Lists

Lists#

Basic lists#

We can create lists using square brackets. Commas separate elements in lists. Lists can contain any type of variable/data.

company1 = "APPL"
company2 = "MMM"
company3 = "MSFT"

list_of_nums = [9, 43, 75, 14, 2019]
list_of_strings = ['this', 'that', 'hello world']
list_of_variables = [company1, company2, company3]
list_of_booleans = [True, False, False, True]
list_of_mixed = [9, 'this', company1, True]

print(list_of_nums)
print(list_of_strings)
print(list_of_variables)
print(list_of_booleans)
print(list_of_mixed)

[9, 43, 75, 14, 2019]
['this', 'that', 'hello world']
['APPL', 'MMM', 'MSFT']
[True, False, False, True]
[9, 'this', 'APPL', True]

Lists can even contain other lists

list_of_lists = [list_of_nums, list_of_variables]
list_of_lists

[[9, 43, 75, 14, 2019], ['APPL', 'MMM', 'MSFT']]

We can also create empty lists.

empty_list = []

Appending to lists#

Suppose we have a list of competitor tickers, and we want to add more tickers to it.

competitors = ['AAPL', 'MSFT', 'TSLA']
more_competitors = ['AMZN', 'FB']

We could just add the lists:

all_competitors = competitors + more_competitors
all_competitors

['AAPL', 'MSFT', 'TSLA', 'AMZN', 'FB']

Another way to add to lists is to use the append method.

all_competitors.append('SNAP')
all_competitors

['AAPL', 'MSFT', 'TSLA', 'AMZN', 'FB', 'SNAP']

Indexing lists#

You can access a specific element in a list by indexing the numerical position: [#]

Here is an example using the all_competitors list.

all_competitors[1]

'MSFT'

Why does this give us the second element? Because in Python, the index starts at zero.

all_competitors[0]

'AAPL'

However, to index the last element in a list, we use -1:

all_competitors[-1]

'SNAP'

Here we access a specific value and store it in a new variable called the_one_i_want.

the_one_i_want = all_competitors[2]
print(all_competitors)
print(the_one_i_want)

['AAPL', 'MSFT', 'TSLA', 'AMZN', 'FB', 'SNAP']
TSLA

To grab a range of values, use the :

comp_subset = all_competitors[0:2]
comp_subset

['AAPL', 'MSFT']

Notice how the endpoints work. There are two elements in comp_subset, coming from list positions 0 and 1. The third element in the list (element [2]) is not included in comp_subset.

When one side of the : does not have a number, Python infers “from the beginning” or “until the end”.

print(all_competitors[:3])
print(all_competitors[-2:])

['AAPL', 'MSFT', 'TSLA']
['FB', 'SNAP']

Other list tools#

Just like .append(), there are other functions (usually called methods) that we can use with lists.

For example, we can remove elements from a list and store them in a new variable using the .pop() method. The following code removes the last item from the list and stores it in the x variable.

list_of_nums = [9, 43, 75, 14, 20, 19, 54, 98, 4]
x = list_of_nums.pop()
print(x)
print(list_of_nums)

4
[9, 43, 75, 14, 20, 19, 54, 98]

Similarly, we can remove the first element (remember indexing starts at 0), third element, etc.

y = list_of_nums.pop(0)
z = list_of_nums.pop(2)
print(y)
print(z)
print(list_of_nums)

9
14
[43, 75, 20, 19, 54, 98]

Just remember that after every .pop() reference the list is shortened.

We can also sort lists using the .sort() method. This modifies the list in place. It is not creating a new list.

list_of_nums.sort()
list_of_nums

[19, 20, 43, 54, 75, 98]

If we want to know how many items there are in a list, we can use the len() function:

list_of_nums = [9, 43, 75, 14, 2019]
list_length = len(list_of_nums)
list_length