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.

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.

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.

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.

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]
{: .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)

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]]

{: .output}

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)

{: .python}

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

{: .output}

## 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](../09-for-loops/)).

## 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.’]

{: .python}

## 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])

{: .python}

zeroth character: c third character: b

{: .output}

## 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.

element[0] = ‘C’

{: .python}

TypeError: ‘str’ object does not support item assignment

{: .error}

*   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](../05-error-messages/).
    *   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])

{: .python}

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]

How Large is a Slice?

If ‘low’ and ‘high’ are both non-negative integers, how long is the list 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 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.