Rearranging bits in python?

Question

Let's say I have A = 0b11110101.

I want to make a new integer like this pseudocode

B[0] = A[4]
B[1] = A[7]
B[2] = A[0]
B[3] = A[6]
B[4] = A[1]
B[5] = A[2]
B[6] = A[3]
B[7] = A[5]

How can I do that?

Maybe I can create an array B first, but at the end I need an integer/binary.

Update #1

I made something like this:

num=0
pc1_tidyHex = []

pc1_tidyHex.append([tidyHex[56], tidyHex[48], tidyHex[40], tidyHex[32], tidyHex[24], tidyHex[16], tidyHex[8]])
num = int(''.join(map(str, pc1_tidyHex)))    
print(pc1_tidyHex)
print(num)

and I got this error:

Traceback (most recent call last):
  File "F:\TUC Aux\python\budi.py", line 47, in <module>
    num = int(''.join(map(str, pc1_tidyHex)))
ValueError: invalid literal for int() with base 10: "['0', '1', '1', '0', '1', '0', '0']"

wiki.python.org/moin/BitManipulation

Serge
– Serge

2017-06-10 00:28:52 +00:00
Commented Jun 10, 2017 at 0:28 — Serge
– Serge, Commented Jun 10, 2017 at 0:28

Skycc · Accepted Answer · 2017-06-10 07:37:46Z

4

convert integer A into binary string from LSB to MSB then manipulate the bit with indexing

bin_map = [5,3,2,1,6,0,7,4]
A = 0b11110101
binA = bin(A)[2:][::-1]
B = int(''.join(binA[i] for i in bin_map), 2)
# B will be integer 175 or binary 0b10101111

edited Jun 10, 2017 at 7:37

answered Jun 10, 2017 at 3:10

Skycc

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2 Comments

Codelearner777 Over a year ago

Hi, thanks for your idea. I think I can use this idea on my next step. What should I do if I want variable B to be an array?

Skycc Over a year ago

To keep B as array, just replace the last line of my code become B = [binA[i] for i in bin_map]

Mohd · Accepted Answer · 2017-06-10 00:27:02Z

1

Once you are done with your array you can use:

num = int(''.join(map(str, B)))

and you will have your integer. If you want it as string, remove the int()

answered Jun 10, 2017 at 0:27

Mohd

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5 Comments

Codelearner777 Over a year ago

Hi, I got an error, I updated my post above. That is the only way that I know how to build an array, this is my first doing python.

Mohd Over a year ago

@Codelearner777 this is because you are appending an array to the array. In that case you can change to num = int(''.join(pc1_tidyHex[0]))

Mohd Over a year ago

And about creating an array, you can directly create the array with the values: pc1_tidyHex = [tidyHex[56], tidyHex[48], tidyHex[40],..etc] but then you need to change to num = int(''.join(pc1_tidyHex))

Yann Vernier Over a year ago

This answer doesn't operate in a bitwise manner. you want to use the base argument of int.

Mohd Over a year ago

@Codelearner777 Don't forget to accept the answer if you find it useful =)

zwer · Accepted Answer · 2017-06-10 01:36:18Z

0

You cannot use string join to turn an array of bits into an integer. You need to do some bit shifting:

A = 0b11110101

b_map = [5, 3, 2, 1, 6, 0, 7, 4]  # map of bits from your `A`, high->low order

B = 0  # initialize as 0
for target_position in b_map:  # loop through our positions
    B = (B << 1) | (A >> target_position & 1)  # shift by one, pick the target bit from A

print("B: {} (binary: {})".format(B, bin(B)))
# B: 175 (0b10101111)

Not exactly sure if that's what you're attempting to do, tho, your question is a bit confusing.

answered Jun 10, 2017 at 1:36

zwer

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1 Comment

Yann Vernier Over a year ago

Sure you can, though it's convoluted and inefficient: int(''.join(str(e) for e in array), 2) compared to something like sum(v<<i for i,v in enumerate(array))

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