B2.3.4 Construct functions and modularization.

--> Time: 45 minutes. Work in pairs, one laptop per pair. <-- 

STAGE 1 - READ (10 min)

Open the FAQ entry "B2.3.4 Construct functions and modularization"
on faq.computersciencewiki.org. Read sections 1 and 3 only
(Functions, and The Principle of Scope). Skip the Java examples.

Copy this into your notebook and fill in the blanks from the reading:

1. A function is a named block of code that performs a ______ task.
2. The inputs to a function are called ______.
3. The word for sending a value back out of a function is ______.
4. A variable created inside a function is ______ to that function,
   which means code outside cannot see it.

STAGE 2 - PREDICT (10 min)

Laptops CLOSED. Write down what each one prints. Then open your
laptop and run them to check.

A)  def double(number):
        return number * 2

    print(double(5))

B)  def shout(word):
        print(word.upper())

    result = shout("hi")
    print(result)

C)  def add(a, b):
        return a + b

    print(add(2, 3) + add(10, 1))

D)  name = "Sam"

    def show():
        print(name)

    show()

Answers:
A) 10
B) HI  then  None
   (shout prints but never returns, so result is empty)
C) 16
D) Sam
   (name was made outside the function, so it is global
   and the function can see it)

STAGE 3 - BUILD (20 min)

Make a new file called my_functions.py. Write these four functions.
Each one is one or two lines. Run the file after EACH function
so you catch mistakes early.

1. greet(name)
   Takes a name. Returns "Hello Ana!" when given "Ana".

2. double(number)
   Takes a number. Returns that number times 2.

3. area(width, height)
   Takes two numbers. Returns width times height.

4. is_long(word)
   Takes a word. Returns True if it is longer than 5 letters,
   otherwise False.

Now add these lines at the bottom and run the file.
You should see exactly the output shown on the right:

    print(greet("Ana"))      ->  Hello Ana!
    print(double(7))         ->  14
    print(area(3, 4))        ->  12
    print(is_long("banana")) ->  True
    print(is_long("cat"))    ->  False

Two rules:
 - Every function must use return, not print.
 - Do not write any function longer than two lines.

STUCK? Here is the shape every one of them follows:

    def name_of_function(input):
        return something

STAGE 4 - EXIT TICKET (5 min)

Answer on paper and hand in:

1. You wrote double(number). Show the line of code you would write
   to double the number 30 and store the answer in a variable
   called big.

2. Why is it better to write double() once and call it five times,
   instead of writing number * 2 in five different places?

3. Look at your area() function. Why does it need two parameters
   when greet() only needs one?

IF YOU FINISH EARLY

Add a fifth function called triple(number) that returns the number
times 3. Then write one line that uses double() and triple()
together on the same number and prints the total.