Sets: no repeats

What you will be able to do

After this lesson you can

• make a set, and remove repeats from a list,
• add and remove values,
• count how many different values there are, and
• show a set in order.

Every value only once

A set is a bag of values in which nothing is repeated. It does not keep an order, so there is no s[0]. A set is useful when you want to know what is there, and not how often or where.

Think of the list of names of the students who came to school today. A student who comes in twice is still one student. A set keeps the different names.

Predict first: the list [3, 1, 3, 2, 1] has repeats. What will len(unique) be? Then run it.

• set(numbers) turned the list into a set with only 1, 2 and 3. So len(unique) is 3.
• A set has no order, so we use sorted(unique) to show it in order. The result of sorted is a list.
• colours.add("blue") puts a value in. colours.add("red") again changes nothing, because red is there.
• "green" in colours asks whether it is in the set. For a set, this is very fast, even if the set has a million values. For a list, Python looks at the items one by one.
• colours.discard("green") takes it out. It does not complain if the value is not there. (The method remove does complain, with a KeyError.)
• {} makes an empty dictionary. An empty set is set(). This is the one place where Python is a little odd.

No order, no positions

Also, a set shows its items in an order that you cannot predict. Never depend on it. Use sorted() for a neat output, and you will get the same output on every computer.

What you can do

However many rolls you make, the set never has more than 6 items: a die has only six faces.

Two jobs that sets do very well

What a set can hold

A set can hold only values that cannot change: numbers, text, tuples. It cannot hold a list or another set, for the same reason that a list cannot be a dictionary key. {[1, 2]} is a TypeError.

Common mistakes

• {} for an empty set. It is an empty dictionary. Use set().
• Expecting an order. A set has none. Use sorted().
• Using s[0]. A TypeError.
• s.remove(x) for a missing value. A KeyError. Use discard.
• Putting a list in a set. A TypeError.
• Forgetting that a set loses the repeats, when you need to count them. Use a dictionary to count.

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 How many different values? You count the different values of a list, and the checker confirms the answers.
2. Build. Open the problem Drop the repeated numbers and the problem How many different words?
3. Stretch. Open the problem Different letters and the problem Is it a vowel?

Remember these

• A set keeps each value once, and has no order. set(list) removes the repeats.
• add puts in, discard takes out without an error. x in s is a quick test.
• set() is an empty set. {} is an empty dictionary.
• A set cannot hold a list. Use sorted(s) to show it in order.

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 len({1, 2, 2, 3, 3, 3})?

2. How do you make an empty set?

3. How do you show a set in order?

4. Which is faster for a big collection, x in a_list or x in a_set?