Dictionaries

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Dictionaries#

The final basic data structure that we will be covering in this course is the dictionary (of type dict). This collection is referred to as a “dictionary” because it consists of key-value pairs. In the case of a literal dictionary, there are a set of words which act as the “keys” and a set of definitions which act as the “values”. In a programming context, it doesn’t have to be words and definitions, but the same principle applies. Let’s consider a simple example:

group_2 = {'Be' : 4,
           'Mg' : 12,
           'Ca' : 20,
           'Sr' : 38,
           'Ba' : 56}

type(group_2)

dict

In this example, we have created a dictionary in which the keys are the symbols of the group \(2\) elements, and the values are their corresponding atomic numbers. The general syntax for creating a dictionary is a comma-separated collection of key : value pairs enclosed by curly brackets:

name_of_dictionary = {key_1 : value_1, key_2 : value_2, key_3 : value_3}

Once a dictionary has been created, its keys can be used just to retrieve its values, much like an index in a list:

group_2['Ca']

20

Note that whilst this process is similar to indexing a list, dictionaries cannot be indexed (with an int) like a list:

group_2[2]

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
Cell In[3], line 1
----> 1 group_2[2]

KeyError: 2

Error

Aha, a new error type! This is a KeyError, which indicates that the key we are trying to use does not exist in the dictionary. Sure enough, the keys here are all strings of group \(2\) elements, so 2 is not a valid key.

Unlike tuples, dictionaries are mutable, so we can add new key : value pairs:

group_2['Ra'] = 88

print(group_2)

{'Be': 4, 'Mg': 12, 'Ca': 20, 'Sr': 38, 'Ba': 56, 'Ra': 88}

We can also modify values that are already in the dictionary. For example, imagine that we wanted to replace the atomic number for Be with its atomic mass:

group_2['Be'] = 9.0122

print(group_2)

{'Be': 9.0122, 'Mg': 12, 'Ca': 20, 'Sr': 38, 'Ba': 56, 'Ra': 88}

Above we have replaced the atomic number, but there is also nothing stopping us from setting the value for the key 'Be' to a tuple or list of both the atomic number and the atomic mass:

group_2['Be'] = (4, 9.0122)

print(group_2)

{'Be': (4, 9.0122), 'Mg': 12, 'Ca': 20, 'Sr': 38, 'Ba': 56, 'Ra': 88}

Exercise#

Create a dictionary describing some of the basic physical properties of silicon. For example, you could have keys like 'Symbol', 'Atomic Number', 'Atomic Mass', 'Melting Point', 'Electronegativity' etc, with appropriate values set for each one. Test your dictionary once you have assigned it to a variable by trying to access and print different values with the appropriate keys.

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