Making and reading lists

What you will be able to do

After this lesson you can

• make a list of numbers, of text, or a mix,
• read one item by its position, from the start or from the end,
• take a slice of a list,
• find out how long a list is and whether a value is in it, and
• read an IndexError.

Why a list?

A class has 30 students, and each has a mark. To keep the marks, you would need mark1, mark2, all the way to mark30. That is a lot of names, and a loop could not go through them. It is much better to keep all the marks in one variable. That is what a list is: many values, one name, in a fixed order.

Making a list

You write a list in square brackets, with commas between the values.

A list shows itself with the square brackets and the commas, exactly as you wrote it. The text items are shown with quotation marks.

Reading by position

Imagine that the list is a row of boxes, and each box has a position number. The positions start at 0, not at 1. This is the same rule as for the letters of a string (Chapter 5). The last box of a list of 4 has the position 3.

The mistake that everybody makes once

What happens if you ask for a position that does not exist?

The numbers under the bars are the positions. A list of 6 items has the positions 0 to 5, and -6 to -1 from the end.

Finding an item

You know the position and want the item: use fruits[2]. You know the item and want its position: use the method index().

Lists and strings are cousins

Almost everything that you learned about strings works for lists: positions, negative positions, slices, len() and in. There is one big difference: a string cannot be changed, and a list can. The next lesson is about that.

Common mistakes

• Counting from 1. The first item is at position 0.
• Asking for position len(a_list). The last position is len(a_list) - 1, or simply -1.
• Forgetting that a slice is a list. fruits[1:2] is ['mango'], with the brackets, and fruits[1] is 'mango'.
• Forgetting the commas between the values.
• Using round brackets ( ) instead of square brackets. That is a different thing, a tuple.

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 Read and slice. You take items and slices out of a list, and the checker confirms each one.
2. Build. Open the problem Read a list and the problem Slice the list.
3. Stretch. Open the problem Find the position and explain what index() does when the item occurs twice.

Remember these

• A list is written in square brackets with commas. It can hold any values, even a mix, or none.
• Positions start at 0. Negative positions count from the end.
• A slice a[start:stop] stops before stop and is always a list.
• len() counts the items. in asks whether a value is in the list. index() gives a position.
• A position that does not exist gives an IndexError.

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 ["a", "b", "c"][1]?

2. What is len([4, 7, 9, 12])?

3. What is [10, 20, 30][-1]?

4. What is [5, 6, 7, 8][1:3]?