Sub programs (functions and procedures) to produce structured code

How to use sub programs (functions and procedures) to produce structured code

2.2.3 Additional Programming Techniques

Additional Programming Techniques — Note 8 of 12

The difference and use of functions and procedures and where to use them effectively

What you need to know

Sub-programs (also called sub-routines) are reusable blocks of code that perform a specific task. There are two types: functions return a value, while procedures perform a task without returning a value. Using sub-programs makes code modular, reusable, and easier to test and maintain.

What to know

A function performs a task and RETURNS a value using the return keyword.
A procedure performs a task but does NOT return a value (it may print output instead).
Parameters are values passed into a sub-program when it is called. They MUST be used within the function.
Sub-programs must be CALLED in the main program to execute. Calling: totalPay = calculatePay(experience, miles).
Using sub-programs makes code modular, reusable, and easier to maintain.
Code does not need to be repeated; the sub-program can be called multiple times.
A procedure is called on its own line: greet("Sarah"). A function call is used inside an assignment or expression, because it gives back a value: total = add(5, 3).

Exam technique

Mistake:DO NOT overwrite parameters by asking for input inside the function. If direction and position are given as parameters, USE them — don't do direction = input() inside the function.
Tip:A function must RETURN its result — not just print it. Use return total, not print(total), inside the function.
Tip:When CALLING a function, string parameters need quotation marks: freeseats("Red"), not freeseats(Red). Store the result: redseats = freeseats("Red").
Tip:You can define the function THEN call it separately: result = newPrice(5, "premium") then print(result).

Example:Refining to use a function means replacing inline code with a function call: totalPay = calculatePay(experience, miles) then print(totalPay).

Code examples

Function vs Procedure

python

# FUNCTION - returns a value
def newPrice(nights, room):
    if room == "basic":
        price = 60 * nights
    else:
        price = 80 * nights
    return price   # RETURN, not print

# CALLING the function:
result = newPrice(5, "premium")
print(result)  # Output: 400

# PROCEDURE - does not return a value
def greet(name):
    print("Hello, " + name + "!")

greet("Alice")  # Output: Hello, Alice!

Calling a function vs calling a procedure

python

# Function: returns a value
def add(x, y):
    return x + y

total = add(5, 3)    # the returned value is stored
print(total)         # 8

# Procedure: does a job, returns nothing
def greet(name):
    print("Hello", name)

greet("Sarah")       # outputs: Hello Sarah

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# FUNCTION - returns a value def newPrice(nights, room): if room == "basic": price = 60 * nights else: price = 80 * nights return price # RETURN, not print # CALLING the function: result = newPrice(5, "premium") print(result) # Output: 400 # PROCEDURE - does not return a value def greet(name): print("Hello, " + name + "!") greet("Alice") # Output: Hello, Alice!

# Function: returns a value def add(x, y): return x + y total = add(5, 3) # the returned value is stored print(total) # 8 # Procedure: does a job, returns nothing def greet(name): print("Hello", name) greet("Sarah") # outputs: Hello Sarah