\(\newcommand{L}[1]{\| #1 \|}\newcommand{VL}[1]{\L{ \vec{#1} }}\newcommand{R}[1]{\operatorname{Re}\,(#1)}\newcommand{I}[1]{\operatorname{Im}\, (#1)}\)

More on working with lists

Concatenating lists with +

You can add lists together.

>>> my_list = [11, 12, 13]
>>> your_list = [5, 6, 7]
>>> long_list = my_list + your_list
>>> long_list
[11, 12, 13, 5, 6, 7]

Adding lists with + makes a new list with the values from the first list followed by the values from the second.

Getting list elements with indexing

You can also index lists to return list elements.

In Python, as for many other languages, the first element in the list is at index 0:

>>> # Indexing a list to get the first element (index 0)
>>> long_list[0]
11
>>> # Indexing to get the second element (index 1)
>>> long_list[1]
12

Getting parts of the list with slicing

You can also take slices out of lists. For example, here we ask for all the elements in the list from (and including) index 1 (the second element).

>>> # Everything from the second element (index 1)
>>> # 1 is the "start index"
>>> long_list[1:]
[12, 13, 5, 6, 7]
>>> # Everything from the fourth element (index 3)
>>> long_list[3:]
[5, 6, 7]

You can also ask for everything up to but not including a particular element. For example, here is everythin up to but not including the fourth element (index 3). Notice the colon followed by the number, which is called the stop index.

>>> # Everything up to but not including the fourth element
>>> # 3 is the "stop index"
>>> long_list[:3]
[11, 12, 13]

So, if we want to split a list at element index 3, we would do this:

>>> # Splitting a list at index 3
>>> first_part = long_list[:3]
>>> first_part
[11, 12, 13]

>>> second_part = long_list[3:]
>>> second_part
[5, 6, 7]

You can specify both the start index and the stop index, like this:

>>> # Everything from index 1, up to, not including, index 3
>>> long_list[1:3]
[12, 13]

Shuffling list element order

We find that we ofen want to shuffle the order of list elements, to have a random order.

Python’s random.shuffle function can do the shuffle for us:

>>> import random

random.shuffle takes a list and shuffles it to a random order. Here I make a small example list and shuffle it a few times to show that the order of the list changes:

>>> # A small example list
>>> a_list = [1, 2, 3, 4, 5]
>>> # Shuffle it
>>> random.shuffle(a_list)
>>> # The shuffled list has a different (random) order
>>> a_list
[5, 1, 4, 2, 3]

>>> # Shuffling again gives a different order
>>> random.shuffle(a_list)
>>> a_list
[4, 2, 1, 3, 5]

>>> random.shuffle(a_list)
>>> a_list
[3, 1, 4, 2, 5]