indexing.rst

Bitarray indexing

Bitarrays can be indexed like usual Python lists. They support slice indexing and assignment:

>>> from bitarray import bitarray
>>> a = bitarray('01000001 01000010 01000011')
>>> a[1::3]
bitarray('10100001')
>>> a[8:20:2] = bitarray('110111')
>>> a
bitarray('010000011110001011100011')
>>> del a[::2]  # remove every second element
>>> a
bitarray('100110001001')
>>> a[::3] = 0  # set every third element to 0
>>> a
bitarray('000010001001')

Integer sequence indexing

As of bitarray version 2.8, indices may also be lists of arbitrary indices (like in NumPy). Negative values are permitted in the index list and work as they do with single indices or slices. For example:

>>> a = bitarray(12)
>>> a.setall(0)
>>> a[[1, 2, 5, 7]] = 1  # set elements 1, 2, 5, 7 to value 1
>>> a
bitarray('011001010000')
>>> a[[-1, -2, 1, 0]]
bitarray('0010')
>>> del a[[0, 1, 5, 8, 9]]
>>> a
bitarray('1000100')
>>> a[[1, 2, 4]] = bitarray('010')  # assign indices to elements
>>> a
bitarray('1010000')

Masked indexing

Also, as of bitarray version 2.8, indices may be bitarrays which are considered masks. For example:

>>> a =    bitarray('1001001')
>>> mask = bitarray('1010111')
>>> a[mask]  # create bitarray with items from `a` whose mask is 1
bitarray('10001')
>>> del a[mask]  # delete items in `a` whose mask is 1
>>> a
bitarray('01')

Note that del a[mask] is equivalent to the in-place version of selecting the reverse mask a = a[~mask].

Also note that masked assignment is not implemented, as a[mask] = 1 would be equivalent to the bitwise operation a |= mask. And a[mask] = 0 would be equivalent to a &= ~mask.