How to make list double every other term, and add 2 digit numbers?

Question

numbers = raw_input("Enter numbers: ")
numbers = [numbers]
numbers = list(numbers[0])
numbers = [int(x) for x in numbers]
numbers[::2] = [2*x for x in numbers[::2]]
for x in numbers: #code stops working as i want here
    if x == 10:
        x = 1
    elif x == 12:
        x = 3
    elif x == 14:
        x = 5
    elif x == 16:
        x = 7
    elif x == 18:
        x = 9
print(numbers)

Basically I want it to take the numbers I give it, put it into a list, split that list, change all elements in the list to integers, and for every other element in the list, double it, and (here's where it stops working) take the possible 2 digit numbers from doubling 5, 6, 7, 8, or 9 and add the digits.

I think the for, if, and elif statements are right but I think their bodies are wrong. What do I do to change x if it's not like a variable? (x = 1)

Or am I doing the last step all wrong? Thanks.

You can't change the variable indicating an item in a collection due to loop scope. — Malik Brahimi, Sep 12 '15 at 15:01
An easier way to add digits would be: `sum(map(int, str(x)))` — Malik Brahimi, Sep 12 '15 at 15:03

Bakuriu · Answer 1 · 2015-09-12T15:09:33.800

1

The assignment x = 1 is simply rebinding the name x to the value 1. This does not modify the contents of numbers.

If you want to change a value contained in a list you must explicitly do so by using:

numbers[index] = value

For example:

for i, x in enumerate(numbers):
    if x >= 10: # not actually required: 0<= x < 10 implies x%10 == x and x//10 == 0
        numbers[i] = x%10 + x//10

Note that if you have a two digit number then x % 10 is equal to the first digit while x // 10 (integer division/quotient) is equal to the second digit.

In fact you could even do:

numbers[i] = sum(divmod(x, 10))

edited Sep 12 '15 at 15:09

answered Sep 12 '15 at 15:03

Bakuriu

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It works. I understand in a for loop that `for x in list:` makes every element (every x) in the list (list) do something, but what does `for i, x in ...:` do? What's the difference/purpose of 2 names (i and x)? – Jacob Wheeler Sep 12 '15 at 15:18
@JacobWheeler See http://stackoverflow.com/questions/522563/accessing-the-index-in-python-for-loops . Also you may be interested in "tuple unpacking" (just google for it). – Bakuriu Sep 12 '15 at 15:20
@JacobWheeler `enumerate` returns pairs containing an index and a value corresponding with `i` and `x` – Malik Brahimi Sep 12 '15 at 15:21
So i is the index and x is the actual element? – Jacob Wheeler Sep 12 '15 at 15:22
And enumerate() applies indexes to the list? – Jacob Wheeler Sep 12 '15 at 15:23

score 0 · Answer 2 · answered Sep 12 '15 at 15:05

0

You can not change the variable indicating an item in a collection due to loop scope. Try this instead:

for i, x in enumerate(numbers): 
    numbers[i] = sum(map(int, str(x)))

answered Sep 12 '15 at 15:05

Malik Brahimi

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Basically the `x` in the loop header is different from the `x` reference in the loop itself. – Malik Brahimi Sep 12 '15 at 15:08
So it's if you turn all int into str how does it sum strings? – Jacob Wheeler Sep 12 '15 at 15:43
Correct me if I'm wrong: `map(a, b)` applies all b to a, so in this case it does str(x) to all int. So if everything is a string, how can `sum` sum strings? Or do I misunderstand? – Jacob Wheeler Sep 12 '15 at 15:51
So `enumerate` turns all elements in numbers to strings, and then you turn them all back to integers with `map`? – Jacob Wheeler Sep 12 '15 at 15:57
So `enumerate` makes all the elements in the list strings? – Jacob Wheeler Sep 12 '15 at 16:01
`map` takes this index and value from `enumerate` to obtain a sum from the sum of the value's digits and it is reassigned according to the index. – Malik Brahimi Sep 12 '15 at 16:03
@JacobWheeler Do you understand now? – Malik Brahimi Sep 12 '15 at 16:10
I don't understand what str(x) is for. – Jacob Wheeler Sep 12 '15 at 16:30
`str` casts the passed integer `x` to a string or a sequence of characters. – Malik Brahimi Sep 12 '15 at 17:31

score 0 · Answer 3 · answered Sep 12 '15 at 18:03

def f(x):
    if x in (10, 12, 14, 16, 18):
        return x-9
    else return x

Instead of the loop at the end,

numbers = [f(n) for n in numbers]

Of course, you can put the definition of f in the list comprehension, but it starts looking a bit cramped.

How to make list double every other term, and add 2 digit numbers?

3 Answers3