Looping over Dictionaries

Now that we know what a dictionary is, it's time to loop through one. Just like we looped through lists, we can loop through dictionaries. But since a dictionary has keys and values, things are a bit different

player = {
    "name": "Tommy Vercetti",
    "health": 100,
    "city": "Vice City"
}

for i in player:
    print(i)

Output

name
health
city

Wait, it only showed the keys? Yes. By default, when we loop through a dictionary, Python gives us the keys. But we can easily get the values too

player = {
    "name": "Tommy Vercetti",
    "health": 100,
    "city": "Vice City"
}

for i in player:
    print(player[i])

Output

Tommy Vercetti
100
Vice City

We used player[i] to grab the value for each key. We took i as the iterator, which goes through each key in the dictionary one by one. This works, but Python has a nicer way

.items() gives us both the key and the value at the same time

player = {
    "name": "Tommy Vercetti",
    "health": 100,
    "city": "Vice City"
}

for key, value in player.items():
    print(f"The key {key} with value {value}")

We see how Python views player.items(), it knows this means a key: value pair, so it automatically assigns key to whatever is on the left of the : ("name", "health", "city") and value to whatever is on the right of the : ("Tommy Vercetti", 100, "Vice City")

If we only need the keys, we can use .keys()

player = {
    "name": "Tommy Vercetti",
    "health": 100,
    "city": "Vice City"
}

for k in player.keys():
    print(k)

Output

name
health
city

If we only need the values, we use .values()

player = {
    "name": "Tommy Vercetti",
    "health": 100,
    "city": "Vice City"
}
for v in player.values():
    print(v)

Output

Tommy Vercetti
100
Vice City

A very common use case: counting things. Let's say we want to count how many times each letter appears in a word

word = "banana"
counter = {}

for letter in word:
    if letter in counter:
        counter[letter] = counter[letter] + 1
    else:
        counter[letter] = 1

print(counter)

Output

{'b': 1, 'a': 3, 'n': 2}

This is called a frequency dictionary or histogram. We check: is the letter already in the dictionary? If yes, increase the count. If not, start it at 1

len() works on dictionaries too

player = {
    "name": "Tommy Vercetti",
    "health": 100,
    "city": "Vice City"
}
print(len(player))

Output 3: the dictionary has 3 key-value pairs

Mission: Crew Scores

You have a dictionary of crew members and their scores from the last mission (already on the right). Loop through it and build a report.

Print, in this order:

1. The name of each member (loop with .keys())
2. The score of each member (loop with .values())
3. Total: then the sum of all scores
4. Average: then the total divided by the number of members (use len())
5. Top: then the name of the member with the highest score

Output

Tommy
Lance
Cortez
Phil
Ken
Sonny
Diaz
Avery
Umberto
Mercedes
88
95
70
90
65
78
84
72
60
83
Total: 785
Average: 78.5
Top: Lance