Python Lists

Python has a number of builtin container types, the most common of which is the list.

A list is an ordered, resizable, mutable, heterogeneous container.

It is also a precursor to Numpy's ndarray. More on that later.

It retains the order of items put into it, unless explicitly sorted
Additional items can be inserted into, or items removed from, the collection at will.
The container can be changed at any time (mutability).
The items don't all have to be the same type (though, in good practice, they should be).

Since it is ordered, it can be used to store time-series data, the lines of a text file, or any other ordered sequence.

Declaring a list literal

Lists are denoted by [] square brackets, and may contain zero-or-more elements

# uci_bootcamp_2021/examples/lists.py

# Declaring a list literal.
data = [1, 4, 2, 3]
print(data)
# [1, 4, 2, 3]

Declaring a list from an existing Iterable

Lists can also be declared from other Iterables.

One common iterable is the builtin range).

range is a Sequence that produces integers in the domain [start, stop), every step values (default 1).

# uci_bootcamp_2021/examples/lists.py

# Declaring a list from an Iterable
# `range` is a special Iterable object that generates a sequence of numbers
# between [start, stop), every `step` values.
data = list(range(1, 42, 3))
print(data)
# [1, 4, 7, 10, 13, 16, 19, 22, 25, 28, 31, 34, 37, 40]

Sorting lists

Lists can also be sorted, provided their elements are sortable.

# uci_bootcamp_2021/examples/lists.py

# Lists can be sorted, too!
data = [191, 113, 161, 5, 13, 33, 231, 213, 32, 234, 47, 125, 101, 98, 169]
# Simplest possible sort invocation, sorts into ascending order.
data.sort()
# Note: `sort` doesn't return the sorted data: it mutates the data in-place.
print(data)
# [5, 13, 32, 33, 47, 98, 101, 113, 125, 161, 169, 191, 213, 231, 234]

# Sort the data in descending order instead.
data.sort(reverse=True)
print(data)
# [234, 231, 213, 191, 169, 161, 125, 113, 101, 98, 47, 33, 32, 13, 5]

Mutating the contents of a list

The elements of a list can be changed by adding, removing, or by changing existing items.

# uci_bootcamp_2021/examples/lists.py

# Items can also be added to the end of the collection.

# NOTE(noinspection): Intentionally writing this in longhand for demonstration purposes.
# noinspection PyListCreation
data = [1, 2, 3]
# Append 4 to the end of the collection
data.append(4)

print(data)
# [1, 2, 3, 4]


# Items can even be mutated in-place.
data[0] = 72
data[1] = data[1] * 4

print(data)
# [72, 8, 3, 4]

# Lists can be mashed together
data2 = [7, 8, 9, 10]
data.extend(data2)
print(data)
# [72, 8, 3, 4, 7, 8, 9, 10]

Min, Max, Sum.

The builtin functions min , max , sum can work with lists.

# uci_bootcamp_2021/examples/lists.py

# Taking the minimum of a list
print(min(data))
# 3

# Taking the maximum of a list
print(max(data))
# 72

# Taking the sum of a list
print(sum(data))
# 121

# Taking the geometric mean of a list
print(sum(data) / len(data))
# 15.125

UCI Bootcamp 2021: Python for Data Science