Binary to Decimal Script in Python

Question

I encountered an issue with my Python script that converts binary to decimals. The caveat with this script is that I can only use basic computational functions (+, -, *, /, **, %, //), if/else, and for/while loops.

Shown below is my script:

x = int(input("Enter your binary input: "))
z=0

while x != 0:

    for y in range (0,20):
        
        if (x % 2 == 0):
            z += 0*2**y
            x = int(x/10)
            
        
        elif (x % 2 != 0):
            z += 1*2**y
            x = int(x/10)

print(z)

While my script works for binaries with short length (e.g., 1101 = 13), it does not work for long binaries (especially those that have a length of 20). For example, inputting a binary of 11111010010011000111, my script returns an output of 1025217 instead of 1025223.

Can anyone point me to my mistake?

Thank you in advance!

Reti43 · Accepted Answer · 2021-05-23 13:37:02Z

2

Floating arithmetic isn't perfect and has a precision limit.

11111010010011000111 / 10 gives 1.1111010010011e+18, which when converted to an integer will give you a wrong result and it snowballs from there.

>>> int(11111010010011000111 / 10)
1111101001001100032

The more binary digits you have, the more "off" your calcuations will be.

In order to avoid the problem you encountered, use floor division, ie, x // 10.

Or you can skip turning x into a number and do the necessary power calculations based off each binary digit and its position.

x = input("Enter your binary input: ")[::-1]

n = 0
# this would be more elegant with `enumerate()`, but I assume you can't use it
# same for `sum()` and a comprehension list
for i in range(len(x)):
    n += int(x[i])*2**i
print(n)

answered May 23, 2021 at 13:37

Reti43

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Comments

xio · Accepted Answer · 2021-05-23 14:00:47Z

0

You can use the following method

to multiple each of the binary digits with its corresponding value of 2 raised to the power to its index (position from right – 1) and sum them up.

binary = input('Binary number: ')
decimal = 0
binary_len = len(binary)

for x in binary:
    binary_len = binary_len - 1
    decimal += pow(2,binary_len) * int(x)
    
print(decimal)

input : 11111010010011000111

output : 1025223

More examples

edited May 23, 2021 at 14:00

answered May 23, 2021 at 13:35

xio

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MiH · Accepted Answer · 2021-05-23 14:58:28Z

0

You should not use int to convert any number if you have to write a number converter. Just use string operations:

digits = input("Enter your binary input: ")
number = 0
for digit in digits:
    number = number * 2 + (digit == "1")
print(number)

output

Enter your binary input: 11111010010011000111
1025223

edited May 23, 2021 at 14:58

MiH

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answered May 23, 2021 at 13:54

Daniel

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