Programming constructs - CCEAProgram Flow

Sequence

Sequence simply means executing each instruction once, in the order it is given.

Example:

Asking for a user's name and printing it on screen

name = input("What is your name?\n")
print("Hello", name)

Outputs:

What is your name?: Mike
Hello Mike

The program was executed one sequence at a time.

Selection

There are times you don't want to execute everything in a sequence. You may want the program to select which set of instructions to execute based on certain conditions.

We can use conditional statements to test a variable against a value and act in one way if the condition is met or another way if it is not. This can be represented as:

IF (Condition evaluates to True): DO THIS
ELSE: DO THIS OTHER THING

Example:

name = input("What is your name?\n")
password = input("Please enter password: ")
if name == "Mike": print("Hello Mike")
if password =="neuromancer": print("Access Granted")
else: print("Access Denied - Wrong Password")

Outputs:

What is your name?: Mike
Hello Mike
Please enter password: neuromancer
Access Granted

or

What is your name?: Mike
Hello Mike
Please enter password: countzero
Access Denied - Wrong Password

IF statements

IF statements are very useful if there are two possible outcomes. However, if your program needs to evaluate multiple conditional statements, you can use:

IF > ELIF (or ELSE IF) > ELSE

Example:

name = input("What is your name?\n")
age = int(input("What age are you? "))
if age < 13: print("You are a child")
elif age >12 AND age <20: print("You are a teenager")
else: print("You are an adult")

Outputs:

What is your name?: Alice
What age are you?: 12
You are a child

or

What is your name?: Alice
What age are you?: 13
You are a teenager

or

What is your name?: Alice
What age are you?: 20
You are an adult