# Split an integer into digits to compute an ISBN checksum

### Question

I'm writing a program which calculates the check digit of an ISBN number. I have to read the user's input (nine digits of an ISBN) into an integer variable, and then multiply the last digit by 2, the second last digit by 3 and so on. How can I "split" the integer into its constituent digits to do this? As this is a basic homework exercise I am not supposed to use a list.

1
51
9/15/2012 8:57:40 PM

Just create a string out of it.

``````myinteger = 212345
number_string = str(myinteger)
``````

That's enough. Now you can iterate over it:

``````for ch in number_string:
print ch # will print each digit in order
``````

Or you can slice it:

``````print number_string[:2] # first two digits
print number_string[-3:] # last three digits
print number_string[3] # forth digit
``````

Or better, don't convert the user's input to an integer (the user types a string)

``````isbn = raw_input()
for pos, ch in enumerate(reversed(isbn)):
print "%d * %d is %d" % pos + 2, int(ch), int(ch) * (pos + 2)
``````

82
6/10/2009 11:44:37 AM

``````while number:
digit = number % 10

# do whatever with digit

# remove last digit from number (as integer)
number //= 10
``````

On each iteration of the loop, it removes the last digit from number, assigning it to `digit`. It's in reverse, starts from the last digit, finishes with the first