Lists

Overview

Teaching: 20 min
Exercises: 15 min

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.

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

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.

print('first item of temperatures:', temperatures[0])
print('fifth item of temperatures:', temperatures[4])

first item of temperatures: 17.3
fifth 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.

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

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.

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

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.

primes = [2, 3, 5, 7, 11]
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, 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.

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('first character:', element[0])
print('fourth character:', element[3])

first character: c
fourth 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.

element[0] = 'C'

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.

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

IndexError: string index out of range

Fill in the Blanks

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

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

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

Solution

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

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

From Strings to Lists and Back

Given this:

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

['t', 'i', 'n']
'gold'

1. Explain in simple terms what list('some string') does.
2. What does '-'.join(['x', 'y']) generate?

Solution

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?

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.)

Solution

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?

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?

Solution

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]

How Large is a Slice?

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

Solution

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

Slice Bounds

What does the following program print?

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

Solution

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().

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

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

Solution

Program A:

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

Program B:

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[:].

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

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

Solution

Program A:

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

Program B:

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.