Lecture 8 — Lists Part 1

Overview

  • So far we’ve looked at working with individual values and variables.
  • This is cumbersome even for just two or three variables.
  • We need a way to aggregate multiple values and refer to them using a single variable.
  • We have done a little bit of this with strings and tuples, but now we are going to get started for real.

This lecture is largely based on Sections 8.1-8.3 of Practical Programming.

Lists are Sequences of Values

  • Gather together values that have common meaning.

  • As a first example, here are scores of 7 judges for the free skating part of a figure skating competition:

    scores = [ 59, 61, 63, 63, 68, 64, 58 ]

  • As a second example, here are the names of the planets in the solar system (including Pluto, for now):

    planets = [ 'Mercury', 'Venus', 'Earth', 'Mras', 'Jupiter',
    'Saturn', 'Neptune', 'Uranus', 'Pluto' ]

  • Notes on syntax:

    • Begin with [ and end with ]
    • Commas separate the individual values
    • The spaces between values are optional and are used for clarity here.
    • Any type of object may be stored in a list, and each list can mix different types.

Why bother?

  • Gather common values together, providing them with a common name, especially when we don’t know how many values we will have.
  • Apply an operation to the values as a group.
  • Apply an operation to each value in the group.
  • Examples of computations on lists:
    • Average and standard deviation
    • Which values are above and below the average
    • Correct mistakes
    • Remove values (Pluto)
    • Look at differences
  • Watch for these themes throughout the next few lectures.

Accessing Individual Values — Indexing

  • Notice that we made the mistake in typing 'Mras'. How do we fix this? We’ll start by looking at indexing.

  • The line

    >>> print(planets[1])

    accesses and prints the string at what’s known as index 1 of the list planets.

  • Each item / value in the list is associated with a unique index

  • Indexing in Python (and most other programming languages) starts at 0.

  • The notation is again to use [ and ] with an integer (non-negative) to indicate which list item.

  • What is the last index in planets?

    • We can find the length using len() and then figure out the answer.

A Memory Model for Lists

We’ll draw a memory model in class that illustrates the relationship among

  • The name of the list
  • The indices
  • The values stored in the list

Practice Problems

We will work on these in class:

  1. What is the index of the first value in scores that is greater than 65?
  2. Write a line of Python code to print this value and to print the previous and next values of the list.
  3. What is the index of the middle value in a list that is odd length? For even length lists, what are the indices of the middle two values?

Changing Values in the List

  • Once we know about indexing, changing a value in a list is easy:

    >>> planets[3] = 'Mars'

  • This makes item 3 of planets now refer to the string 'Mars'

  • Now we can check the output:

    >>> print(planets)

    to make sure we got it right.

  • Strings are similar in many ways.

    >>> s = 'abc'
>>> s[0]
'a'
>>> s[1]
'b'

  • Big difference: you can change a part of a list; you cannot change part of a string!

    >>> s[1] = 'A'
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'str' object does not support item assignment

All Indices Are Not Allowed

  • If t is a list, then the items are stored at indices from 0 to len(t)-1.

  • If you try to access indices at len(t) or beyond, you get a run-time error. We’ll take a look and see.

  • If you access negative indices, interesting things happen:

    >>> print(planets[-1])
Pluto

  • More specifically, for any list t, if i is an index from 0 to len(t)-1 then t[i] and t[i-len(t)] are the same spot in the list.

Functions on Lists: Computing the Average

  • There are many functions (methods) on lists. We can learn all about them using the help command.

    • This is just like we did for strings and for modules, e.g.

      >>> import math
>>> help(math)

>>> help(str)

  • Interestingly, we can run help in two ways, one

    help(list)

    gives us the list methods, and the second

    help(planets)

    tells us that planets is a list before giving us list methods.

  • First, let’s see some basic functions on the list values.

  • The basic functions max, sum and min may be applied to lists as well.

  • This gives us a simple way to compute the average of our list of scores.

    >>> print("Average Scores = {:.2f}".format( sum(scores) / len(scores) ))
Average Scores = 62.29
>>> print("Max Score =", max(scores))
Max Score = 68
>>> print("Min Score =", min(scores))
Min Score = 58

  • Exploring, we will look at what happens when we apply sum, max and min to our list of planet names. Can you explain the result?

Functions that modify the input: Sorting a list

  • We can also sort the values in a list by sorting it. Let’s try the following:

    >>> planets = [ 'Mercury', 'Venus', 'Earth', 'Mras', 'Jupiter', \
'Saturn', 'Neptune', 'Uranus', 'Pluto' ]
>>> planets
['Mercury', 'Venus', 'Earth', 'Mras', 'Jupiter', 'Saturn', 'Neptune', 'Uranus', 'Pluto']
>>> planets.sort()
>>> planets
['Earth', 'Jupiter', 'Mercury', 'Mras', 'Neptune', 'Pluto', 'Saturn', 'Uranus', 'Venus']

  • Note that we did not assign the value returned by sort to a new variable. This is the first function we have learned outside of our Image module that modifies the input but returns nothing. Try the following and see what happens:

    >>> scores = [ 59, 61, 63, 63, 68, 64, 58 ]
>>> new_scores = scores.sort()
>>> scores
[58, 59, 61, 63, 63, 64, 68]
>>> new_scores
>>>
    • Ok, what is the value of the variable new_scores? It is unclear. Let’s try in a different way.
    >>> print(scores)
[58, 59, 61, 63, 63, 64, 68]
>>> print(new_scores)
None
>>>

  • So, the function returns nothing! But, it does change the value of the input list.

  • It does so because lists are containers, and functions can manipulate what is inside containers. Functions cannot do so for simple types like integer and float.

  • If we want a new list that is sorted without changing the original list then we use the sorted() function:

    >>> scores = [ 59, 61, 63, 63, 68, 64, 58 ]
>>> new_scores = sorted(scores)
>>> scores
[ 59, 61, 63, 63, 68, 64, 58 ]
>>> new_scores
[58, 59, 61, 63, 63, 64, 68]
>>>

More Functions: Appending Values, Inserting Values, Deleting

  • Now, we will see more functions that can change the value of a list without returning anything.
  • Armed with this knowledge, we can figure out how to add and remove values from a list:
    • append()
    • insert()
    • pop()
    • remove()
  • These operations are fundamental to any “container” — an object type that stores other objects.
    • Lists are our first example of a container

Lists of Lists

  • Note that lists can contain any mixture of values, including other lists.

  • For example, in

    >>> L = [ 'Alice', 3.75, ['MATH', 'CSCI', 'PSYC' ], 'PA' ]
    • L[0] is the name,
    • L[1] is the GPA
    • L[2] is a list of courses
    • L[2][0] is the 0th course, 'MATH'
    • L[3] is a home state abbreviation

  • We will write code to print the courses, to change the math course to a stats course, and to append a zipcode.

Additional Practice Problems

  1. Write three different ways of removing the last value — 'Pluto' — from the list of planets. Two of these will use the method pop.
  2. Write code to insert 'Asteroid belt' between 'Mars' and 'Jupiter'.

Summary

  • Lists are sequences of values, allowing these values to be collected and processed together.
  • Individual values can be accessed and changed through indexing.
  • Functions and methods can be used to return important properties of lists like min(), max(), sum().
  • Functions and methods can be also used to modify lists, but not return anything.