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Counting with a Dictionary

~10 mincode completion

Counting how often each value appears is one of the most useful things you can do with a dictionary, and you will do it constantly once you reach data work.

The pattern:

counts = {}
for item in items:
    counts[item] = counts.get(item, 0) + 1

Read that middle line carefully. counts.get(item, 0) says "how many have I seen so far, or 0 if this is the first". Then you add 1 and store it back. Without the .get default, the very first time you see a value you would get a KeyError.

Iterating a dictionary:

for key in counts:              # just the keys
for key, value in counts.items():   # both at once

Your task:

Write count_labels(labels) that takes a list of labels and returns a dictionary mapping each label to how many times it appeared.

["cat", "dog", "cat"] returns {"cat": 2, "dog": 1}.

Example Tests

Repeated labels are counted, not overwritten

Input: {"labels":["cat","dog","cat"]}

Expected: {"cat":2,"dog":1}

Every label appearing once gives every count as 1

Input: {"labels":["a","b","c"]}

Expected: {"a":1,"b":1,"c":1}

One label repeated is counted correctly

Input: {"labels":["spam","spam","spam","spam"]}

Expected: {"spam":4}

Python

def count_labels(labels):
    """
    Count how many times each label appears.

    Args:
        labels: a list of strings

    Returns:
        A dict mapping label -> count.
    """
    # YOUR CODE HERE
    pass

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