Python supports a container type called a dictionary.
This is also known as an “associative array”, “map” or “hash” in other languages.
In a list, we use a number to look up an element:
names="Martin Luther King".split(" ")
names[1]
‘Luther’
In a dictionary, we look up an element using another object of our choice:
me = { "name": "James", "age": 39, "Jobs": ["Programmer", "Teacher"] }
print(me)
{‘name’: ‘James’, ‘age’: 39, ‘Jobs’: [‘Programmer’, ‘Teacher’]}
print(me['Jobs'])
[‘Programmer’, ‘Teacher’]
print(type(me))
<class ‘dict’>
The things we can use to look up with are called keys:
me.keys()
dict_keys([‘name’, ‘age’, ‘Jobs’])
The things we can look up are called values:
me.values()
dict_values([‘James’, 39, [‘Programmer’ , ‘Teacher’]])
When we test for containment on a dict we test on the keys:
'Jobs' in me
True
For values this needs to be done differently
'James' in me
False
'James' in me.values()
True
The way in which dictionaries work is one of the coolest things in computer science: the “hash table”. This is way beyond the scope of this course, but it has a consequence:
You can only use immutable things as keys.
good_match = {("Lamb", "Mint"): True, ("Bacon", "Chocolate"): False}
but:
illegal = {[1,2]: 3}
—————————————————————————
TypeError Traceback (most recent call last)
<ipython-input-12-cca03b227ff4> in
—-> 1 illegal = {[1,2]: 3}
TypeError: unhashable type: ‘list’
Supplementary material: You can start to learn about the ‘hash table’ in this YouTube Video
This material is very advanced, but, I think, really interesting!
Another consequence of the way dictionaries work is that there’s no guaranteed order among the elements:
my_dict = {'0': 0, '1':1, '2': 2, '3': 3, '4': 4}
print(my_dict)
print(my_dict.values())
{‘4’: 4, ‘0’: 0, ‘1’: 1, ‘2’: 2, ‘3’: 3}
dict_values([4, 0, 1, 2, 3])
A set is like a list which cannot contain the same element twice.
name = "James Hetherington"
unique_letters = set(name)
unique_letters
{‘ ‘, ‘H’, ‘J’, ‘a’, ‘e’, ‘g’, ‘h’, ‘i’, ‘m’, ‘n’, ‘o’, ‘r’, ‘s’, ‘t’}
print("".join(unique_letters))
mth sareJHngoi
To see more clearly how join works:
"".join(['a', 'b', 'c'])
‘abc’
Similarly to a dictionary, a set has no particular order for the elements it contains. It is really useful for checking or storing unique values.
alist = [1, 2, 3]
is_unique = len(set(alist)) == len(alist)
print(is_unique)
True
Supplementary material: For more information about sets please see the Python documentation https://docs.python.org/3/library/stdtypes.html#set
If we have a dictionary, x
x = {'a':1, 'b':2}
We can access the value associated with a key
x['a']
1
However, if the key does not exist an error is generated.
x['fish']
—————————————————————————
KeyError Traceback (most recent call last)
<ipython-input-21-81af752ce649> in <module>()
—-> 1 x[‘fish’]
KeyError: ‘fish’
Using the get() method, we can access the value via the key
x.get('a')
1
x.get('fish')
If the key is not available the default value None will be returned.
print(x.get('fish'))
None
Alternatively, we can define the value to be returned.
x.get('fish', 'tuna') == 'tuna'
True
In this example, the default value for key fish is tuna.