nmmdl nmmdl - 3 years ago 74
Python Question

Why doesn't a.insert(-1, x) mimic a.append(x)?

So I have the following Python code, which appends numbers 1-10 to the list


values = []

for value in range(1, 11):


And, as expected, gives us
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Although impractical, just out of curiosity, I attempted to recreate this result using
instead of

values = []

for value in range(1, 11):
values.insert(-1, value)


Which gave me the result
[2, 3, 4, 5, 6, 7, 8, 9, 10, 1]

I tried this with other ranges as well, and the same thing happens every time: It is in ascending order, except for the smallest number being at the end.

From the Python Documentation Tutorial, I now know that
a.insert(len(a), x)
can be used to mimic
. However, I still do not understand why inserting the values to the last position in a list works except for the smallest value.

Answer Source

The fundamental thing to understand is that -1 is not the same as len(a). It is indeed the same as len(a) - 1:

In [396]: x = [1, 2, 3]

In [397]: x[-1]
Out[397]: 3

In [398]: x[len(x) - 1]
Out[398]: 3 

When the list has size 1 and more, then len(a) - 1 will always point to the spot just before the last element (or, technically, to the spot at which the last element currently is), which is where list.insert will put your new item.

In [400]: x.insert(-1, 4); x
Out[400]: [1, 2, 4, 3]

As you've already figured out, len(a) points to the spot just after the last element, which is where list.append inserts elements. So, -1, or len(a) - 1 will point to the spot of the last element, and list.insert(-1, ...) will push the last element to the right, and place the new element in the last element's old position.

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