Filtering Lists by Condition

The Filtering Puzzle

Suppose a dictionary records how many times each name appears in a dataset. You want only the names whose counts are greater than 10. The key challenge is that a for loop over a dictionary gives you keys. To test a value, you must first use the current key with the index operator, such as counts[key], to retrieve that key's value.

Three Steps in Every Check

1. The for statement retrieves a key from the dictionary.
2. The index operator retrieves the value associated with that key.
3. The if statement compares that value with the filtering condition.

counts = { "Alice": 14, "Bob": 21, "Chuck": 1 } for name in counts: if counts[name] > 10: print(name, counts[name])

Selecting Without Changing

Filtering means selecting the entries that satisfy a condition. The selected entries keep their existing keys and values. The loop may print those entries immediately, or it may place them into another collection for later use. Transforming is a different operation: it changes entries or prepares modified results. A filtering loop should therefore be read as a selection step, not as a value-changing step.

Building a Result Collection

Printing is useful when you only need to inspect the matches. When the matches must be used later, create a separate result collection. A new dictionary can receive the original key and its original value whenever the condition is true.

counts = {
    "Alice": 14,
    "Bob": 21,
    "Chuck": 1
}

filtered = {}

for name in counts:
    if counts[name] > 10:
        filtered[name] = counts[name]

print(filtered)

{'Alice': 14, 'Bob': 21}

The same selection idea can also be used when the desired result is a list, such as a list of keys that pass the value test. The essential pattern remains unchanged: iterate over keys, retrieve each value with the index operator, test the value, and add only matching results to the separate collection.

Mistakes Beginners Make

• Comparing the key instead of the value

  The loop variable contains a key. It does not contain the dictionary value being filtered.

  Fix: Retrieve the value first: if counts[name] > 10:

• Forgetting the index operator

  The comparison tests the key directly rather than the value associated with that key.

  Fix: Use the key inside the dictionary lookup: if counts[key] == 10:

• Using assignment where comparison is required

  The condition needs to compare a retrieved value with a threshold.

  Fix: Use a comparison operator such as ==, >, or another appropriate comparison.

• Modifying the dictionary being iterated over

  The filtering pattern is intended to select entries into a separate result collection.

  Fix: Build a new dictionary, such as filtered, instead of modifying the source dictionary during iteration.

Practice the Pattern

Pattern to Remember

1. A for loop over a dictionary gives you keys.
2. Use the index operator, such as counts[key], to retrieve the value connected to the current key.
3. Test the retrieved value with a condition; the loop body runs only for entries that pass.
4. Filtering selects entries without requiring their values to change.
5. Print matches immediately or collect them in a new dictionary or list for later use.