Index out of range error in Python

I assume you want to add elements to change_array, if so, you need to use change_array.append(your_element).

change_array = [] is an empty array, therefore there is no 0 element. 0 element is the first element in arrays.

array = ["one", "two"]

array[0] is "one", array[1] is "two"

@Ekinydre gave you a good solution, but I just wanted to elaborate on the error you got.

When you have a list like change_array = [1, 2, 3], then change_array[i] accessed element i (lists use 0-based indices in Python). Also, += in Python is not an append operator, but an increment operator. So change_array[i] += 10 you are adding 10 to the list element at position i.

You could just append to change_array as @Ekinydre suggests, but given your code it may be safer (though less pythonic) to do something like the following:

#define variables for a loop
#create a list of 0 of length end_year_index
change_array = [0]*end_year_index
roc = 0

#iterate to find the change percentage
for i in range (0, end_year_index+1):
    if i == 0:
       change_array[i] += 0
    else:
        roc = ((population[i] - population[i-1])/ (population[i-1]))
        change_array[i] += roc

A slightly more pythonic way could look like this:

change_array = [(population[i] - population[i-1]) / population[i-1] \
                    for i in range(1, end_year_index+1)]

Note: this last solution won't have the initial 0 at the beginning of change_array.