numpy boolean array with 1 bit entries

Question

Is there a way in numpy to create an array of booleans that uses just 1 bit for each entry?

The standard np.bool type is 1 byte, but this way I use 8 times the required memory.

On Google I found that C++ has std::vector<bool>.

Answer 1

To do this you can use numpy's packbits and unpackbits:

import numpy as np # original boolean array A1 = np.array([     [0, 1, 1, 0, 1],     [0, 0, 1, 1, 1],     [1, 1, 1, 1, 1], ], dtype=bool)  # packed data A2 = np.packbits(A1, axis=None)  # checking the size print(len(A1.tostring())) # 15 bytes print(len(A2.tostring())) #  2 bytes (ceil(15/8))  # reconstructing from packed data. You need to resize and reshape A3 = np.unpackbits(A2, count=A1.size).reshape(A1.shape).view(bool)  # and the arrays are equal print(np.array_equal(A1, A3)) # True 

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Prior to numpy 1.17.0, the first function is straight-forward to use, but reconstruction required additional manipulations. Here is an example:

import numpy as np # original boolean array A1 = np.array([     [0, 1, 1, 0, 1],     [0, 0, 1, 1, 1],     [1, 1, 1, 1, 1], ], dtype=np.bool)  # packed data A2 = np.packbits(A1, axis=None)  # checking the size print(len(A1.tostring())) # 15 bytes print(len(A2.tostring())) #  2 bytes (ceil(15/8))  # reconstructing from packed data. You need to resize and reshape A3 = np.unpackbits(A2, axis=None)[:A1.size].reshape(A1.shape).astype(np.bool)  # and the arrays are equal print(np.array_equal(A1, A3)) # True 

Answer 2

You want a bitarray:

efficient arrays of booleans -- C extension

This module provides an object type which efficiently represents an array of booleans. Bitarrays are sequence types and behave very much like usual lists. Eight bits are represented by one byte in a contiguous block of memory. The user can select between two representations; little-endian and big-endian. All of the functionality is implemented in C. Methods for accessing the machine representation are provided. This can be useful when bit level access to binary files is required, such as portable bitmap image files (.pbm). Also, when dealing with compressed data which uses variable bit length encoding, you may find this module useful...