[Tutor] Problems padding a string with 0's while deconcatonating

Ismael Garrido ismaelgf at adinet.com.uy
Tue Dec 6 01:04:20 CET 2005

Josh Yagy wrote:

>Wow, that code is much more compact, thanks for the help! But as far as the original question goes, I think I worded my problem wrong. The output you got with your binary string is almost what I Want, but not quite. I need each subset of binary strings to be n bits long (in the dummy problem you ran, 4 bits). The last subset is two bits short, but for the sake of the cryptosystem, I can't pad it with zeros at the right, I need to pad it to the left. for instance:
>Your output was:
> print binaryString(b, 4) -> ['0101', '0011', '0001', '1100', '0011', '11']
>What I'm looking for is for the output to be:
>print binaryString(b, 4) -> ['0001', '0100', '1100', '0111', '0000', '1111']
>notice that my desired output is the same as yours, only all the bits are moved back two places to fill up the last subset, and the extra space is filled with 0's in the first subset. 
>Thanks again for the help

Why don't you pad it before processing?

def padleft(b, bits):
    bitsshort = len(b)%bits
    if bitsshort:  #so that if len(b) == bits you don't get extra 0s
        amount = bits - bitsshort
        return "0"*amount + b
    return b

 >>> padleft("10011", 5)
 >>> padleft("101101", 4)
 >>> padleft("1010101", 6)


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