Storing data: lists and dictionaries

Step 1 Warm-up recall

From M6.L2: what does a function give back?

Your answer

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Step 2 Learning goal

By the end of this lesson you can:

• read an item from a list by position (Foundation);
• look up a value by key (Core);
• choose a structure for given data (Core).

Step 3 A familiar example

A queue of people is a list: order matters. A phone directory is a dictionary: you find a number by a name, not by a position.

Step 4 Clear explanation

A data structure is a way of organising data so it can be used. A list stores items in order, and each item has a position (its index). Many programming languages count positions from 0.

In the figure, the list 4, 8, 15, 16 has length 4; the item at position 0 is 4, the item at position 2 is 15 and the last item is at position 3. A dictionary stores pairs, each with a key and a value, and you look up a value by its key.

Looking up the key Raj gives 102. A table of records keeps several facts about each thing, one row per thing. To choose a structure, ask how you will find an item.

Using a dictionary for lookup by name avoids searching a long list from the start, which links back to the cost of linear search in Module 5. The entry on object-oriented programming shows a more advanced way of grouping data with the functions that use it.

Step 5 Worked example (4 marks)

The list is 4, 8, 15, 16. Give the item at position 1, the item at position 3 and the length.

1. Positions start at 0: 4 is at 0, 8 at 1, 15 at 2, 16 at 3. (1 mark)
2. Position 1 holds 8. (1 mark)
3. Position 3 holds 16. (1 mark)
4. Answer: 8, 16 and length 4. (1 mark)

Step 6 Learner practice (instant-marked)

Step 7 Recap

• A list is ordered and found by position from 0.
• A dictionary is found by key.
• Choose a structure by how you will look items up.

Step 8 Exit ticket (ungraded)

For the list 7, 2, 9, 5 give the item at positions 0 and 3.

Your answer

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Step 9 Go further (Extension)

Read about object-oriented programming and note how it groups data with functions.