gogurt - 1 year ago 79
Python Question

# Drawing a random number from set of numbers excluding those given in certain sets

Suppose I have a two arrays:

``````import numpy as np
a = np.random.randint(0,10,10)
b = np.random.randint(0,10,10)
``````

I want to generate another length-10 array whose i-th entry is a random integer drawn from the set (
`{0...9}`
minus the elements
`a[i]`
and
`b[i]`
).

Being a relative rookie when it comes to NumPy, I thought the easiest way to do this might be:

1. obtain the set difference
`x = {0...9} - (a[i] union b[i])`
for each
`i`

2. do
`np.random.choice(x[i], 1)`
for each
`i`

But I'm finding this a little tricky because I can't figure out how to map
`setdiff1d`
elementwise across 2 arrays. Is there an obvious way to do this in NumPy (i.e. ideally without having to resort to Python sets etc.)?

Here is one way:

``````In [87]: col = np.array((a, b)).T

In [88]: r = np.arange(10)

In [89]: np.ravel([np.random.choice(np.setdiff1d(r, i), 1) for i in col])
Out[89]: array([7, 8, 8, 6, 6, 8, 6, 5, 5, 6])
``````

Or as a numpytonic approach:

``````In [101]: def func(x):
return np.random.choice(np.setdiff1d(r, x), 1)
.....:

In [102]: np.apply_along_axis(func, 1, col).ravel()
Out[102]: array([6, 7, 9, 6, 4, 6, 7, 4, 0, 7])
``````
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