Boxes with Names: Variables

Name: Boxes with Names: Variables
Author: StemPy Lab

Every program needs a place to keep what it knows: a score, a name, a price. In this course you learn to keep values in variables, to change them step by step, to follow a program by hand, to tell the four kinds of value apart, to choose names that other people can read, and to put it all together in a small project. Every lesson ends with practice that checks your answers for you.

5 lessons, about 3 h 10 min5 simulations14 problems10 practice questionsPython in every lesson

Created by StemPy Lab

Course outline (5 lessons)

Lesson 1about 40 min
Making a variable
A variable is a name that holds a value. The = sign stores the value on its right in the name on its left, so age = 12 is correct and 12 = age is an error. We use the name, without quotation marks, to get the value back. A variable can be given a new value, and then the old value is forgotten. A variable must be made before it is used, or Python shows a NameError.
1 simulation2 practice questions4 Python programs5 problems2 Python labs
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Lesson 2about 40 min
Changing a variable and tracing a program
A new value of a variable can be worked out from its old value: score = score + 10. The short forms +=, -=, *= and /= do the same. The order of the lines matters, so we follow a program with a trace table, one row for each line. To swap two variables we keep the first value in a third variable, or we write a, b = b, a. A swap written as a = b and then b = a does not work, because the first value is lost.
1 simulation2 practice questions5 Python programs2 problems2 Python labs
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Lesson 3about 45 min
The four types of value
Every value has a type. An int is a whole number, a float has a decimal point, a str is text inside quotation marks and a bool is True or False. The function type() tells which one a value is. The same + sign adds numbers and joins text, so text that looks like a number must be changed with int() or float() before a sum. A float can show a very small error, as in 0.1 + 0.2, so we round when we show it.
1 simulation2 practice questions6 Python programs4 problems2 Python labs1 question set
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Lesson 4about 40 min
Choosing good names
A name can have letters, digits and underscores, but it cannot start with a digit, contain a space or a hyphen, or be a Python keyword. Small and capital letters are different, so score and Score are two variables. A good name says what the variable holds, and the usual style is small letters with underscores, as in total_marks. Never give a variable the name of a function that Python already has, such as str or print. A comment, which starts with #, explains the program to a person.
1 simulation2 practice questions4 Python programs3 problems1 Python lab
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Lesson 5about 25 min
Project: a shop bill
In this project we write a shop bill in four steps: the facts, the cost of each kind of item, the bill before tax, the tax and the total, and the display. We keep each fact in a variable, so that one change updates the whole bill. We check the answer by hand, change the facts to test it, and read an error message when it breaks. The project uses everything in this course: variables, updates, types and good names.
1 simulation2 practice questions1 Python program1 Python lab
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Variables: Predict the Output

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Boxes with Names: Variables

Course outline (5 lessons)

Making a variable

Changing a variable and tracing a program

The four types of value

Choosing good names

Project: a shop bill

Test Your Learning

Your teacher

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