Basic python: how to increase value of item in list

Question

This is such a simple issue that I don't know what I'm doing wrong. Basically I want to iterate through the items in an empty list and increase each one according to some criteria. This is an example of what I'm trying to do:

list1 = []
for i in range(5):
    list1[i] = list1[i] + 2*i

This fails with an list index out of range error and I'm stuck. The expected result (what I'm aiming at) would be a list with values:

[0, 2, 4, 6, 8]

Just to be more clear: I'm not after producing that particular list. The question is about how can I modify items of an empty list in a recursive way. As gnibbler showed below, initializing the list was the answer. Cheers.

Answer 1

Ruby (for example) lets you assign items beyond the end of the list. Python doesn't - you would have to initialise list1 like this

list1 = [0] * 5

Answer 2

So when doing this you are actually using i so you can just do your math to i and just set it to do that. there is no need to try and do the math to what is going to be in the list when you already have i . So just do list comprehension:

list1 = [2*i for i in range(5)]

Since you say that it is more complex, just don't use list comprehension, edit your for loop as such:

for i in range(5):
    x = 2*i
    list1[i] = x

This way you can keep doing things until you finally have the outcome you want, store it in a variable, and set it accordingly! You could also do list1.append(x) , which I actually prefer because it will work with any list even if it's not in order like a list made with range

Edit: Since you want to be able to manipulate the array like you do, I would suggest using numpy! There is this great thing called vectorize so you can actually apply a function to a 1D array:

import numpy as np
list1 = range(5)
def my_func(x):
    y = x * 2
vfunc = np.vectorize(my_func)
vfunc(list1)
>>> array([0, 2, 4, 6, 8])

I would advise only using this for more complex functions, because you can use numpy broadcasting for easy things like multiplying by two.

Answer 3

Your list is empty, so when you try to read an element of the list (right hand side of this line)

list1[i] = list1[i] + 2*i

it doesn't exist, so you get the error message.

Answer 4

You may also wish to consider using numpy . The multiplication operation is overloaded to be performed on each element of the array. Depending on the size of your list and the operations you plan to perform on it, using numpy very well may be the most efficient approach.

Example:

>>> import numpy
>>> 2 * numpy.arange(5)
array([0, 2, 4, 6, 8])

Answer 5

I would instead write

for i in range(5):
    list1.append(2*i)

Answer 6

Yet another way to do this is to use the append method on your list. The reason you're getting an out of range error is because you're saying:

list1 = []
list1.__getitem__(0) 

and then manipulate this item, BUT that item does not exist since your made an empty list.

Proof of concept:

list1 = []
list1[1]
IndexError: list index out of range

We can, however, append new stuff to this list like so:

list1 = []
for i in range(5):
    list1.append(i * 2)