Dictionaries

The final basic data structure that we will be covering in this course is the dictionary (of type dict). Dictionaries contain an ordered list of key-value pairs - much like in a real dictionary where we have words and definitions.

Dictionary basics

To see how dictionaries work, look at the following example which contains the symbols and atomic numbers of the group two elements

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

type(group_2)
print(group_2)

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

NoteSyntax

We write dictionaries using braces {} and insert a set of key-value pairs separated by a colon :, with commas , between pairs.

The general syntax for a dictionary is

name_of_dictionary = {
    key1: value1,
    key2: value2
}

We could, equivalently, write everything on one line

name_of_dictionary = {key1: value1, key2: value2}

but this becomes quite hard to read when lots of pairs are present.

Dictionary indexing

Once a dictionary has been created, each key can be used to retrieve the corresponding value, similarly to the use of an index with a list. We use square brackets [] immediately after the dictionary name and, for example, obtain the atomic number of calcium with

group_2['Ca']

20

NoteTest Yourself

See what happens when you try and index this dictionary using an int, by attempting to access the first element of group_2 with

group_2[0]

TipResult

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
Cell In[7], line 1
----> 1 group_2[0]

KeyError: 0

Python fails with an error - in this case a KeyError which tells us that the key 0 does not exist. The keys in this dictionary are all str, so using an int wont work.

Modifying dictionaries

Dictionaries are mutable - we can add new key-value pairs as follows

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

or perhaps we could change the value for Be to a tuple containing both the atomic number and 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}

of course, we should probably go through and change all of the entries to follow this pattern - otherwise the dictionary is a bit of a mess of information!

Exercise

Use your knowledge of dictionaries to solve the following problem

Create a dictionary containing the basic physical properties of silicon, by editing the following code so that it executes properly.

silicon_properties = {}

print(f'Silicon\'s atomic mass is {silicon_properties['amass']:.2f} u')
print(f'Silicon\'s atomic symbol is {}')
print(f'Silicon\'s electronegativity is {:.2f}')

Use python to add a new entry to your dictionary containing the melting point of silicon in Kelvin - print this number as a float with two decimal places.

Tip

Don’t just edit the print commands to contain the data, make a dict and index it!