if and else

What you will be able to do

After this lesson you can

• write an if statement, and an if with an else,
• explain what the colon and the indent do,
• say which lines run and which do not, for a given value, and
• write a program that asks and decides.

if: do something only when a condition is True

An if statement has a condition, the question, and some lines that run only when the answer is True.

What did the program do?

1. It asked the question: is age >= 18?
2. If True, it ran the lines under if.
3. If False, it ran the lines under else.
4. In both cases, it then went on to the last line, which belongs to neither part.

Only one of the two parts runs. Never both.

Three details that you must get right

The first line of an if ends with a colon :. The lines that belong to the if are indented, by four spaces. The words if and else are written in small letters.

The indent is how Python knows which lines belong to the if. In many languages, curly brackets do this job. In Python, it is the white space. This is a strong rule, so be careful: the lines in one block must have the same indent, and the lines that come after the block go back to the left edge.

What happens when you forget the indent?

An age of exactly 18 goes to the if part, because 18 >= 18 is True.

An if without an else

An else is not necessary. Sometimes you only want to do something extra in a special case.

Ask, then decide

A program that decides on what the user types is a real program. Remember: input() gives text, so we change it with int().

Type 15 in the Input box and run it. The program says that you are not an adult yet, and it works out how many years remain. Run it with 30. Run it with 18: the condition age >= 18 is True for 18 exactly. This kind of test, at the edge, is important, and we study it in a later lesson.

A useful pattern: even or odd

An even number leaves a remainder of 0 when divided by 2.

Common mistakes

• Forgetting the colon at the end of the if or the else line.
• Forgetting the indent, or mixing different indents in one block.
• Using = instead of == in the condition.
• Putting the else at a different indent from the if. The two words must line up.
• Writing a condition after else. else never has a condition. For that, use elif, in the next lesson.
• Forgetting that the unindented lines always run.

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 Can you vote? The checker runs your decision on four ages, and says which ones fail.
2. Build. Open the problem Can you vote? and the problem Is the number even or odd?
3. Stretch. Open the problem Fix the indentation. Then write a program that asks for a number and says whether it is positive or not.

Remember these

• if condition: and then indented lines. The lines run only when the condition is True.
• else: runs when the condition is False. Only one of the two parts runs.
• The colon and the indent are part of the rule. Four spaces for each block.
• number % 2 == 0 tests for an even number.

Check yourself

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

1. How many parts of if and else run in one run?

2. Which of the lines after an if block run in every case?

3. What does if x = 5: do?

4. How do you test that n is odd?