Ranges and the word in

What you will be able to do

After this lesson you can

• check that a number is inside a range, in two ways,
• ask whether a value is inside a group with in and not in,
• see why letter == "a" or "e" is a trap, and
• write a check for vowels, weekdays or any other group.

Is the number inside a range?

A value is in a range when it is at least the smallest and at most the biggest. Two comparisons joined with and say it exactly. Python also lets you write it as one chain, as you do in mathematics.

Two things to watch.

1. The edges. <= includes the end value, < does not. Test with the end values, as you learned in the last chapter.
2. The comment at the bottom shows the mistake marks >= 50 and <= 75. Remove the # to see the SyntaxError, and then put the # back. Each comparison needs a variable on its left.

The short chain and the long way with and always give the same answer.

The word in

Often a value must be one of several allowed choices. The word in asks: is this value inside that group?

A trap that catches many beginners

You want to test that a letter is a vowel. In English you say "if the letter is a or e or i or o or u". So you write it like that:

Python reads the condition as five separate pieces: letter == "a", then "e", then "i", "o", "u". The first piece is False. But the others are not comparisons. They are bare pieces of text, and a piece of text that is not empty counts as True. So the whole or is True, whatever the letter is.

There are two correct ways.

Version 1: letter == "a" or letter == "e" or letter == "i" or letter == "o" or letter == "u"

Version 2: letter in "aeiou"

Put either one after the word if and a colon.

The second is short, clear and easy to extend. Whenever you catch yourself writing a long line of or with the same variable, think of in.

Common mistakes

• x == 1 or 2. The 2 alone is always True. Write x == 1 or x == 2, or x in [1, 2].
• Forgetting that in is case sensitive for text. Use letter.lower() in "aeiou".
• in with a number and a text: 5 in "12345" is a TypeError.
• Edges of a range that are one off.
• Writing not letter in "aeiou" and not knowing it is the same as letter not in "aeiou". Both work. The second reads better.

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 Vowel or not? and use in to write the test.
2. Build. Open the problem Vowel or not? and the problem Weekend or school day?
3. Stretch. Open the problem Find the trap and explain, in one sentence, why the program says yes for every letter.

Remember these

• A range is two comparisons joined with and, or the chain low <= x <= high.
• x in group asks whether x is inside the group. x not in group is the opposite.
• Never write x == a or b. Write x == a or x == b, or x in (a, b).

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" in "banana"? And "nan" in "banana"?

2. Which test is wrong: x == 1 or x == 2, x == 1 or 2, x in [1, 2]?

3. Write a test for "the age is from 13 to 19 inclusive".