Questions:

  • How can I store multiple values?

Objectives:

  • Explain why programs need collections of values.
  • Write programs that create flat lists, index them, slice them, and modify them through assignment and method calls.

Keypoints:

  • 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.
  • Use del to remove items from a list entirely.
  • The empty list contains no values.
  • Lists may contain values of different types.
  • Character strings can be indexed like lists.
  • Character strings are immutable.
  • Indexing beyond the end of the collection is an error.

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.

  • Just like strings.
print('zeroth item of pressures:', pressures[0])
print('fourth item of pressures:', pressures[4])

zeroth item of pressures: 0.273
fourth item of pressures: 0.276

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.
primes = [2, 3, 5]
print('primes is initially:', primes)
primes.append(7)
primes.append(9)
print('primes has become:', primes)

primes is initially: [2, 3, 5]
primes has become: [2, 3, 5, 7, 9]
  • append is a method of lists. A method is like a function, but tied to a particular object.
  • Use object_name.method_name to call methods.
  • We will meet other methods of lists as we go along - you can use help(list) for a preview.
  • extend is similar to append, but it allows you to combine two lists. For example:
teen_primes = [11, 13, 17, 19]
middle_aged_primes = [37, 41, 43, 47]
print('primes is currently:', primes)
primes.extend(teen_primes)
print('primes has now become:', primes)
primes.append(middle_aged_primes)
print('primes has finally become:', primes)

primes is currently: [2, 3, 5, 7, 9]
primes has now become: [2, 3, 5, 7, 9, 11, 13, 17, 19]
primes has finally become: [2, 3, 5, 7, 9, 11, 13, 17, 19, [37, 41, 43, 47]]

Note that while extend maintains the "flat" structure of the list, appending a list to a list makes the result two-dimensional.

Use del to remove items from a list entirely.

  • del list_name[index] removes an item from a list and shortens the list.
  • Not a function or a method, but a statement in the language.
print('primes before removing last item:', primes)
del primes[4]
print('primes after removing last item:', primes)

primes before removing last item: [2, 3, 5, 7, 9, 11, 13, 17, 19, [37, 41, 43, 47]]
primes after removing last item: [2, 3, 5, 7, 11, 13, 17, 19, [37, 41, 43, 47]]

The empty list contains no values.

  • Use [] on its own to represent a list that doesn't contain any values.
    • "The zero of lists."
  • Helpful as a starting point for collecting values (which we will see in the next episode).

Lists may contain values of different types.

  • A single list may contain numbers, strings, and anything else.
goals = [1, 'Create lists.', 2, 'Extract items from lists.', 3, 'Modify lists.']

Character strings can be indexed like lists.

  • Get single characters from a character string using indexes in square brackets.
element = 'carbon'
print('zeroth character:', element[0])
print('third character:', element[3])

zeroth character: c
third character: b

Character strings are immutable.

  • Cannot change the characters in a string after it has been created.
    • Immutable data types can't be changed after creation.
    • In contrast, lists are mutable: they can be modified in place.
  • Python considers the string to be a single value with parts, not a collection of values.
element[0] = 'C'

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/var/folders/5q/mny3pg2n7h5g21h3v32rfj9wpykqrf/T/ipykernel_11747/1676686116.py in <module>
----> 1 element[0] = 'C'

TypeError: 'str' object does not support item assignment

Indexing beyond the end of the list or string 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.
    • This cannot be detected as the code is parsed because the index might be calculated based on data.
print('99th element of element is:', element[99])

Do the quick-test.

Back to Python part one.