添加程序不起作用

Question

I'm trying to write a simple binary adding program in Python (I know Python can do it already, I'm just doing it to practice basic computing concepts). I got it to work pretty well, the only thing that is weird is that when one of the numbers is longer than the other and starts in 0, the program doesn't return the expected result:

#Binary Adding Machine
def add(a,b):
    #create empty variables
    result=""
    state=0
    #equalize string lengths
    if a>=b:
        c=a
        b="0"*(len(a)-len(b))+b
    else:
        c=b
        a="0"*(len(b)-len(a))+a
    #add strings together into result, in reverse order
    for i in range(1,(len(c)+1)):
        if state==0:
            if a[-i]==b[-i]=="0":
                result+="0"
                state=0
            elif ((a[-i]=="0" and b[-i]=="1") or (a[-i]=="1" and b[-i]=="0")):
                result+="1"
               state=0
            elif a[-i]==b[-i]=="1":
                result+="0"
                state=1
        elif state==1:
            if a[-i]==b[-i]=="0":
                result+="1"
                state=0
            elif ((a[-i]=="0" and b[-i]=="1") or (a[-i]=="1" and b[-i]=="0")):
                result+="0"
                state=1
            elif a[-i]==b[-i]=="1":
                result+="1"
                state=1
    #add another "1" if final state is 1
    if state==1:
        result+="1"
        state=0
    #reverse string
    return result[::-1]

print(add("110101","1111010"))
print(add("1","100000"))
print(add("1","1"))
print(add("100","100"))
print(add("000100100","100"))
print(add("100100100","100"))

If you run the program the following numbers will be printed:

10101111
100001
10
1000
1000
100101000

The second to last line should return 000101000 but instead it returns 1000 . It works fine if the number starts in 1, though, as we can see in the last line.

Can anyone recognize why this is happening?

Thank you very much.

Answer 1

change

if(a >= b) to if(len(a) >= len(b))

your condition means that python is supposed to compare ascii values and not its length. As you know 0 is less than 1, in that particular case it won't give you what you expect. Length comparison is what you want.

And as Marcin suggested, there are better ways to do this.

Answer 2

This is not a direct answer to your problem, ie why your specific code does not work, but rather an alternative implementation of your add function. It might be useful to you for checking results of your code, or for others who have similar problem.

def add2(a,b):
    return "{0:010b}".format(int(a,2) + int(b,2))


print(add2("110101","1111010"))
print(add2("1","100000"))
print(add2("1","1"))
print(add2("100","100"))
print(add2("000100100","100"))
print(add2("100100100","100"))

The output is:

0010101111
0000100001
0000000010
0000001000
0000101000
0100101000

Answer 3

def add(a,b):
'''add two binary numbers'''
if len(a) > len(b):
    high =  list(a)
    low  =  list(b)
else:
    high =   list(b)
    low  =   list(a) 
# using integers
low     =   map(int,low)
high    =   map(int,high)
# turn
low.reverse()
high.reverse()    

for x in range(len(low)):
    ''' add one too the longer 'number' in the position x, 
        if the smaller number contains an 1 in the same position
    '''
    if low[x] == 1:
        high[x] += 1
''' if in the bigger number is a two, add 1 to the higher position and set it to zero
    if no higher position exists, create one.
'''
for y in range(len(high)):
    if high[y] > 1:
        try:
            high[y+1] +=1
            high[y]   = 0
        except:
            high.append(1)
            high[y]   = 0
'''turn, make strings and return one string'''
high.reverse()
high = map(str,high)
return ''.join(high)     

if name == ' main ':

print(add("110101","1111010"))
print(add("1","100000"))
print(add("1","1"))
print(add("100","100"))
print(add("000100100","100"))
print(add("100100100","100"))