A decision inside a decision

What you will be able to do

After this lesson you can

• put one if inside another and keep the indents right,
• follow which message appears for given values,
• write the same decision in a flat form with and and elif, and
• choose the clearer form.

Decisions inside decisions

Sometimes the second question makes sense only after the first answer. At a club door, the guard first asks about your age. Only if you are old enough does the guard look for a ticket. Python does the same, with an if inside an if.

1. The outer question: age >= 18? It is True for 20.
2. So Python goes inside, and asks the inner question: has_ticket? It is True.
3. So the output is Welcome in.

The indents

Look at the indents. The inner if is indented by four spaces under the outer if. The lines of the inner block are indented by eight. The else of the inner if lines up with the inner if, and the else of the outer if lines up with the outer if. The indent tells Python which else belongs to which if.

A good way to check: draw a vertical line down from each if. The else that belongs to it must touch the same line.

Each end of the tree is one print(). There are three ends, so three tests reach every one of them.

The same decision, written flat

A nested decision is often possible to write flat, using and and elif.

Which one is better? There is no rule. Use the one that is clearer to read.

• Nested is clear when the second question really belongs to the first, as at a door, and when the inner part has a few lines of its own.
• Flat is clear when each branch is a separate reason. It stays at one level of indent, so a long program stays easy to read.

A word of advice: if you notice a third level of if inside if inside if, stop. Something is wrong. Try to make it flat, or split the code into functions (Chapter 12).

Testing a nested decision

With two questions there are four combinations, and each should be tested.

The last two rows give the same answer. That is fine, but they show an important thing: the ticket does not change the answer when the person is too young.

Common mistakes

• A wrong indent that attaches the else to the wrong if.
• Nesting too deep. Three levels are very hard to read.
• Repeating a test. If you ask age >= 18 in the outer if, you do not need to ask it again in the inner one.
• Forgetting one combination in the tests.

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 club door and write the nested decision. The checker tries all four combinations.
2. Build. Open the problem A login check and the problem Fix the condition.
3. Stretch. Write the same login check in the flat form, and say which one is clearer.

Remember these

• An inner if is asked only when the outer condition is True. It has one more indent.
• The indent decides which else belongs to which if.
• Many nested decisions can be written flat with and and elif. Choose the clearer one.
• Test every combination.

Check yourself

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

1. In the club program, when is the ticket question asked?

2. How many tests do you need for two yes/no questions?

3. How can you make a nested decision flat?