Lists

Last updated on 2023-07-24 | Edit this page

Estimated time: 20 minutes

Overview

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.

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.

PYTHON 

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

OUTPUT 

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.

PYTHON 

print('zeroth item of pressures:', pressures[0])
print('fourth item of pressures:', pressures[4])

OUTPUT 

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.

PYTHON 

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

OUTPUT 

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.

PYTHON 

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

OUTPUT 

primes is initially: [2, 3, 5]
primes has become: [2, 3, 5, 7]
  • 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.
  • We will meet other methods of lists as we go along.
    • Use help(list) for a preview.
  • extend is similar to append, but it allows you to combine two lists. For example:

PYTHON 

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)

OUTPUT 

primes is currently: [2, 3, 5, 7]
primes has now become: [2, 3, 5, 7, 11, 13, 17, 19]
primes has finally become: [2, 3, 5, 7, 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 means the last element in primes will itself be a list, not an integer. Lists can contain values of any type; therefore, lists of lists are possible.

Use del to remove items from a list entirely.

  • We use del list_name[index] to remove an element from a list (in the example, 9 is not a prime number) and thus shorten it.
  • del is not a function or a method, but a statement in the language.

PYTHON 

primes = [2, 3, 5, 7, 9]
print('primes before removing last item:', primes)
del primes[4]
print('primes after removing last item:', primes)

OUTPUT 

primes before removing last item: [2, 3, 5, 7, 9]
primes after removing last item: [2, 3, 5, 7]

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.

PYTHON 

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.

PYTHON 

element = 'carbon'
print('zeroth character:', element[0])
print('third character:', element[3])

OUTPUT 

zeroth character: c
third character: b

Character strings are immutable.

  • Cannot change the characters in a string after it has been created.
    • Immutable: 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.

PYTHON 

element[0] = 'C'

ERROR 

TypeError: 'str' object does not support item assignment
  • Lists and character strings are both collections.

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.

PYTHON 

print('99th element of element is:', element[99])

OUTPUT 

IndexError: string index out of range

Fill in the Blanks

Fill in the blanks so that the program below produces the output shown.

PYTHON 

values = ____
values.____(1)
values.____(3)
values.____(5)
print('first time:', values)
values = values[____]
print('second time:', values)

OUTPUT 

first time: [1, 3, 5]
second time: [3, 5]

PYTHON 

values = []
values.append(1)
values.append(3)
values.append(5)
print('first time:', values)
values = values[1:]
print('second time:', values)

How Large is a Slice?

If start and stop are both non-negative integers, how long is the list values[start:stop]?

The list values[start:stop] has up to stop - start elements. For example, values[1:4] has the 3 elements values[1], values[2], and values[3]. Why ‘up to’? As we saw in episode 2, if stop is greater than the total length of the list values, we will still get a list back but it will be shorter than expected.

From Strings to Lists and Back

Given this:

PYTHON 

print('string to list:', list('tin'))
print('list to string:', ''.join(['g', 'o', 'l', 'd']))

OUTPUT 

string to list: ['t', 'i', 'n']
list to string: gold
  1. What does list('some string') do?
  2. What does '-'.join(['x', 'y', 'z']) generate?
  1. list('some string') converts a string into a list containing all of its characters.
  2. join returns a string that is the concatenation of each string element in the list and adds the separator between each element in the list. This results in x-y-z. The separator between the elements is the string that provides this method.

Working With the End

What does the following program print?

PYTHON 

element = 'helium'
print(element[-1])
  1. How does Python interpret a negative index?
  2. If a list or string has N elements, what is the most negative index that can safely be used with it, and what location does that index represent?
  3. If values is a list, what does del values[-1] do?
  4. How can you display all elements but the last one without changing values? (Hint: you will need to combine slicing and negative indexing.)

The program prints m.

  1. Python interprets a negative index as starting from the end (as opposed to starting from the beginning). The last element is -1.
  2. The last index that can safely be used with a list of N elements is element -N, which represents the first element.
  3. del values[-1] removes the last element from the list.
  4. values[:-1]

Stepping Through a List

What does the following program print?

PYTHON 

element = 'fluorine'
print(element[::2])
print(element[::-1])
  1. If we write a slice as low:high:stride, what does stride do?
  2. What expression would select all of the even-numbered items from a collection?

The program prints

PYTHON 

furn
eniroulf
  1. stride is the step size of the slice.
  2. The slice 1::2 selects all even-numbered items from a collection: it starts with element 1 (which is the second element, since indexing starts at 0), goes on until the end (since no end is given), and uses a step size of 2 (i.e., selects every second element).

Slice Bounds

What does the following program print?

PYTHON 

element = 'lithium'
print(element[0:20])
print(element[-1:3])

OUTPUT 

lithium

The first statement prints the whole string, since the slice goes beyond the total length of the string. The second statement returns an empty string, because the slice goes “out of bounds” of the string.

Sort and Sorted

What do these two programs print? In simple terms, explain the difference between sorted(letters) and letters.sort().

PYTHON 

# Program A
letters = list('gold')
result = sorted(letters)
print('letters is', letters, 'and result is', result)

PYTHON 

# Program B
letters = list('gold')
result = letters.sort()
print('letters is', letters, 'and result is', result)

Program A prints

OUTPUT 

letters is ['g', 'o', 'l', 'd'] and result is ['d', 'g', 'l', 'o']

Program B prints

OUTPUT 

letters is ['d', 'g', 'l', 'o'] and result is None

sorted(letters) returns a sorted copy of the list letters (the original list letters remains unchanged), while letters.sort() sorts the list letters in-place and does not return anything.

Copying (or Not)

What do these two programs print? In simple terms, explain the difference between new = old and new = old[:].

PYTHON 

# Program A
old = list('gold')
new = old      # simple assignment
new[0] = 'D'
print('new is', new, 'and old is', old)

PYTHON 

# Program B
old = list('gold')
new = old[:]   # assigning a slice
new[0] = 'D'
print('new is', new, 'and old is', old)

Program A prints

OUTPUT 

new is ['D', 'o', 'l', 'd'] and old is ['D', 'o', 'l', 'd']

Program B prints

OUTPUT 

new is ['D', 'o', 'l', 'd'] and old is ['g', 'o', 'l', 'd']

new = old makes new a reference to the list old; new and old point towards the same object.

new = old[:] however creates a new list object new containing all elements from the list old; new and old are different objects.

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.
  • 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.