PR1 – Preparation for Week 4 – Flow Control

Flow Control – If… Then.. Else…

Before we start with if statements, we have to understand a bit better how boolean expressions work.

Boolean Expressions

Boolean are a data structure which may only have two values known as True / False. They are important for controlling the path of instructions.

For example, check if a value is higher than 0.

a = -1
a > 0
False

There are other boolean operators, that allow you to create more complete expressions. For example, if you want to check if a value is between 0 and 1 (inclusive).

b = 0.4
b >= 0 and b <= 1
True

The operation and only returns True if both instances are True.

Now imagine, you want to check the opposite, check is a number is not between 0 and 1 (inclusive). You can’t write:

c = 2
c < 0 and c > 1
False

you have one of the other two boolean operators.

c = 2
c < 0 or c > 1
True

The operation or returns True if any of instances are True.

Alternatively you can take the first expression, and negate it

d = 2
not(d >= 0 and d <= 1)
True

The operation not inverts the value of a boolean. Note that you may have to use parenthesis, to assure the correct order of operations, similar to the multiplication and addition.

d = 2
not d >= 0 and d <= 1
False

If statement

Until now all the instructions that you placed, are executed. If statements allow to conditional execute instructions sets.

n = 10 
if n > 0:
  print("Positive result")

It also allows you to branch your code, and chain multiple instructions, even use alternative branches:

n = 10 
if n > 0:
  print("Positive result")
  print("Yes!")
else:
  print("Negative result")
  print("Oh, no...")

Note, that by using if statements you are creating a new scope, which follows the same rule set in the functions lecture. Please revise the scope section, if you don’t recall.

You can obviously combine this to create more complete functions, for example.

def maximum(a,b):
  """
  Function that returns the highest number between a and b.
  """
  if a > b:
    return a
  else:
    return b

if…elif…elif…elif…else

There are times that, it is relevant to chain multiple conditions, the elif works exactly like you would chain multiple if else if else statements

def filmRating(recommendedMinimumAge: int)
    result = ""
  	if recommendedMinimumAge >= 18:
    	result = "R18"
  	elif recommendedMinimumAge >= 16:
    	result = "R16"
  	elif recommendedMinimumAge >= 12:
    	result = "R12"
  	elif recommendedMinimumAge >= 9:
    	result = "R9"
  	elif recommendedMinimumAge >= 6:
    	result = "R6"
    else:
    	result = "AL"
    return result
  
filmRating(10)
'R9'

Further explanation