Question

I am trying to create a simple program that reads a string of 4 digits, makes sure it is indeed 4 digits, makes sure there are no non-digits, then separates the first two digits from the last two and adds them together. I can make it all work but I still got this error:

ValueError: invalid literal for int() with base 10:

This only happens when I try a string such as ‘456f’.

What can I change to fix this?

Code:

    s = input('please type a 4-digit integer \n')
valid = True
for c in s:
    if len(s)!= 4:
        valid = False
    if not c.isdigit():
        print (c, 'is not a valid input')
number = int(s)
firstOne = number // 100
secondOne = number % 100
sum = firstOne + secondOne
x = '/'
if valid == True:
    print('your integer is ' + str(number), x, 'first two digits are ' + str(firstOne), x, 'second two digits are ' + str(secondOne), x, 'sum of two new numbers is ' + str(sum))
else:
    print(len(s), 'is an invalid amount of digits')

Answer 1

Let’s focus on this code:

for c in s:
    if len(s)!= 4:
        valid = False
    if not c.isdigit():
        print (c, 'is not a valid input')
number = int(s)

The first thing to say is that the len() check should be moved outside the character loop.

if len(s)!= 4:
    valid = False
for c in s:
    ...

The next comment to make is that whilst you are detecting non-digits, you continue executing code as if nothing is wrong. You presumably intend to set valid to False.

if not c.isdigit():
    print (c, 'is not a valid input')
    valid = False

Now, the main part of the problem. You need to skip the conversion to int when invalid input is detected.

if valid:
    number = int(s)
    ...

If you want to continue with such an approach your code would look like this:

valid = True
s = input('please type a 4-digit integer \n')
if len(s)!= 4:
    valid = False
    print(len(s), 'is an invalid amount of digits')

if valid:
    for c in s:
        if not c.isdigit():
            valid = False
            print (c, 'is not a valid input')

if valid:
    number = int(s)
    firstOne = number // 100
    secondOne = number % 100
    sum = firstOne + secondOne
    x = '/'
    print('your integer is ' + str(number), x, 'first two digits are ' + str(firstOne), x, 'second two digits are ' + str(secondOne), x, 'sum of two new numbers is ' + str(sum))

Having said all of that, I’d probably reorganise the code quite a bit to deal with the errors as soon as they are detected. Code is much easier to understand if you can organise your error handling that way.

s = input('please type a 4-digit integer \n')
if len(s)!= 4:
    sys.exit(str(len(s)) + ' is an invalid amount of digits')
for c in s:
    if not c.isdigit():
        sys.exit(c + ' is not a valid input')
number = int(s)
firstOne = number // 100
secondOne = number % 100
sum = firstOne + secondOne
x = '/'
print('your integer is ' + str(number), x, 'first two digits are ' + str(firstOne), x, 'second two digits are ' + str(secondOne), x, 'sum of two new numbers is ' + str(sum))

Now, that’s a start in the right direction, but you can continue in this vein making the code better and better. Sven’s answer gives you an excellent illustration of where such a process would ultimately lead.