Lists

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 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.

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

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

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

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]
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]]

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.

primes = [2, 3, 5, 7, 9]
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]

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

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.']

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])

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
Cell In[10], line 1
----> 1 print('99th element of element is:', element[99])

NameError: name 'element' is not defined

Exercise

1. 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]

2. If values is a list that has 30 elements, how many elements does the list values[2:27] have?

3. Given this:

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

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?

4. 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?

5. 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)

Takeaway

• 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.

Continue

6. Lists
7. Loops
8. Functions