Initialize the first index of a list in Python

Question

I am converting something I wrote in C++ to Python. Here is a snippet of what I am trying to rewrite in python:

std::vector<int> dates(numberOfPayments.size(), 0);
dates[0] = NDD[0] - '0';
for (int i = 1; i < dates.size(); ++i)
{
    dates[i] = (dates[i - 1] + 12 - numberOfPayments[i - 1]) % 12;
}

The problem I am having is that I cannot set the first index of my list in python to something. I try this:

dates = []
dates[0] = NDD_month[0]
for i in range(len(first_payments)):
    dates[i] = (dates[i-1] + 12 - first_payments[i-1]) % 12
print(dates)

But I get this error:

IndexError: list assignment index out of range

Anyone know how to fix this?

List that you initialized is empty. Try dates.append(). Also in the next line you would probably like to loop from 1 instead of 0. — victini
– victini, Commented Nov 19, 2018 at 23:57

Aurora Wang · Accepted Answer · 2018-11-20 00:06:03Z

1

You're having this problem because you're trying to access a index that was not allocated yet.

To append things to a list you should use append (edited to fix loop):

dates = []
dates.append(NDD_month[0])
for i in range(1, len(first_payments)):
    dates.append((dates[i-1] + 12 - first_payments[i-1]) % 12)
print(dates)

edited Nov 20, 2018 at 0:06

answered Nov 19, 2018 at 23:53

Aurora Wang

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2 Comments

Aurora Wang Over a year ago

I want to highlight the fact that I only fixed the append part. Your loop is starting in 0 and your logic will access a index -1. It will not break an error because Python allow access to negative indexes (it accesses your list backwards), but I don't think this is what you intended to do, is it?

user9366862 Over a year ago

Please see this post stackoverflow.com/questions/53384396/…

ahota · Accepted Answer · 2018-11-19 23:51:53Z

1

Since you initialized date with [], it is empty with a size of 0. You will need to use append() to add elements to it.

answered Nov 19, 2018 at 23:51

ahota

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Comments

iElden · Accepted Answer · 2018-11-19 23:51:36Z

0

You can declare your dates like that:

dates = [NDD_month[0]]

for i in range(len(first_payments)):
    dates[i] = (dates[i-1] + 12 - first_payments[i-1]) % 12
print(dates)

answered Nov 19, 2018 at 23:51

iElden

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