Adrian - 1 year ago 68
Python Question

# Looking for the fastest way to find the exact overlap between two arrays of equal length in numpy

I am looking for the optimal (fastest) way to find the exact overlap between two arrays in numpy. Given two arrays x and y

``````x = array([1,0,3,0,5,0,7,4],dtype=int)
y = array([1,4,0,0,5,0,6,4],dtype=int)
``````

What I want to get is, an array of the same length that contains only the numbers from both vectors that are equal:

``````array([1,0,0,0,5,0,0,4])
``````

First I tried

``````x&y
array([1,0,0,0,5,0,6,4])
``````

Then I realised that this is always true for two numbers if they are > 0.

``````result = numpy.where(x == y, x, 0)
``````

Have a look at `numpy.where` documentation for explanation. Basically, `numpy.where(a, b, c)`, for a condition `a` returns an array of shape `a`, and with values from `b` or `c`, depending upon whether the corresponding element of `a` is true or not. `b` or `c` can be scalars.

By the way, `x & y` is not necessarily "always true" for two positive numbers. It does bitwise-and for elements in `x` and `y`:

``````x = numpy.array([2**p for p in xrange(10)])
# x is [  1   2   4   8  16  32  64 128 256 512]
y = x - 1
# y is [  0   1   3   7  15  31  63 127 255 511]
x & y
# result: [0 0 0 0 0 0 0 0 0 0]
``````

This is because the bitwise representation of each element in `x` is of the form `1` followed by `n` zeros, and the corresponding element in `y` is `n` 1s. In general, for two non-zero numbers `a` and `b`, `a & b` may equal zero, or non-zero but not necessarily equal to either `a` or `b`.

Recommended from our users: Dynamic Network Monitoring from WhatsUp Gold from IPSwitch. Free Download