Table of Contents
Introduction
def calculate(a, b):
answer = a + b
print(f"{a} + {b} = {answer}")
num1 = 10
num2 = 15
calculate(num1, num2)
Explain exactly what you think will happen when this code is executed.
Run the code and check if you are right.
Subroutines
A subroutine is a sequence of instructions to perform a specific task with an identifiable name.
def calculate(a, b):
answer = a + b
print(f"{a} + {b} = {answer}")
Subroutines can be called upon whenever they are required.
def just defines the subroutine. It is not executed unless the subroutine is called.
Parameters are values passed into the subroutine. In the calculate function above, the parameters are a and b.
A subroutine call has arguments. The values held in the arguments are passed to the parameters.
def calculate(a, b):
answer = a + b
print(f"{a} + {b} = {answer}")
num1 = 10
num2 = 5
calculate(num1, num2) # 10 + 5 = 15
What exactly do you think are the arguments passed to the parameters?
Control Flow
The control flow differs when a subroutine call is encountered.
Instead of running statements one after another, the program transfers control ‘out of line’ to the subroutine, and the code in the subroutine is executed.
def greet(name):
print("Hello, " + name + "!")
print("Starting program...")
greet("Alice")
print("Ending program...")
In this example, the greet function is a subroutine that takes a single argument, name. When the greet function is called, control is transferred to the code inside the function, and the message "Hello, Alice!" is printed to the console. Once the function has finished executing, control is transferred back to the main program, and the message "Ending program..." is printed.
Procedures and functions
Subroutines come in two categories:
- Procedures
- Functions
A procedure executes a specific set of commands when it is called. It does not return a value.
def calculate(a, b):
answer = a + b
print(f"{a} + {b} = {answer}")
calculate(5, 10)
A function is used to calculate or process a value based on arguments given. It always returns a value.
def calculate(a, b):
answer = a + b
return answer
answer = calculate(5, 10)
Note: In Python, there is no distinction between a function and a procedure.
It is important to note that when the return line is executed. The function will terminate, returning the value.
Any code after the return is executed is ignored.
Project: Create a Calculator
def add(x, y):
return x + y
def subtract(x, y):
return x - y
def multiply(x, y):
return x * y
def divide(x, y):
return x / y
num1 = float(input("Enter first number: "))
num2 = float(input("Enter second number: "))
operator = input("Would you like to +, -, / or * ? ")
if operator == "+":
result = add(num1, num2)
print(f"The result is {result}")
elif operator == "-":
result = subtract(num1, num2)
print(f"The result is {result}")
elif operator == "*":
result = multiply(num1, num2)
print(f"The result is {result}")
elif operator == "/":
result = divide(num1, num2)
print(f"The result is {result}")
else:
print("Invalid operator")
Questions:
- What exactly do you think will happen when you run the program?
- How can you improve the program?
Scope
The scope of a variable is the section of the program where the variable can be accessed and modified.
Variables can either have global scope or local scope.
Global scope refers to a variable that can be accessed and modified from anywhere in the program, even from inside subroutines.
Local scope refers to variables that can only be accessed and modified within the code block that they were declared.
Not all programming languages handle scope in the same way.
Python has been designed to discourage the modification of global variables, you’ll find out why during this lesson. Variables are local unless otherwise declared.
A variable declared in the main part of the program can be accessed globally by all subroutines.
It cannot be modified unless you explicitly state that the program should modify the global variable.
def example():
print(number)
number = 5 # global variable
example()
print(number)
What exactly will happen when you run this program?
def example():
number = 10
print(number) # outputs 10
number = 5
example()
print(number) # outputs 5
The intention of this subroutine is to modify the global variable number. However, it doesn’t execute as expected.
A new variable is declared with local scope. A global variable cannot be modified unless it is explicitly stated in the program.
Here is a subroutine that will access and modify the global variable.
def example():
global number
number = 10
print(number) # outputs 10
number = 5
example()
print(number) # outputs 10
Global variables are generally seen as bad practice by programmers. This is because it can be difficult to track the value of the global variable if it is modified in other areas of the program. It is also harder to test a function in isolation if it accesses global variables.
If you are tempted to use a global variable, it is a good idea to try and think carefully whether parameter passing would be a better approach.
Constants
A constant is a fixed value that doesn’t change through the execution of the program.
Constants can be helpful in programming when you need to ensure that a value stays the same throughout execution. One example of this might be the value of pi.
Not all programming languages deal with constants in the same way.
In C for example, you need to declare a constant:
const float pi=3.14;
If the program is instructed to change the value of a declared constant, then it will produce an error.
In Python it is different. Python does not have a specific feature for declaring constants. If you wish to use a constant, then you follow a naming convention.
All constants are declared using capital letters.
PI = 3.14
The Python language will still treat this as a variable and it will not produce an error message if the value is changed during execution.
The naming convention is used to let programmers know that this variable is to be used as a constant.
Example
A game designer wants to ensure that all new levels begin with the background moving at the same speed.
This start speed should be the same for all new levels.
INITIAL_SPEED = 5
def level_1():
speed = INITIAL_SPEED
print("Go faster? Y/N")
answer = input()
if answer == "Y":
speed = speed + 5
level_1()
At the start of each new level, the speed is set by assigning the constant SPEED to the variable speed.
INITIAL_SPEED = 5
def level_1():
speed = INITIAL_SPEED
print("Go faster? Y/N")
answer = input()
if answer == "Y":
speed = speed + 5
level_1()
As each level gets harder, the speed will increase at different rates. The speed at the beginning of each level will always be the same.
The programmer could just assign 5 to speed at the beginning of each level. This would remove the need for the constant.
def level_1():
speed = 5
print("Go faster? Y/N")
answer = input()
if answer == "Y":
speed = speed + 5
level_1()
Question:
What would happen if the programmer decided that the start speed should be 10 instead?
They would need to go to every part of the program that referenced the start speed and change each individual value to 10.
By having a constant, the programmer can simply change the value once and it will change the value everywhere.
INITIAL_SPEED = 5
def level_1():
speed = INITIAL_SPEED
print("Go faster? Y/N")
answer = input()
if answer == "Y":
speed = speed + 5
level_1()
Conclusion
In this lesson, we have learned about subroutines, their parameters and arguments, and how they affect control flow. We have also explored the difference between procedures and functions, and how to create a simple calculator program using functions. Additionally, we have discussed the concept of scope, including global and local scope, and the best practices for using global variables. Finally, we have learned about constants and their importance in programming. Understanding these concepts is crucial for writing efficient and well-structured code.
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