Lists and loops

What you will be able to do

After this lesson you can

• go through a list with a for loop,
• find a total, an average, the biggest and the smallest,
• count the items that pass a test,
• build a new list with a loop, and
• use two lists together by position.

Go through a list

In the loops chapter, you wrote for letter in "python":. A list works in the same way. In each round, the loop variable holds the next item.

Then three ready-made functions do the same work in one line each.

• sum(marks) adds the items.
• max(marks) is the biggest item.
• min(marks) is the smallest item.

The average is the sum divided by the number of items: sum(marks) / len(marks).

So why did we write the loop at all? Because the loop is the pattern that you will need when no ready-made function exists: for example, "the total of the marks that are at least 60". Learn the pattern, and use the functions when they fit.

Counting, and building a new list

All three have the same shape: start before the loop, change inside, use after.

These four tools save you a loop each. Behind the scenes, each one goes through the list once.

A warning: do not change a list while you loop over it

If you remove items from a list inside a for loop over the same list, Python moves on by position, and some items are skipped. The safe way is to build a new list of the items that you want to keep, as in pattern 3.

When you need the position

Sometimes you need both the item and its position. Two lists in the same order work like a table: position 0 of names belongs to position 0 of marks.

A rule for choosing:

• Only the items: for item in a_list:.
• Items and positions: for i, item in enumerate(a_list):.
• Two lists together: for i in range(len(a_list)):, reading both at position i.

Common mistakes

• Dividing by the wrong number. The average is the sum divided by len(marks).
• Starting the total inside the loop, so it starts again in every round.
• max([]) or sum-then-divide on an empty list. An empty list gives an error (max) or a division by zero. Check len(a_list) > 0 first.
• Forgetting append. squares + [n * n] does not change squares. Use squares.append(n * n).
• Two lists of different lengths. One position is missing in one of them, and the loop gives an IndexError.

Practice

Start with two quick questions. Each New numbers gives you a fresh one, so you can practise as long as you like.

Then the labs and the problems:

1. Warm-up. Open the lab Facts about a list of marks. You find the total, the average and the extremes.
2. Build. Open the problem The total and the average and the problem How many passed?
3. Stretch. Open the problem Remove the repeats and build a list without the repeated items.

Remember these

• for item in a_list: goes through the items.
• Use sum(), max(), min() and len() for quick answers.
• Count with a counter and an if. Build with [] and append.
• Do not change a list while you loop over it. Build a new one.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What is sum([2, 4, 6])?

2. What is max([5, 9, 7]) - min([5, 9, 7])?

3. How do you find the average of a list called marks?

4. Which loop gives the position and the item together?