ex4.rst

Exercise 4: Variables And Names

Now you can print things with print and you can do math. The next step is to learn about variables. In programming a variable is nothing more than a name for something so you can use the name rather than the something as you code. Programmers use these variable names to make their code read more like English, and because they have lousy memories. If they didn't use good names for things in their software, they'd get lost when they tried to read their code again.

If you get stuck with this exercise, remember the tricks you have been taught so far of finding differences and focusing on details:

1. Write a comment above each line explaining to yourself what it does in English.
2. Read your .py file backwards.
3. Read your .py file out loud saying even the characters.

.. literalinclude:: ex/ex4.py
    :linenos:

Note

The _ in space_in_a_car is called an underscore character. Find out how to type it if you do not already know. We use this character a lot to put an imaginary space between words in variable names.

What You Should See

.. literalinclude:: ex/ex4.txt
    :language: console

Extra Credit

When I wrote this program the first time I had a mistake, and python told me about it like this:

.. literalinclude:: ex/ex4.err

Explain this error in your own words. Make sure you use line numbers and explain why.

Here's more extra credit:

1. I used 4.0 for space_in_a_car, but is that necessary? What happens if it's just 4?
2. Remember that 4.0 is a "floating point" number. Find out what that means.
3. Write comments above each of the variable assignments.
4. Make sure you know what = is called (equals) and that it's making names for things.
5. Remember _ is an underscore character.
6. Try running python as a calculator like you did before and use variable names to do your calculations. Popular variable names are also i, x, and j.