Module assignment: ForgeStack receipt

Why this assignment exists

You have now met the main tools of this module. You can declare variables of several types, read numbers and lines from the input, combine them with arithmetic, convert them with casts, protect a fixed rate with const, and check the results for overflow. Knowing each tool is different from using them together on a task with a clear answer. This assignment asks you to do that. It is a small point-of-sale program, the kind of program a shop runs when it prints a bill, and every line of its output depends on a decision you make about types.

Read the whole assignment before you write any code. Then write the program in small steps, compile after each step, and check the output against the examples. The rubric at the end tells you what a 60 percent, an 80 percent and a 100 percent solution look like, so you can judge your own work honestly.

The task

A small cafe prints a receipt for each customer. The cafe's point-of-sale program reads the store name and three items that the customer bought. Each item has a name, a price in cents and a quantity. The program works out the subtotal, adds the tax, and prints the receipt.

Your program must do these things, in this order:

1. Read the store name as one full line, which may contain spaces.
2. Read three items. Each item is one line with three values: the item name, which is one word; the price of one unit in cents; and the quantity bought.
3. Work out the amount for each item, which is its price times its quantity, in cents.
4. Work out the subtotal, which is the sum of the three amounts.
5. Work out the tax. The tax rate is 18 percent, and it is rounded to the nearest cent, with half a cent rounding up.
6. Work out the total, which is the subtotal plus the tax.
7. Print the receipt, as described in the output section below.

The whole program must work in whole numbers until the last step. You will print money with a dollar sign and two digits after the point, but the calculations themselves use cents only. This is the rule from the money lesson, and the judge checks it with large inputs.

The input

The input has exactly four lines. The first line is the store name, which may contain spaces. The next three lines are the items, one per line. Each item line contains three values separated by spaces, in this order: name, price in cents, quantity.

ForgeStack Cafe
Coffee 120 2
Toast 85 1
Juice 250 3

Each price is a whole number of cents from 0 to 1000000000. Each quantity is a whole number from 0 to 1000000000. The name of an item is one word, with no spaces. The store name is up to 60 characters.

The input can contain a quantity of zero. An item bought zero times still appears on the receipt, with a line amount of zero.

The output

The receipt has seven lines. Print them exactly in this form, with one space between the words and no extra spaces at the end of a line. The first line is the store name, exactly as read. The next three lines are the items. Each shows the name, then x and the quantity, then the amount in dollars and cents. The last three lines are the subtotal, the tax and the total.

ForgeStack Cafe
Coffee x2 $2.40
Toast x1 $0.85
Juice x3 $7.50
Subtotal $10.75
Tax 18% $1.94
Total $12.69

Look at the amounts carefully. Two digits always follow the point, so 85 cents prints as $0.85, and 5 cents would print as $0.05. The dollar part has no padding, so 1075 cents prints as $10.75, not $010.75. A tool in the <iomanip> header, setw with setfill, pads the cents to two digits. You will use it in the reference solution in the hidden part of this page.

The tax line says Tax 18%. Print the number from your const variable, so the label and the rate can never disagree.

Examples

Here is the example from the task. Your output must match it exactly, character for character.

Input:
ForgeStack Cafe
Coffee 120 2
Toast 85 1
Juice 250 3
 
Output:
ForgeStack Cafe
Coffee x2 $2.40
Toast x1 $0.85
Juice x3 $7.50
Subtotal $10.75
Tax 18% $1.94
Total $12.69

Work it out by hand before you run it. The coffee line is 120 times 2, which is 240 cents. The toast line is 85 times 1, which is 85 cents. The juice line is 250 times 3, which is 750 cents. The subtotal is 240 plus 85 plus 750, which is 1075 cents. The tax is 18 percent of 1075, which is 193.5 cents, and it rounds half up to 194 cents. The total is 1075 plus 194, which is 1269 cents.

Notice how the tax rounding works. The formula is (subtotal * 18 + 50) / 100, which adds 50 before the whole-number division. Half a cent is 50 on the scale of 100, so the addition moves every exact half up to the next cent. This is the same idea as the cast trick in the casting lesson, done in whole numbers so that no decimal error can creep in.

Here is a second example with a zero quantity. It checks that your program does not skip an item that was bought zero times.

