Project: a shop bill

What you will be able to do

After this lesson you can

• turn a real problem into a plan of small steps,
• write the plan as a program with variables and good names,
• check a program by hand, and by changing the facts, and
• find and fix a mistake by reading the message.

The problem

A shop sells pens and notebooks. A customer buys 3 pens at 12 rupees each and 2 notebooks at 45 rupees each. The shop adds a tax of 5 percent. We want a program that shows the bill.

Step 1: plan on paper, before you type

Good programmers think first. Write the plan in plain words. Do the bill by hand for these numbers.

1. Pens: 3 times 12 is 36 rupees.
2. Notebooks: 2 times 45 is 90 rupees.
3. Before tax: 36 plus 90 is 126 rupees.
4. Tax: 5 percent of 126 is 126 times 5 divided by 100, which is 6.3 rupees.
5. The total: 126 plus 6.3 is 132.3 rupees.

You now know the right answer: 132.3. When the program shows the same, it is probably right. This is the most important habit in this course: know the answer before you run the program.

Step 2: choose the variables

Look at the plan. Which values do we have to keep? Each fact gets its own variable, with a name that says what it is.

• The facts that we are given: pen_price, pen_count, notebook_price, notebook_count, tax_percent.
• The values that we work out: pen_total, notebook_total, subtotal, tax, total.

Why not write 36 and 90 in the program? Because the shop will change a price tomorrow. With variables, you change one line, and the whole bill is right. With numbers written into every line, you must find and change each one, and you will forget one.

Step 3: write the program

Here is the whole program.

The program has four parts.

1. The facts. Five variables with numbers. These are the only lines that you change when the shop changes a price.
2. The cost of each kind of item. Price times count.
3. The bill. The subtotal, the tax and the total, each one worked out from the earlier ones.
4. The display. print() for each line. The spaces after the words make the colons line up, so the bill is easy to read.

Each slider is one fact at the top of the program. Changing a fact never needs a change in the formulas below it.

Step 4: test it with other facts

A program that works for one set of numbers may still be wrong. Test it with new facts, and work out the answer by hand each time.

Change pen_count = 3 to pen_count = 10. By hand: 10 times 12 is 120, the subtotal is 120 plus 90, which is 210, the tax is 10.5, and the total is 220.5. Run it, and check.

Now change tax_percent = 5 to tax_percent = 0. The tax must be 0, and the total must be the same as the subtotal. Does it show that?

Step 5: when it breaks

Programmers meet errors all the time. A good programmer reads the message instead of being afraid of it. Suppose you type pen_totl in one place instead of pen_total. What does the message say?

• NameError: name 'pen_totl' is not defined tells you the name that Python cannot find. Look for it in the program, and compare it letter by letter with the variable that you made.
• The line number in the message tells you where.

Try it. Change pen_total to pen_totl in the line subtotal = pen_total + notebook_total. Run it, read the message, and fix it.

Make it your own

A project is yours only when you change it. Choose one of these, and do it without looking at a solution.

1. Add a third item, such as erasers, with its own price and count. The subtotal must include it.
2. Add a discount. If the customer gets 10 percent off the subtotal, where does the discount go in the plan? Work it out by hand first.
3. Split the bill. Three friends share the total. Add a variable friends and show what each person pays, rounded to two decimal places with round().

Practice

Start with two quick questions. Each New numbers gives you a fresh one, so you can practise as long as you like.

Remember these

• Plan on paper first. Work out the answer by hand, so that you know what is right.
• Keep every fact in a variable with a good name, so that one change updates everything.
• Build a program in steps: the facts, the parts, the result, and the display.
• Test with other numbers. Read an error message from the bottom up.

Check yourself

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

1. Why do we keep tax_percent in a variable and not write 5 in the formula?

2. In the bill, which line would you change if the pen price went up to 15?

3. What is the first thing to do when a program gives a wrong answer?