How to manipulate digits within an integer/string?

Question

I am looking for a general way to refer to particular digits in a integer or string, I need to be able to perform different operations on alternating digits and sum the result of all of those returned values.

Any help is much appreciated

Oh and I am a complete beginner so idiot-proof answers would be appreciated.

I will elaborate, is there any inbuilt function on Python that could reduce an integer into a list of it's digits, I have looked to no avail and I was hoping someone here would understand what I was asking, sorry to be so vague but I do not know enough of Python yet to provide a very in-depth question.

Solution

If you're starting with an integer, first convert it to a string; you can't address the digits within an integer conveniently:

>>> myint = 979
>>> mystr = str(myint)
>>> mystr
'979'

Address individual digits with their index in square brackets, starting from zero:

>>> mystr[1]
'7'

Convert those digits back to integers if you need to do math on them:

>>> int(mystr[1])
7

And if you're just doing a numerological summation, list comprehensions are convenient:

>>> sum( [ int(x) for x in mystr ] )
25

Just keep in mind that when you're considering individual digits, you're working with strings, and when you're doing arithmetic, you're working with integers, so this kind of thing requires a lot of conversion back and forth.

Comments

[George Burrows]

Thank you very much, I did not know you could refer to items in something such as mystr in the same way you referred to items in a List, thanks for the basic explanation also. :)

[jfs]

you could drop [] inside sum() (use genexpr instead of listcomp).

[George Burrows]

Also is it possible to use the same start, stop, stride format in this instance? or is that reserved for lists? I.e. if I wanted to print the alternating digits from the end of an integer list1 = 0123456789, I could do print list1[-1::2] to return 8, 6, 4, 2, 0?

[Russell Borogove]

@GeorgeBurrows - lists, tuples, and strings are all sequences. The elements of a string sequence are strings of one character each. This is great when you want to pick characters out of a string, and confusing as heck when you accidentally pass a string to a function that expects a list.

[Russell Borogove]

0123456789 without quotes is an integer, not a string, and the leading zero makes it octal, not decimal, so that's a problem. And your start-stop-stride numbers are wrong, but '0123456789'[-2::-2] is '86420'. Try starting a python shell (just type python at the command line) and experiment yourself.

[George Burrows]

Ok, I think I understand now, I will give it a go and see what happens, thanks to everyone that helped. :)

[George Burrows]

How would you suggest applying this if I have the input in this format?: test_num('482898477382') ? As now I cannot simply refer to the string without typing the string in again and putting something along the lines of input = '482898477382'?

[AlG]

If your input is a string you can omit the str() call and truncate it to [int(y) for y in x]. I'll update my answer with an example.

[George Burrows]

This may be because I am new at this but to refer to mystr in such a manner would you have to explicitly put mystr = '47833893949' or could you refer to the string as mystr if it was defined as mystr('47833893949')? Thanks for the help :)

[George Burrows]

Sorry I phrased my question a bit awkwardly, if defined the string as test('12345') could I then refer to the string '12345' simply as test? So then to turn the string '12345' into an integer I could simply put integer = int(test)?

[AlG]

test('12345') would try to call the function test() with the parameter '12345'. To create a variable you'd do test = '12345' then, yes, you could do asInt = int(test)

[retracile]

This assumes that mystr is the string representation of a number. If you have an integer, str(mynumber) will give you the string representation. I don't know what you mean by "if it was defined as mystr('47833893949')"; that's not a variable definition, that's a function call.

[George Burrows]

I have to define a function to check if a string input meets certain criteria, and the function has to be defined as: is_true(n) where n is the string, I need to change n into an integer, which from what I can gather would involve using for example integer = int(n) and I would have to type out n again, when the function should be able to have the number inputted just once and then it should be able to perform all the operations on n without needing any extra input. So ideally I would type n in the first time, and then it would give me an answer back of true or false if n meets these criteria.