CS 022A Lecture 10: CSS22A #9: Dictionaries
16 Dec 2019
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● Dictionary
○ A dictionary is a way of storing pairs of data as key: value
○ Dictionaries are similar to what their name suggests: an index of words and for
each word a definition
○ In python, the word is called a key and the definition is a value
○ Each pair of data consisting of a key and a value is called an item
○ Keys are associated with the values
○ When storing items in a dictionary we separate them with commas
○ The keys and values in a dictionary are not numbered or stored in order
○ The syntax for creating a dictionary is similar to creating a list but we use curly
brackets rather than square ones
● Python dictionary vs physical dictionary
○ Key ←→ value
○ Word ← → definition
○ Looking at the key in a python dictionary is similar to the looking for a particular
word in a physical dictionary
○ The key is used to look up the value
○ Values are mapped to keys
● Dictionary syntax
○ Key-value pairs are separated by a colon and the pairs are separated by
commas and all this is enclosed in a pair of curly braces
○ The keys must be unique values can repeat
○ For keys: only immutable objects
○ For values: either immutable or mutable objects can be used
○ The key-value pairs in a dictionary are not ordered in any manner if you want a
particular order you will have to sort the dictionary before using it
● Creating a dictionary
○ Three methods for creating a dictionary
■ Method: initialize items during definition: d ={“a” :1, “b” :2}
■ Method 2: create an empty dictionary then add key/value pairs to it
● Step 1 : create an empty dictionary a pair of curly brackets is
used: emptyDict{ }
● Step 2: add elements using square brackets on the left hand side
of an assignment
■ Method 3: using the built in dict() function to construct a dictionary the
argument to dict() are a sequence of key-value pairs
■ Dictionary entries are not stored in a particular order
● The first pair added in could be the last pair shown
● Look up -- retrieve data from the dictionary
○ The main use for dictionaries is to look up value associated with a particular key
○ Dictionary elements are accessed via keys
○ To look up a value use the dictionary name followed by the key in square bracket
○ Instead of using the index of the element use the key for the value to retrieve
