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Oct 22nd, 2007

Convert negative decimal to binary

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Hi, I'm having trouble trying to figure out a code that converts negative decimal numbers to binary, as well as specifying the number of bits. For example. convert -18 using 8 bits. This should come out as 10010010 doing it manually, I think. I'd appreciate the help, thanks.

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ALJ

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Oct 22nd, 2007

Re: Convert negative decimal to binary

Working with negative numbers in binary you need to use twos complement method. It is a bit confusing to start with (well it was for me). I use it a lot for checksum calculations. Hope this helps.

http://www.rsu.edu/faculty/PMacphers...rams/twos.html

David

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Oct 23rd, 2007

Re: Convert negative decimal to binary

you can try Ex-Or ing the positive number with 10000000... it will work for numbers less than 01111111

jai.mahadeokar

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Oct 23rd, 2007

Re: Convert negative decimal to binary

Check the preample of:
http://wiki.python.org/moin/BitwiseOperators

bumsfeld

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Oct 24th, 2007

Re: Convert negative decimal to binary

Click to Expand / Collapse Quote originally posted by ALJ ...

Hi, I'm having trouble trying to figure out a code that converts negative decimal numbers to binary, as well as specifying the number of bits. For example. convert -18 using 8 bits. This should come out as 10010010 doing it manually, I think. I'd appreciate the help, thanks.

Hum... 10010010 is a bit-sign representation (sign bit followed by the 7 bit representation of 18) wich is very unusual. If you want the complement to 2 reprrésentation, you have to know that whith 8 bits, the binary for N negative is the same than the positive for 255 + N + 1, 238 for -18. I assume you know how to convert a positive value to binary. Look at this, I think it works for any BITS value:

Python Syntax (Toggle Plain Text)
BITS = 8
MAXFORBITS = 1
for i in range (0,BITS):MAXFORBITS *= 2
print MAXFORBITS
testvaleur = -18
print (MAXFORBITS + testvaleur) % MAXFORBITS
testvaleur = 18 
print (MAXFORBITS + testvaleur) % MAXFORBITS
BITS = 8
MAXFORBITS = 1
for i in range (0,BITS):MAXFORBITS *= 2
print MAXFORBITS
testvaleur = -18
print (MAXFORBITS + testvaleur) % MAXFORBITS
testvaleur = 18 
print (MAXFORBITS + testvaleur) % MAXFORBITS

Pierre Maurette

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Oct 27th, 2007

Re: Convert negative decimal to binary

So in the manual, it says

Quote ...

Of course, Python doesn't use 8-bit numbers. It USED to use however many bits were native to your machine, but since that was non-portable, it has recently switched to using an INFINITE number of bits. Thus the number -5 is treated by bitwise operators as if it were written "...1111111111111111111010".

How?! Specifically, how do they do this efficiently?

Jeff

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Oct 28th, 2007

Re: Convert negative decimal to binary

Oh, nevermind. I thought it was saying something else. All it's saying is that when it performs right-shifts, it rolls in the MSB from the left. (Arithmetic instead of Logical shift)

I thought it was saying that it somehow represented negatives as if they had infinite precision, which would be impressive.

Jeff

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Jan 28th, 2010

Re: Convert negative decimal to binary

Click to Expand / Collapse Quote originally posted by ALJ ...

Hi, I'm having trouble trying to figure out a code that converts negative decimal numbers to binary, as well as specifying the number of bits. For example. convert -18 using 8 bits. This should come out as 10010010 doing it manually, I think. I'd appreciate the help, thanks.

This may help you out:

Signed Numbers (2 methods)
Signed‐and‐Magnitude
Leading digit is the sign: 0 = positive; 1 = negative
+9 = 00001001
-9 = 10001001
+23 = 00010111
-23 = 10010111
2’s Compliment
Decimal to Binary
Invert all 0’s and 1’s, and add 1.
+9 = 00001001
-9 = 11110110
00000001 +
--------
11110111
+23 = 00010111
-23 = 11101000
00000001 +
--------
11101001
Binary to Decimal
Check the sign bit. If the sign bit is 0, number is positive, and you’re done. If it is 1, then number is negative, so invert
the bits and add 1.
00001101 = positive number
= + 13 (Done)
11110010 negative number
00001100 invert
00000001 + add one
--------
00001101 = - 13
11111111 negative number
00000000 invert
00000001 + add one
--------
00000001 = - 1
10000000 negative number
01111111 invert
00000001 + add one
--------
10000000 = - 128

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