrays have all zeros. Or maybe there are zeros on the borders only. Try np.unique(merge_arr) to see if there are really only zeros in an array.

FJ_Abbasi Over a year ago

well the input arrays don't have zeros.. they have data like [[[[215 185 118 ..., 60 25 18] [228 185 90 ..., 42 18 25] [219 172 100 ..., 18 18 81] ..., [240 172 100 ..., 118 118 134] [209 100 70 ..., 81 81 118] [126 25 60 ..., 60 51 90]]

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