converts signed decimal numbers to signed binary and vice versa

Question

Can't Write Decimal To Binary and vice versa converter that works with the positive and negative numbers.

This Decimal To Binary Converter works fine

# decimal to binary 
def to2sCompStr(num):
    bitWidth = 8
    num &= (2 << bitWidth-1)-1 
    formatStr = '{:0'+str(bitWidth)+'b}'
    ret =  formatStr.format(int(num))
    return ret


print(to2sCompStr(-100))
#output 10011100


# Binary To decimal
binary = 10011100 
print(int(binary, 2))
# output 156, not -100

Binary To Decimal Working only on positive number, not negative

There is at least the definition line missing from your code block. The indentation of the last line is also incorrect, and you should show its output and the desired output. Finally, it's not that clear what you're asking -- can you add a clear question? — Matt Hall, Oct 24 '22 at 12:56
@kwinkunks i want to make converter that converts decimal number to binary and vice versa even it's with negative sign. — PR0X1M4, Oct 24 '22 at 13:03
Please provide enough code so others can better understand or reproduce the problem. — Community, Oct 24 '22 at 13:43
You still need to show your code, with sample input and expected output. See https://stackoverflow.com/help/how-to-ask — Matt Hall, Oct 24 '22 at 13:43

Booboo · Accepted Answer · 2022-10-24T18:43:20.577

The call to int function with the base argument set to 2 in order to convert binary strings does not recognize input in two's-complement notation. It cannot assume that the leftmost "bit" (actually a character '0' or '1') of the input string is a sign bit. It does, however, recognize a leading '-' character.

Therefore, your to2sCompStr needs to negate negative numbers before calling int() and then prefix the result with a '-':

# decimal to binary
def to2sCompStr(num):
    bitWidth = 8
    if num < 0:
        negative = True
        num = -num
    else:
        negative = False
    num &= (2 << bitWidth-1)-1
    formatStr = '-{:0'+str(bitWidth)+'b}' if negative else '{:0'+str(bitWidth)+'b}'
    return formatStr.format(int(num))


binary = to2sCompStr(-100)
print(binary)
#output -01100100


print(int(binary, 2))
# output -100

Prints:

-01100100
-100

The alternative is not to use the int builtin function:

BITWIDTH = 8
MAX_POSITIVE_NUM = 2 ** (BITWIDTH - 1) - 1
MIN_NEGATIVE_NUM = -(2 ** (BITWIDTH - 1))

# decimal to binary:

def to2sCompStr(num):
    if not (MIN_NEGATIVE_NUM <= num <= MAX_POSITIVE_NUM):
        raise ValueError(f'Invalid input: {num}')
    num &= (2 << BITWIDTH-1)-1
    formatStr = '{:0'+str(BITWIDTH)+'b}'
    return formatStr.format(int(num))

# binary to decimal:

def strTo2Int(num):
    n = int(num, 2)
    return n if num[0] == '0' else -(2**len(num) - n)

for n in (0, 1, -1, 100, -100):
    base2 = to2sCompStr(n)
    base10 = strTo2Int(base2)
    print(n, base2, base10)

Prints:

0 00000000 0
1 00000001 1
-1 11111111 -1
100 01100100 100
-100 10011100 -100

converts signed decimal numbers to signed binary and vice versa

1 Answers1