Input:
Night Owl
Tea 30 0
Cake 45 0
Milk 60 4
 
Output:
Night Owl
Tea x0 $0.00
Cake x0 $0.00
Milk x4 $2.40
Subtotal $2.40
Tax 18% $0.43
Total $2.83

Rules for this module

Use only what this module has taught. That means variables, the basic types, cin and getline, arithmetic, casts, auto, const, and the printing tools you have met. Do not use if, loops or your own functions in this assignment, because later modules teach them, and a solution that depends on them is not what this module is checking. Repeat the three item reads and the three item printing lines, since that is the honest result of the rules.

Keep one declaration per line, and give each variable a name that says what it holds. Use const for the tax rate. Do not write the number 18 or the number 50 more than once, except in the tax formula, where the 50 is the half-cent rounding and the 18 comes from the constant.

Use long long for every money value, from the line amounts to the total. The largest input in the judge's tests, a line of a billion cents times a quantity of a billion, is far beyond the range of an int. A program that uses int for money can pass the examples and then fail the judge, which is the reason for this rule.

Requirements checklist

Tick each item when you have verified it yourself. Use the Run button after each step, and compare your output with the examples.

• The program compiles with g++ -std=c++17 and no errors.
• The store name is read with getline, so a name with spaces is kept whole.
• Each item is read with cin >>, in the order name, price, quantity.
• The line amount for each item is price times quantity, in long long.
• The subtotal is the sum of the three line amounts.
• The tax uses the formula (subtotal * TAX_PERCENT + 50) / 100, with TAX_PERCENT declared as a const.
• The total is the subtotal plus the tax.
• Every money amount prints with a dollar sign, the dollars, a point, and exactly two digits of cents.
• A quantity of zero prints x0 and $0.00 and does not break the output.
• The output matches the first example exactly, including the spaces and the final newline.
• The output matches the second example exactly.
• The program handles the large test, in which a line amount is about one hundred billion cents, and prints correct values.
• The code has short comments that explain each calculation, and no unused variables.

Grading rubric

Your grade depends on what the judge sees and on what the code shows. The three levels below describe what each grade looks like in practice, so you can judge the work you have done.

A solution that uses int for money can reach 80 percent on the small examples, but it cannot reach 100 percent, because the large test overflows. A solution that prints the tax without the half-cent rounding can be correct on the first example by luck, and it still loses the rounding marks on other inputs. Test with your own numbers, and look for the cases where a rounding step could go the wrong way.

Test your own cases

Make up three inputs of your own before you submit. Choose one with a large price, one with a quantity of zero, and one where the tax is exactly half a cent. For the half-cent case, find a subtotal where subtotal * 18 ends in 50 when divided by 100. Work out the expected answer by hand, run your program, and compare. If your answer differs, find the first line where the two disagree, and trace the values that produced it.

Hints

The hints below are hidden until you open them. Try each step on your own first, and open the hint only when you are stuck.

Reference solution

The solution is shown below. Try the assignment yourself first. When you open this solution, run it with the examples in the Input box, so you see the output and can compare it with your own.

Run the solution with the large test below. The output should be exactly as shown, which checks the long long arithmetic and the padding together.

Input:
Vault
Gold 999999999 100
Silver 1 5
Bronze 0 1000000000
 
Output:
Vault
Gold x100 $999999999.00
Silver x5 $0.05
Bronze x1000000000 $0.00
Subtotal $999999999.05
Tax 18% $179999999.83
Total $1179999998.88

Notice that the tax in the large test is the half-cent formula at work on a big number. The subtotal in cents is 99999999905, and 18 percent of that is 17999999982.9, which rounds up to 17999999983 cents. A program that used int for the subtotal would already have wrapped by the time it reached the tax.

What this module taught

The assignment brings the whole module together. Here is the list of what it used, and the place in the module where each tool was introduced.

• A program starts at main, and it is compiled with g++ -std=c++17 into an executable that prints the output exactly as the judge expects.
• Variables have a type, a name and a value. You declare them, initialise them and change them by assignment.
• The basic types are int and long long for whole numbers, double for decimals, char for one character, bool for true or false, and string for text.
• cin >> reads values and skips spaces, and getline reads a whole line. A leftover newline can trap a later getline.
• A cast, auto, const and the limits of each type decide whether a calculation is correct, and money belongs in whole cents.
• Compiler messages are read from the top, one at a time, and a judge compares your output exactly.