Lists

Last updated on 2023-05-08 | 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.

  • Scenario: You have set up an Arduino to do temperature measurements in a storage room for rare books.
  • Doing calculations with a hundred variables called temperature_001, temperature_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 

temperatures = [17.3, 17.5, 17.7, 17.5, 17.6]
print('temperatures:', temperatures)
print('length:', len(temperatures))

OUTPUT 

temperatures: [17.3, 17.5, 17.7, 17.5, 17.6]
length: 5

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

  • Just like strings.

PYTHON 

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

OUTPUT 

zeroth item of temperatures: 17.3
fourth item of temperatures: 17.6

Lists’ values can be replaced by assigning to them.

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

PYTHON 

temperatures[0] = 16.5
print('temperatures is now:', temperatures)

OUTPUT 

temperatures is now: [16.5, 17.5, 17.7, 17.5, 17.6]

Appending items to a list lengthens it.

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

PYTHON 

print('temperatures is initially:', temperatures)
temperatures.append(17.9)
temperatures.append(18.2)
print('temperatures has become:', temperatures)

OUTPUT 

temperatures is initially: [16.5, 17.5, 17.7, 17.5, 17.6]
temperatures has become: [16.5, 17.5, 17.7, 17.5, 17.6, 17.9, 18.2]
  • 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.

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.

PYTHON 

primes = [2, 3, 5, 7, 11]
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, 11]
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: cannot 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[1:3]
print('second time:', values)

OUTPUT 

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

How Large is a Slice?

If ‘low’ and ‘high’ are both non-negative integers, how long is the list values[low:high]?

The list’s length would be equal to high - low.

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 

['t', 'i', 'n']
'gold'
  1. Explain in simple terms what list('some string') does.
  2. What does '-'.join(['x', 'y']) generate?
  1. It creates a list of the some strings characters as elements.
  2. It creates a string composed of x and y, separated by a hyphen character(-).

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.)

OUTPUT 

m
  1. A negative index begins at the final element.
  2. -(N) corresponds to the first index, which is the [0] index.
  3. It removes the final element of the list.
  4. You could do the following: print(values[0:-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?

OUTPUT 

furn
eniroulf
  1. stride indicates both the number of steps, and from which end: positive starts from first element, negative from the last element.
  2. element[1::2]

Slice Bounds

What does the following program print?

PYTHON 

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

OUTPUT 

lithium 
''

There is no 20th index, so the entire string is captured.
There is no element after the -1 index.

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:

OUTPUT 

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

Program B:

OUTPUT 

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

sorted(letters) returns a sorted copy of the list, while letters.sort() sorted the list in place. Thus, it was already sorted, and calling a further sort returns None.

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:

OUTPUT 

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

Program B:

OUTPUT 

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

new = old is assigning old to new, whereas new = old[:] is a slice assignment, which will only return a copy of old.

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.