python - Negative list index?

105
votes

Possible Duplicate:
Explain slice notation

I'm trying to understand the following piece of code:

# node list
n = []
for i in xrange(1, numnodes + 1):
    tmp = session.newobject();
    n.append(tmp)
link(n[0], n[-1])

Specifically, I don't understand what the index -1 refers to. If the index 0 refers to the first element, then what does -1 refer to?

2
pythonlist
I think this question covers the same ground in the end as Good Primer for Python Slice Notation - Daenyth
I think that's right.. thanks! - Dawood

2 Answers

196
votes

Negative numbers mean that you count from the right instead of the left. So, list[-1] refers to the last element, list[-2] is the second-last, and so on.

11
votes

List indexes of -x mean the xth item from the end of the list, so n[-1] means the last item in the list n. Any good Python tutorial should have told you this.

It's an unusual convention that only a few other languages besides Python have adopted, but it is extraordinarily useful; in any other language you'll spend a lot of time writing n[n.length-1] to access the last item of a list.