The Alcorn Method · Python

If this, otherwise that: tiered prices

A complete step-by-step sheet, the way we write one for a learner. You will build two small programs that charge one price up to a cutoff and a cheaper price after it. Everything you need is on this page. Nothing here assumes you remember last week.

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· This is a demonstration sheet on an original problem. Learners' real coursework sheets stay private, between them and their mentor.

Start here, every time

Open IDLE (or Thonny, or VS Code). You should see a window where you can type Python.
File → New File. Save it right away as tickets.py in a folder called python-class.
To run: press F5 (IDLE) or the green Run button. The output appears in the other window.

Rules for this sheet

Ask with input( ) and wrap it in int( ). No typing the number into the code.
One decision, made with if and else. Not two ifs.
Spell a variable the same way every time it appears.
Money prints with a dollar sign and exactly two decimals.

Words you need

variable a labeled jar that holds one value. tickets is a jar with a number in it.
input( ) the program asks a question and waits for you to type.
int( ) turns what you typed into a whole number so math works on it.
if / else a fork in the road. One side runs when the test is true, the other side when it is not.
indent four spaces at the start of a line. It means 'this line lives inside the fork'.
.format( ) dresses a number for printing: ${0:,.2f} means dollar sign, commas, two decimals.

Part 1 · Practice

Game tickets

The first 4 tickets are $25 each. Every ticket after the 4th is $20. Build it in four steps. Check the box when the screen matches.

Part 1 · Step 1Ask for the number
Type these two lines. The first asks a question and turns the answer into a whole number. The second prints it back so you know the program heard you.
```
tickets = int(input("How many tickets? "))
print(tickets)
```
You should see
```
How many tickets? 3
3
```
Part 1 · Step 2Decide which price applies
Delete the print line. Add the fork: if the number is 4 or fewer, every ticket is $25. Otherwise the first 4 cost $100 and the rest are $20 each. Note the colon at the end of the if line and the else line, and the four spaces at the start of the lines inside.
```
tickets = int(input("How many tickets? "))
if tickets <= 4:
    cost = 25 * tickets
else:
    cost = 100 + 20 * (tickets - 4)
print(cost)
```
You should see
```
How many tickets? 6
140
```
Part 1 · Step 3Dress the number as money
Replace the last line. The {0:,.2f} part means: put the first value here, with commas, with exactly two decimals.
```
print("Total is ${0:,.2f}".format(cost))
```
You should see
```
How many tickets? 6
Total is $140.00
```
Part 1 · Step 4Test both roads
Run it three times. Each answer below comes from the real program; if yours differs, compare your if line and your else line to Step 2, character by character.
You should see
```
How many tickets? 3
Total is $75.00

How many tickets? 4
Total is $100.00

How many tickets? 10
Total is $220.00
```

Part 2 · Your task

Study hall T-shirts

Save a new file as shirts.py. The first 10 shirts are $12 each. After 10, each extra shirt is $9. Same four steps, same shape. Where you see ____, fill in the blank yourself; the "You should see" box tells you whether you got it right.

Part 2 · Step 1Ask for the number
Same shape as Part 1, Step 1. Fill the blank with the name you will use for the count. Use the same spelling every time it appears.
```
____ = int(input("How many shirts? "))
print(____)
```
You should see
```
How many shirts? 5
5
```
Part 2 · Step 2Decide which price applies
Delete the print line. First 10 shirts are $12 each. After that, the first 10 cost $120 and each extra shirt is $9. Fill the four blanks: the cutoff, the single price, the flat amount, and the extra price.
```
shirts = int(input("How many shirts? "))
if shirts <= ____:
    cost = ____ * shirts
else:
    cost = ____ + ____ * (shirts - 10)
print(cost)
```
You should see
```
How many shirts? 14
156
```
Part 2 · Step 3Dress the number as money
Replace the last line so the output says Order total, with the dollar sign, commas, and two decimals.
```
print("Order total is ${0:,.2f}".format(cost))
```
You should see
```
How many shirts? 14
Order total is $156.00
```

Part 2 · Step 4Test both roads, and a big one

Run it three times. The last one checks that the comma shows up.

You should see

How many shirts? 5
Order total is $60.00

How many shirts? 10
Order total is $120.00

How many shirts? 120
Order total is $1,110.00

If you see red text

Reading an error is a skill. Find the last line of the red text in the left column.

What you see	What it means	What to do
`NameError: name 'ticket' is not defined`	You spelled the variable two different ways. Python thinks they are two different jars.	Find every copy of the name and make the spelling match the first line exactly.
`SyntaxError: expected ':'`	The if line or the else line is missing its colon at the end.	Add the colon. Both lines need one.
`IndentationError: expected an indented block`	The line after if or else is not pushed in.	Put exactly four spaces at the start of each line inside the fork.
`TypeError: can't multiply sequence by non-int of type 'float'`	You forgot int( ) around input( ). The answer came in as text, not a number.	Make the first line match Step 1: int(input(...)).
`ValueError: invalid literal for int()`	You typed something that is not a whole number when the program asked, like 'six' or '6.5'.	Run it again and type digits only.
`It prints 140 instead of $140.00`	You are still on Step 2's print line.	Do Step 3: replace the last line with the .format version.

Final check before you submit

Run the program once with a small number and once with a big number. Both print a line that starts with Order total is $.
Count the decimals with your finger: exactly two.
Type 120. The output has a comma: $1,110.00.
Every copy of the variable name is spelled the same way.
Save the file. Close it. Open it again. Run it. It still works.

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If this, otherwise that: tiered prices

Start here, every time

Rules for this sheet

Words you need

Game tickets

Study hall T-shirts

If you see red text

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