Converting lower and upper 8 bits into one value with python

Question

I have pairs like these: (-102,-56), (123, -56). First value from the pairs represents the lower 8 bits and the second value represents the upper 8 bits, both are in signed decimal form. I need to convert these pairs into a single 16 bit values.

I think I was able to convert (-102,-56) pair by:

l = bin(-102 & 0b1111111111111111)[-8:]
u = bin(-56 & 0b1111111111111111)[-8:]
int(u+l,2)

But when I try to do the same with (123, -56) pair I get the following error:

ValueError: invalid literal for int() with base 2: '11001000b1111011'.

I understand that it's due to the different lengths for different values and I need to fill them up to 8 bits.

Am I approaching this completely wrong? What's the best way to do this so it works both on negative and positive values?

UPDATE: I was able to solve this by:

low_int = 123
up_int = -56
(low_int & 0xFF) | ((up_int & 0xFF) << 8)

What is the expected output when you input (-102, -56)? And for (123, -56)? — Be Chiller Too, May 27 '21 at 10:09
That I don't know unfortunately. All I know is what I described in the question. — Gvantsa, May 27 '21 at 10:10

Be Chiller Too · Answer 1 · 2021-05-27T10:18:22.207

0

You can try to shift the first value 8 bits: try to use the logic described here https://stackoverflow.com/a/1857965/8947333

edited May 27 '21 at 10:18

answered May 27 '21 at 10:02

Be Chiller Too

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score 0 · Answer 2 · answered May 27 '21 at 10:19

0

Just guessing

l, u = -102 & 255, -56 & 255
# shift 8 bits to left
u << 8 + l

answered May 27 '21 at 10:19

Alexander Volkovsky

2,588
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score 0 · Answer 3 · answered May 27 '21 at 10:35

Bitwise operations are fine, but not strictly required.

In the most common 2's complement representation for 8 bits:

-1 signed == 255 unsigned
-2 signed == 254 unsigned
...
-127 signed = 129 usigned
-128 signed = 128 usigned

simply the two absolute values always give the sum 256.

Use this to convert negative values:

if b < 0:
    b += 256

and then combine the high and low byte:

value = 256 * hi8 + lo8

Converting lower and upper 8 bits into one value with python

3 Answers3