2.9. Figuring out an Invoice

We can use variables to solve problems like those we might solve in a spreadsheet. Imagine that you had a spreadsheet with an invoice for an office supply company.

A spreadsheet with order information

Here’s a program to compute the total price for the invoice:

quantity1 = 2 unitPrice1 = 7.56 total1 = quantity1 * unitPrice1 quantity2 = 4 unitPrice2 = 4.71 total2 = quantity2 * unitPrice2 invoiceTotal = total1 + total2 print(invoiceTotal)

How many variables are in this program?

• 7
• Yes, quantity1, unitPrice1, total1, quantity2, unitPrice2, total2, invoiceTotal.
• 6
• There are three variables per line, two lines, and one total.
• 5
• There are three variables per line, two lines, and one total.
• 2
• There are three variables per line, two lines, and one total.

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We don’t really have to create new variables quantity2 and unitPrice2. We only use those to compute the total for the line, and then we could reuse those variable names.

Activity: CodeLens 2.9.3 (cspnamenumbers_invoice3)

How many variables are in this program?

• 7
• We have two fewer variables now.
• 6
• We have a total for each line (two), a quantity, a unitPrice, and an invoiceTotal.
• 5
• The variables are quantity, unitPrice, total1, total2, and invoiceTotal.
• 2
• We have a total for each line (two), a quantity, a unitPrice, and an invoiceTotal.

Let’s say that apples are $0.40 apiece, and pears are $0.65 apiece. Modify the program below to calculate the total cost (it should print 3.55).

numApples = 4 numPears = 3 costPerApple = costPerPear = totalCost = print(totalCost)

You are welcome to try out the following answers by copying and pasting them into the program above before answering this question:

Which line of code will compute the correct totalCost if put into the program above?

• totalCost = numApples + numPears
• That does not consider the cost of the apples or pears.
• totalCost = (costPerApple * numApples) + (costPerPear * numPears)
• We need to multiply the cost per apple times the number of apples and add it to the cost per pear times the number of pears.
• totalCost = (costPerApple * numPears) + (costPerPear * numApples)
• That gets the costs backwards
• totalCost = (0.4 * numApples) + (0.65 + numPears)
• That would work, but giving names to numbers makes code easier to understand.

Write the code to calculate and print how many paperclips you can buy if each paperclip is $0.05 and you have $4.00 in your pocket. It should print 80.

Create variables to hold each value. Calculate numPaperclips as budget / costPerClip. Be sure to print the result.

# DECLARE VARIABLES AND ASSIGN VALUES costPerClip = .05 budget = 4.00 # 2. CREATE FORMULA numPaperclips = budget / costPerClip # 3. PRINT RESULT print(numPaperclips)

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