python 3.x - Why am I not getting an expected output using logical operators and indexing? -

i having trouble achieving expected output. trying create byte adder using logical operators such and, xor , or. have taken minimal code required reproduce problem out of code, assume finalfirstvalue = "1010" , finalsecondvalue = "0101".

secondvalueindex = (len(finalsecondvalue) - 1) carry, finalans = false, [] in range(-1, -len(finalfirstvalue) - 1, -1):     andone = (bool(finalfirstvalue[i])) & (bool(finalsecondvalue[secondvalueindex]))     xorone = (bool(finalfirstvalue[i])) ^ (bool(finalsecondvalue[secondvalueindex]))     andtwo = (bool(carry)) & (bool(xorone))     xortwo = (bool(carry)) ^ (bool(xorone))     orone = (bool(andone)) | (bool(andtwo))     carry = (bool(orone))     finalans.append(xortwo)     secondvalueindex -= 1  answer = ''.join(str(e) e in finalans) print (answer) 

actual output: falsetruetruetrue expected output: truetruetruetrue

the code follows change zeroes , ones.

because missing single boolean feel issue indexing. although i've played around bit , not had luck.

i need carry out these operations on 2 variables mentioned @ start, right elements, , move left 1 next loop , on.

first mistake representing binary numbers string values.

finalfirstvalue = "1010" finalsecondvalue = "0101"

secondvalueindex = (len(finalsecondvalue) - 1) == 3

so in second loop result as

(finalsecondvalue[secondvalueindex]) == '0'

if check in idle

>>> bool('0') true >>>  

because '0' not actual 0 non-empty string return true.

you need cast result int before checking them bool

like this

(bool(int(finalsecondvalue[secondvalueindex])))

edit 2 adding variable lenghts

full adder verification using bin() function

a="011101" b="011110" if a>b:     b=b.zfill(len(a)) if a<b:     a=a.zfill(len(b)) finalfirstvalue = finalsecondvalue = b carry, finalans = 0, [] secondvalueindex = (len(finalsecondvalue))  in reversed(range(0, len(finalfirstvalue))):     xorone = (bool(int(finalfirstvalue[i]))) ^ (bool(int(finalsecondvalue[i])))     andone = (bool(int(finalfirstvalue[i]))) & (bool(int(finalsecondvalue[i])))     xortwo = (carry) ^ (xorone)     andtwo = (carry) & (xorone)      orone = (andone) | (andtwo)     carry = (orone)     finalans.append(xortwo)   finalans.reverse() answer=(''.join(str(e) e in finalans)) print(str(carry)+answer) print(bin(int(a,2) + int(b,2)))    #verification