Str in Python's map and sum

Question

Why do you need to use the function 'str' in the following code?

I am trying to count the sum of digits in a number.

My code

for i in number:
    sum(map(int, str(i))

where number is the following array

[7,79,9]

I read my code as follows

1. loop though the array such that
2. count sum of the integer digits
3. by getting given digits in a number by map increasingly
4. such that each object (given number) is converted to String // This does not make sense

Manual says this for str

Type:           type
Base Class:     <type 'type'>
String Form:    <type 'str'>
Namespace:      Python builtin
Docstring:
    str(object) -> string

    Return a nice string representation of the object.
    If the argument is a string, the return value is the same object.

Answer 1

What happens when you try that code without using str()?

The str() is used to convert the integer into a sequence of characters, so that map() can iterate over the sequence. The key point here is that a "string" can be treated as a "sequence of characters".

Answer 2

Given 79 you need to get [7, 9] in order to sum up this list.

What does it mean to split a number into digits? It means to represent the number in a numerical system with some base (base 10 in this case). E. g. 79 is 7 * 10**1 + 9 * 10**0.

And what is the simplest (well, at least in this context) way to get such a representation of a number? To convert it to a string of decimals!

Your code does exactly that:

>>> str(79)
'79'

# Another way to say this is [int(c) for c in str(79)]
>>> map(int, str(79))
[7, 9]

>>> sum(map(int, str(79)))
16

Answer 3

Why do you need to use the function 'str' in the following code?

Because map takes an iterable, like a list or a tuple or a string.

The code in question adds upp all the numbers in an integer. And it does it by a little clever hack. It converts the number into a sequence of numbers by doing

map(int, str(i))

This will convert the integer 2009 to the list [2, 0, 0, 9]. The sum() then adds all this integers up, and you get 11.

A less hacky version would be:

>>> number = [7,79,9]
>>> for i in number:
...     result = 0
...     while i:
...         i, n = divmod(i, 10)
...         result +=n
...     print result
... 
7
16
9

But your version is admittedly more clever.