Convert binary (0|1) numpy to integer or binary-string?

Question

Is there a shortcut to Convert binary (0|1) numpy array to integer or binary-string ? F.e.

b = np.array([0,0,0,0,0,1,0,1])   
  => b is 5

np.packbits(b)

works but only for 8 bit values ..if the numpy is 9 or more elements it generates 2 or more 8bit values. Another option would be to return a string of 0|1 ...

What I currently do is :

    ba = bitarray()
    ba.pack(b.astype(np.bool).tostring())
    #convert from bitarray 0|1 to integer
    result = int( ba.to01(), 2 )

which is ugly !!!

Answer 1

One way would be using dot-product with 2-powered range array -

b.dot(2**np.arange(b.size)[::-1])

Sample run -

In [95]: b = np.array([1,0,1,0,0,0,0,0,1,0,1])

In [96]: b.dot(2**np.arange(b.size)[::-1])
Out[96]: 1285

Alternatively, we could use bitwise left-shift operator to create the range array and thus get the desired output, like so -

b.dot(1 << np.arange(b.size)[::-1])

If timings are of interest -

In [148]: b = np.random.randint(0,2,(50))

In [149]: %timeit b.dot(2**np.arange(b.size)[::-1])
100000 loops, best of 3: 13.1 µs per loop

In [150]: %timeit b.dot(1 << np.arange(b.size)[::-1])
100000 loops, best of 3: 7.92 µs per loop

---

Reverse process

To retrieve back the binary array, use np.binary_repr alongwith np.fromstring -

In [96]: b = np.array([1,0,1,0,0,0,0,0,1,0,1])

In [97]: num = b.dot(2**np.arange(b.size)[::-1]) # integer

In [98]: np.fromstring(np.binary_repr(num), dtype='S1').astype(int)
Out[98]: array([1, 0, 1, 0, 0, 0, 0, 0, 1, 0, 1])

Answer 2

def binary_converter(arr):
    total = 0
    for index, val in enumerate(reversed(arr)):
        total += (val * 2**index)
    print total


In [14]: b = np.array([1,0,1,0,0,0,0,0,1,0,1])
In [15]: binary_converter(b)
1285
In [9]: b = np.array([0,0,0,0,0,1,0,1])
In [10]: binary_converter(b)
5

or

b = np.array([1,0,1,0,0,0,0,0,1,0,1])
sum(val * 2**index for index, val in enumerate(reversed(b)))

Answer 3

Using numpy for conversion limits you to 64-bit signed binary results. If you really want to use numpy and the 64-bit limit works for you a faster implementation using numpy is:

import numpy as np
def bin2int(bits):
    return np.right_shift(np.packbits(bits, -1), bits.size).squeeze()

Since normally if you are using numpy you care about speed then the fastest implementation for > 64-bit results is:

import gmpy2
def bin2int(bits):
    return gmpy2.pack(list(bits[::-1]), 1)

If you don't want to grab a dependency on gmpy2 this is a little slower but has no dependencies and supports > 64-bit results:

def bin2int(bits):
    total = 0
    for shift, j in enumerate(bits[::-1]):
        if j:
            total += 1 << shift
    return total

The observant will note some similarities in the last version to other Answers to this question with the main difference being the use of the << operator instead of **, in my testing this led to a significant improvement in speed.