John Crow - 5 months ago 54

Python Question

What is the best way to delete the elements from one numpy array in another? Essentially I'm after

`np.delete()`

`import numpy as np`

a = np.array([2,1,3])

print a

b = np.array([4,1,2,5,2,3])

b = np.delete(b, a) # doesn't work as desired

print b # want [4,5,2]

Iterating over the elements of

`a`

Answer

You can use `np.argmax`

to find the first True element along a set of rows or columns. So, for example, you can do a broadcasted version of this operation this way:

```
>>> a = np.array([2,1,3])
>>> b = np.array([4,1,2,5,2,3])
>>> np.delete(b, np.argmax(b == a[:, np.newaxis], axis=1))
array([4, 5, 2])
```

Of course, as with many numpy vectorized operations, the speed comes at the cost of allocating an array of size `len(a) * len(b)`

, so depending on your inputs this may not be appropriate.