Counting with dictionaries

Python · Unit 7: Dictionaries

A dictionary is great for counting: each different thing is a key and its value is how many times it showed up.

If the key is already there, add one; if not, create it with 1.

counts = {}
for letter in "banana":
    if letter in counts:
        counts[letter] += 1
    else:
        counts[letter] = 1
print(counts)

Prints

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

The rest of the explanation is in the lesson, which is part of the full plan.

Exercises in this lesson

You do them in the app, which checks them on the spot and explains why.

1. Predict the output
What does this code print?
2. Complete the code
Complete what happens the first time a letter shows up.
3. Complete the code
Complete it to count how many times each fruit shows up.
4. Predict the output
What does this code print?
5. Find the bug
This program should count how many times each animal shows up. Which line has the bug?
6. Predict the output
What does this code print?
7. Put the lines in order
Put in order the function that returns how many times each character of a text shows up.
8. Multiple choice
counts already has the words of a text counted. What does this code do?
9. Find the case that fails
tally(text) should count each letter ignoring case: A and a are the same letter. Which call does it fail with?
10. Write the code
Write most_repeated(words), which takes a list of words and returns the one that repeats the most. If there's a tie, the one that showed up first wins. The list is never empty.

It opens in your browser. This lesson is part of the full plan; the first unit of each course is free.