Collections

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About Collections

• An important part of a programming language is the ability to store and manipulate collections of values. For example, a string is a sequence of characters.

• A collection of values can either be ordered or unordered. An ordered collection is a sequence of values. A sequence has a first element, second element, and so on. A string is a sequence of characters.

• Every collection can be iterated using a for loop:

for element in collection:
    do something with the element

• The for loop will repeat the statements in its action block once for each element in the collection

• The variable defined in the for (in the example above we called it element) will be assigned to the current element value before each iteration of the block

• Sequences will visit their elements in order

• Every collection can be used with the len built-in function.

• The collection types built-in to Python are

• Sequences str, list, range, and tuple

• Unordered collections set, as well as dict and a dict's keys and values

About Sequences

• str, list, range, and tuple are all sequences.

• Every sequence has access to

• for and len, because they are collections

• sequence[ i ] to retrieve a particular element from index i

• sequence[ i : j ] to retrieve a subsequence from index i through j, not including j

• sequence[ i : ] to retrieve a subsequence from index i through the end of the sequence

• sequence[ : j ] to retrieve a subsequence from the beginning of sequence through j, not including j

• list is a mutable sequence, meaning you can assign its elements new values

• You change a value like this: sequence[ i ] = 1112

• You remove a value like this: del sequence[ i ]

• You can also change or remove entire slices (e.g. del sequence[ i : j ] or sequence[ i : j ] = [ 1, 2, 3 ]) but that is rarely done in practice

• str, range, and tuple are immutable, meaning you cannot change their values

Collection literals

• A literal str is delimited by matching quotes: either " or ' or ''' or """. Its elements are always characters. Example: 'Hi'

• A literal list is delimited by square brackets and separated by commas. Example: [ 1, 2, 'three', 4.0 ]

• A literal tuple is delimited by parentheses and separated by commas. Example: ( 1, 2, 'three', 4.0 )

• A literal range is created using the range type as a function. Examples: range( 5 ), range( 1, 4 ), range( 3, 9, 2 )

• A literal set is delimited by curly braces and separated by commas. Example: { 1, 2, 'three', 4.0 }

• A literal dict is delimited by curly braces and separated by commas (like a set) but also has colons separating key-value pairs. Example: { 1:'one', 'two':2 }

Casting and concatenating collections

• You can often cast one sequence type to another by using type-names as functions.

• All values of any type (collection or not) can be case into a str

• All collections can be cast into a list, tuple, or set

• You cannot cast into a range or dict

• Some collections can be concatenated with other values of the same type using the + operator

• '12' + '34' creates '1234'

• [ 1, 2 ] + [ 3, 4 ] creates [ 1, 2, 3, 4 ]

• ( 1, 2 ) + ( 3, 4 ) creates ( 1, 2, 3, 4 )

• + does not work on ranges, sets, or dicts

About dicts

• The human brain is great with associations; e.g., we typically associate apple with red and January 1st with New Year's Day.

• A dict is Python's software equivallent of a collection of associations.

• In particular, a dict is unnordered collection of associations.

• The elements of dict are called mappings. A mapping associates one object (called the key) with another object (called the value).

• The literal for an empty dict is {}.

• A literal mapping is written using the : to separate the key with its value. For example, the variable flora_to_color defined below is a dict that associates flora with colors.

flora_to_color = { 'spruce': 'green', 'apple': 'red', 'shamrock': 'green',

  'banana': 'yellow', 'potato': 'brown', 'eggplant': 'purple' }

• The len() function works on a dict. len( flora_to_color ) is 6 because the definition flora_to_color included six mappings.

• The brackets [] operator in conjunction with assignment can be used to put a mapping into a dict. The statement below adds the mapping lemon to yellow.

flora_to_color[ 'lemon' ] = 'yellow'

• The keys of a dict are always distinct. If there had already a mapping with 'lemon' as a key, the above assignment would replace it.

• As our flora_to_color example shows, the values of a dict are not necessarily unique.

• More detailed information on a dict is always available in the module documentation section of the website.