Comparing values

What you will be able to do

After this lesson you can

• ask a question about two values with the six comparison signs,
• say what each one gives, True or False,
• tell = and == apart, and
• compare numbers and text.

Programs that choose

Think about your day. If it rains, you take an umbrella. If the bell rings, you go to class. If you are hungry, you eat. All of these are decisions, and each one begins with a question that has a yes or no answer.

A program needs the same. A game asks: is the score more than the high score? A website asks: is the password right? A calculator asks: is the divisor zero? To ask such questions, Python has comparisons.

The six comparison signs

A comparison asks a question about two values. The answer is always True or False, the fourth type of value that you met in the variables course.

Notice the two-sign forms. <= is the less-than sign followed directly by an equals sign, and the other way round does not exist. There is no space between the two characters. != is an exclamation mark and an equals sign.

One equals sign or two?

This is the most common mistake of beginners, and it is worth a moment.

When you write a decision, you ask a question, so you need two signs. If you write one, Python stops with a SyntaxError. This is a help: it tells you that you made the usual mistake.

a != b is always the opposite of a == b, and a >= b the opposite of a < b. Only one of each pair is True.

Comparing text

You can also compare text. Look at the next lines of the program.

• "apple" == "apple" is True.
• "apple" == "Apple" is False. A capital letter is a different character, so the two pieces of text are different.
• "apple" < "banana" is True. Text is compared in alphabetical order, from the first letter on. a comes before b.
• "Zebra" < "apple" is also True. In the computer, all the capital letters come before the small letters. So an alphabetical sort with capitals and small letters mixed gives surprises. A common solution is to make both small first, with lower().
• 5 == 5.0 is True. A whole number and a decimal number with the same value are equal.
• "5" == 5 is False. Text is never equal to a number. This is the reason why you must change what a user types with int().

A comparison is a value

The answer of a comparison is a value, and a value can be kept in a variable. For example, is_adult = age >= 18 stores True or False. A name that starts with is_ or has_ is a good name for such a variable, because it reads like a question.

Common mistakes

• Writing = when you want to ask. Use ==.
• Writing =< or =>. The right forms are <= and >=, with the equals sign second.
• Comparing text with a number. "18" >= 18 gives a TypeError. Change the text first.
• Forgetting the capitals. "yes" == "Yes" is False. Use answer.lower() == "yes".
• Putting spaces inside the sign. = = is wrong, == is right.

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 True or False? Write your guess for six comparisons. The checker shows the true answers.
2. Build. Open the problem Which comparisons are true? and the problem One equals sign or two?
3. Stretch. Open the problem Comparing text. Say what each comparison gives, before you run it.

Remember these

• A comparison gives True or False. The signs are ==, !=, <, >, <= and >=.
• One = stores a value. Two == ask whether two values are equal.
• Text is compared letter by letter, and capital letters come before small letters.
• Keep the answer in a variable if you need it later, for example is_adult = age >= 18.

Check yourself

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

1. What are 7 > 7 and 7 >= 7?

2. What is the difference between x = 3 and x == 3?

3. What does "Cat" == "cat" give?

4. Which one is right: => or >=?