Lists

Overview

Teaching: 10 min
Exercises: 10 min

Questions

• How can I store multiple values?

Objectives

• Explain why programs need collections of values.

• Write programs that create flat lists, index them, and modify them through assignment and method calls.

We have been working with one dataset at a time. With programming however, we have the ability to reproduce the same work on multiple sets of data. Let’s look at some concepts that will allow us to later group together a collection of file names for us to repeat analysis on.

A list stores many values in a single structure.

• Doing calculations with a hundred variables called pressure_001, pressure_002, etc., would be at least as slow as doing them by hand.
• Use a list to store many values together.
  • Contained within square brackets [...].
  • Values separated by commas ,.
• Use len to find out how many values are in a list.

pressures = [0.273, 0.275, 0.277, 0.275, 0.276]
print('pressures:', pressures)
print('length:', len(pressures))

pressures: [0.273, 0.275, 0.277, 0.275, 0.276]
length: 5

Use an item’s index to fetch it from a list.

• First item starts at 0.
• Items can be referenced counting from the end with negative numbers.

print('first item:', pressures[0])
print('fifth item:', pressures[4])
print('last item:', pressures[-1])

zeroth item: 0.273
fourth item: 0.276
last item: 0.276

Indexing beyond the end of the collection is an error.

• Python reports an IndexError if we attempt to access a value that doesn’t exist.
  • This is a kind of runtime error.
  • Cannot be detected as the code is parsed because the index might be calculated based on data.

print('100th item:', pressures[99])

IndexError: string index out of range

Lists’ values can be replaced by assigning to them.

• Use an index expression on the left of assignment to replace a value.

pressures[0] = 0.265
print('pressures is now:', pressures)

pressures is now: [0.265, 0.275, 0.277, 0.275, 0.276]

Appending items to a list lengthens it.

• Use list_name.append to add items to the end of a list.

pressures.append(0.269)
print('pressures:', pressures)

pressures: [0.265, 0.275, 0.277, 0.275, 0.276, 0.269]

• append is a method of lists.
  • Like a function, but tied to a particular object.
• Use object_name.method_name to call methods.
  • Deliberately resembles the way we refer to things in a library.
• There are many other methods of lists.
  • Use help(list) to view.

Key Points

• A list stores many values in a single structure.

• Use an item’s index to fetch it from a list.

• Lists’ values can be replaced by assigning to them.

• Appending items to a list lengthens it.

• Indexing beyond the end of the collection is an error.