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, 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.

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 query it from a list.

• Items in a list are assigned an ‘index’ value to represent their relative order
• GotCha: The index starts at the value 0, not 1, so be careful about off-by-one bugs in your code

print('First item of pressures is zeroth index:', pressures[0])
print('Fourth index is fifth 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 directly.

• 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.

• Trying to assign to an index that doesn’t exist will return an error

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

IndexError: string index out of range

• Use list_name.append to add items to the end of a list (i.e., beyond currently assigned indices)

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.
  • 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:

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

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.']

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).

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: 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

Ranges of indices can be specified using the : character (Called ‘slicing’)

To retrieve multiple points in a list, specify the start index and the end index plus one

primes = [2, 3, 5, 7, 9, 11, 13, 17, 19]
print(primes[2:5])

[5, 7, 9]

If you want to get all of the items from the beginning of the list until some index, leave the left side of the colon blank

primes = [2, 3, 5, 7, 9, 11, 13, 17, 19]
print(primes[:5])

[2, 3, 5, 7, 9]

Similarly, leaving the right side blank will return all items until the end of the list

primes = [2, 3, 5, 7, 9, 11, 13, 17, 19]
print(primes[5:])

[11, 13, 17, 19]

Or get really fancy by specifying a ‘stride’ parameter, which allows you to jump items.

primes = [2, 3, 5, 7, 9, 11, 13, 17, 19]

# Only take every other item in the slice
print(primes[1:6:2])

[3, 7, 11]

Fill in the Blanks

Copy the following into a blank notebook cell and fill in the blanks so that the program produces the output shown lower down.

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]

How Large is a Slice?

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

Solution

values = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
low = 3
high = 7
print(len(values[low:high]))

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?

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.)

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?

Slice Bounds

What does the following program print?

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

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)

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)

Key Points

• A list stores many values in a single structure.

• Use an item’s index to query 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.