A dictionary in Python is what in other languages we would call an Ordered Hashmap. Its a set of keys and associated values. If you know the key, the machine can get the value extremely quickly.
A dictionary looks like this:
cat_powers = {
'Floofy': 'Flight',
'Jammy': 'Dodging',
'Flakey': 'Speed'
}Get an element from a dictionary like this:
cat_powers[‘Jammy’]
If you try to access a key that doesn’t exist you’ll get a KeyError.
You can insert a new value using the array syntax:
cat_powers['Michael'] = 'Lasers'Note that you can also overwrite a value if you reuse a key, so be careful.
If you want just the keys, or just the values:
cat_powers.keys()
# ['Floofy', 'Jammy', 'Flakey']
cat_powers.values()
# ['Flight', 'Dodging', 'Speed']If you want both, you can convert the Dictionary to an Array of Tuples with the items method:
cat_powers.items()
# [('Floofy', 'Flight'), ('Jammy', 'Dodging'), ('Flakey', 'Speed')]Note that the arrays you get out will be ordered in Python version 3.7 and above. This is actually pretty useful. Hashmaps are not inherantly ordered, they’re just a bag of keys and values, but modern Python keeps them ordered for you.
Because they’re ordered, we can pop from them, like an array.
Assume I have a dictionary of cats, each with a power:
cats = {“alfy”: “guns”, “bert”: “lasers”, “chewy”: “guns”}
We can see that Alfy and Chewy have the same power. I want to collect all the cats together that have the same power, using that power as the key. The output should look like this:
{‘guns’: [‘alfy’, ‘chewy’], ‘lasers’: [‘bert’]}
Please write code to do this.
First let’s create a new dict to write the result to:
out = {}
Now we convert cats to an array of tuples, and use parallel assignment to get the keys and values. If the value is already a key in the new array, append the cat to that value. If not, create an array to hold it.
for key, value in cats.items():
if value in out:
out[value].append(key)
else:
out[value] = [key]
print(out)