Text in, numbers out

What you will be able to do

After this lesson you can

• explain why input() gives text, and what goes wrong if you forget it,
• change text to a whole number or to a decimal number,
• change a number back to text, and
• read a ValueError and say what the user typed to cause it.

What the user types is text

Here is the most common mistake in every first program that asks questions.

input() always gives text. It does not matter what the user types. The digits 15 come as the text "15". This is why a program that adds two typed numbers shows 34 for 3 and 4.

Text to a whole number: int()

To calculate, we must change the text into a number. For a whole number, use int().

Type 15 in the Input box and run it. The best way is to do both steps in one line: int(input("...")). The input is read first, as text, and then int() turns it into a number. Now age + 1 is a real sum, 16, and the type is <class 'int'>.

The same + sign does two different jobs. Before you calculate with an answer from input(), turn it into a number.

Text to a decimal number: float()

For a number with a decimal point, use float().

Type 1.6 in the Input box and run it. float() can read 1.6. It can also read a whole number such as 7, and it gives 7.0. But int() cannot read a decimal point, as we see next.

Number to text: str()

Sometimes you must go the other way. You can join a number to text only when the number is changed to text first.

When the text cannot be a number

What if the user types something that is not a number?

Type 3.5 in the Input box and run it. Python stops with ValueError: invalid literal for int() with base 10: '3.5'. In plain words: the text 3.5 cannot be made into a whole number. Run it again, with the word ten. Same error. A ValueError means "this value is the right kind of thing, but a wrong one".

Later, in the course on try and except, you will learn how to catch this error, so that your program does not stop. For now, be careful with what you type, and use float() when a decimal point is possible.

A table to remember

Common mistakes

• Forgetting int() or float(). The program shows 1515 instead of 30.
• Using int() on a number with a decimal point. It gives a ValueError. Use float().
• Joining text and a number with +. "Age: " + 15 is a TypeError. Use str(15), or a comma.
• Converting at the wrong time. int(input("Age? ") + 1) is wrong, because the brackets close in the wrong place. Write int(input("Age? ")) + 1.

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 Add two numbers. The program is given two answers. Convert them and show the sum.
2. Build. Open the lab Age in ten years, and the problem Your age in ten years.
3. Stretch. Open the problem A ValueError. Say what the user typed, and fix the program so that it accepts it.

Remember these

• input() always gives text.
• int() makes a whole number, float() a decimal number, str() text.
• int(input("...")) asks and converts in one line.
• A word, or a number with a decimal point given to int(), gives a ValueError.

Check yourself

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

1. The user types 4 and 5. What does input() + input() give?

2. What do you write to read a whole number from the user?

3. Which function reads the answer 2.75?

4. What is the error if you write int("hello")?