Question

If there is an binary number:10011100
It is 156 in decimal.

I want to use mathematics way to make binary to decimal.

For example:
binary: 10011100

the first number is 1: 2**7
the forth number is 1: 2**4
the fifth number is 1: 2**3
the sixth number is 1: 2**2

then 2**7+2**4+2**3+2**2 = 156

I think, I need to use string.find() method.

>>> my_str = '10011100'   
>>> my_str = my_str[::-1]   
>>> print(my_str)   
00111001   
>>> my_str.find('1')   
2   
>>>  

I just can find the first ‘1’.

How to find all the index of ‘1’?

Answer 1

Why do you want to retrieve the indexes? You can simply iterate over the bits like this:

num = sum(2**i for i, bit in enumerate(my_str) if bit == '1')

Anyway, you can get the indexes like this if you prefer two separate steps:

indexes = [i for i, bit in enumerate(my_str) if bit == '1']
num = sum(2**i for i in indexes)