More than two choices: elif

What you will be able to do

After this lesson you can

• choose between three or more possibilities with elif,
• explain why the first True condition wins,
• put the checks in the right order, and
• find a bug that comes from the wrong order.

When two choices are not enough

A grade is not only pass or fail. It can be A, B, C or D. A ticket price depends on the age in several steps. For such a decision with many outcomes, you use elif, which means else if.

The picture shows a ladder of questions. Python starts at the top, and goes down one step at a time. The first condition that is True decides, and the rest of the ladder is skipped.

The first True wins

Because Python stops at the first True, the order of the checks matters. Look at this program, which has a mistake.

The rule for the order: put the most special (the hardest) condition first. For a grade, start with the highest marks. Then each elif only has to ask about its own step, because the higher ones have already been taken out.

The check for B does not need marks < 90. Python reaches it only when the check for A was False.

Why the conditions can be short

In the correct program, the second check is marks >= 75. It does not need to say marks >= 75 and marks < 90. Why? Because the first check already caught everything from 90 up. If we reach the second check, the marks must be less than 90. Each step uses what the earlier steps have already ruled out.

A ticket office price

Another example. A ticket is free for children under 5, costs 50 rupees under 13, 100 rupees under 60, and 70 rupees for older people.

A ladder is only one if

A common question is: when should I use elif, and when should I write a new if? Use elif when the choices exclude each other: exactly one of them is right. Use separate if statements when each condition is a separate question, and more than one can be true.

Common mistakes

• The wrong order. A weak condition first catches everything.
• Writing else if with two words. Python's word is elif.
• A condition after else. else has none.
• A new if where you need elif. Then two parts can run for the same value.
• Edges that are one off, as in < and <=. Always test the values at the edges.

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 The grade. The checker tests your ladder with seven marks, and shows the ones that fail.
2. Build. Open the problem A grade from the marks and the problem The price of a ticket.
3. Stretch. Open the problem Order of the checks. Find the bug, and say which input shows it.

Remember these

• elif means else if. Python stops at the first condition that is True.
• else at the end catches all that is left.
• Put the most special condition first.
• Each elif can be short, because the earlier conditions have ruled things out.

Check yourself

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

1. In a ladder of one if, two elif and an else, how many parts run?

2. Marks are 75. Which grade does the ladder of this lesson give, and why?

3. Why is marks >= 50 a bad first check for a grade ladder?

4. When do you use separate if statements instead of elif?