Counting and Grouping

A dictionary is how you count things, and how you split a table into groups.

Counting with a dictionary

The key is the thing being counted and the value is how many you have seen. .get with a default of 0 is what keeps the first sighting from raising.

subjects = ["maths", "computing", "maths"]
counts = {}
for s in subjects:
    counts[s] = counts.get(s, 0) + 1
print(counts)

Grouping is counting with lists

Same shape, except the value is a list you append to instead of a number you add to. This is the whole idea behind every group-by you will meet later.

rows = [("maths", 92), ("computing", 88), ("maths", 79)]
groups = {}
for subject, score in rows:
    groups.setdefault(subject, []).append(score)
print(groups)

Exercise

Try It Yourself

Write count_subjects(rows) that takes a list of (subject, score) pairs and returns a dictionary of how many rows each subject has.

def count_subjects(rows):
    # Return {subject: how many rows}
    return {}

print(count_subjects([("maths", 92), ("computing", 88), ("maths", 79)]))

Press Run to see output

Check Your Understanding

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