How to use numpy.where with logical operators

python numpy where logical-operators
rferdinand picture rferdinand · Nov 27, 2012 · Viewed 57.9k times · Source

I'm trying to find the indices of all elements in an array that are greater than a but less than b. It's probably just a problem with my syntax but this doesn't work:

numpy.where((my_array > a) and (my_array < b))

How should I fix this? Or is there a better way to do it?

Thanks!

Answer

Mark Dickinson picture Mark Dickinson · Nov 27, 2012

Here are two ways:

In [1]: my_array = arange(10)

In [2]: where((my_array > 3) & (my_array < 7))
Out[2]: (array([4, 5, 6]),)

In [3]: where(logical_and(my_array > 3, my_array < 7))
Out[3]: (array([4, 5, 6]),)

For the first (replacing and with &), be careful to add parentheses appropriately: & has higher precedence than the comparison operators. You can also use *, but I wouldn't recommend it: it's hacky and doesn't make for readable code.

In [4]: where((my_array > 3) * (my_array < 7))
Out[4]: (array([4, 5, 6]),)

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