Karthik - 3 months ago 25

Python Question

I want to get the intersecting (common) rows across two 2D numpy arrays. E.g., if the following arrays are passed as inputs:

`array([[1, 4],`

[2, 5],

[3, 6]])

array([[1, 4],

[3, 6],

[7, 8]])

the output should be:

`array([[1, 4],`

[3, 6])

I know how to do this with loops. I'm looking at a Pythonic/Numpy way to do this.

Answer

For short arrays, using sets is probably the clearest and most readable way to do it.

Another way is to use `numpy.intersect1d`

. You'll have to trick it into treating the rows as a single value, though... This makes things a bit less readable...

```
import numpy as np
A = np.array([[1,4],[2,5],[3,6]])
B = np.array([[1,4],[3,6],[7,8]])
nrows, ncols = A.shape
dtype={'names':['f{}'.format(i) for i in range(ncols)],
'formats':ncols * [A.dtype]}
C = np.intersect1d(A.view(dtype), B.view(dtype))
# This last bit is optional if you're okay with "C" being a structured array...
C = C.view(A.dtype).reshape(-1, ncols)
```

For large arrays, this should be considerably faster than using sets.