Introduction to Conditional Statements

Introduction

Programs so far have executed every line of code in the text file.
Conditional Statements allow us to execute specific blocks of code based on certain conditions.
We can use this functionality to make a program that reacts to user input.

Structure of a Conditional Statement

A conditional statement is a logical statement in the form "if-then".
"If today is Tuesday then I buy groceries"
Hypothesis
Conclusion

Creating a Conditional Statement

To create a conditional statement in Python, you simply use the keyword "if".


            computers = ['hp', 'toshiba', 'dell', 'lenovo']
            for computer in computers:
              # Conditional statement checking for the string 'hp'.
              if computer == 'hp'
                print(computer.upper())
              else
                print(computer.title())


            HP
            Toshiba
            Dell
            Lenovo

Assignment Operator vs. Comparision Operator

Since we first started programming we have been using the equals sign, =, as an assignment operator
To compare two values to check if they are the same we can use the double-equal sign, ==, the comparison operator
This comparison returns a value of either True or False

Case Sensitivity

String comparisons are case-sensitive
the string "hello" is different from the strings "Hello", "heLLo" or "helLO" or any other string that isn't exactly identical to it
To work with this we need to have strings with a consistent case, we will choose to make all the strings lower case in order to compare them.

Example


              # Use the assignment operator to store the string 'hp' in the variable 'computer'
              computer = 'HP'
            
              # Comparison between 'HP' and 'hp'
              print(computer == 'hp')
              # Comparison between 'hp' and 'hp'
              print(computer.lower() == 'hp')


              False
              True

Checking for Inequality

There are some cases where checking if something is not true is more efficient
We can use the negation operator denoted by an exclamation mark, !
This operator reverses the truth value.
This means that !True = False and !False = True.
Similarly we can use != to denote "not equal to".

Example


              my_computer = 'dell'
              # Check if the computer is not an HP
              if my_computer != 'hp':
                print("My computer is not an HP")
              else:
                print("You have an HP computer!")


              My computer is not an HP

Numerical Comparisons

Numerical comparisons are pretty straightforward since we do not have to check the case on them


            num = 45
            # Check for equality
            print(num == 45)
            # Check for inequality
            print(num != 45)
            # Check if the number is less than 20
            print( num < 20)
            # Check if the number is greater than 40
            print(num > 40)


            True
            False
            False
            True

Compound Conditional Statements

Sometimes we would like to check if two or more conditions are simultaneously true with the and logical operator. Here is a truth table that demonstrates that.

The AND Operator

In its most simplest form the AND logical operator is only true when BOTH conditions are true.

P	Q	P AND Q
T	F	F
F	T	F
F	F	F
T	T	T

Example

Here is a small program that demonstrates how to use the AND conditional statement in Python which conveniently has the keyword and.


              car_1 = 'mazda'
              car_2 = 'honda'
              # The AND operator evaluates to FALSE since one of the cars is a Mazda
              if car_1 == 'honda' and car_2 == 'honda':
                print('Both cars are Hondas!')
              else:
                print('One of the cars is not a Honda!')


              One of the cars is not a Honda!

The OR Operator

Another type of logical operator is the OR operator. In short, the OR logical operator is only false when both statements are false; otherwise it is true.

P	Q	P OR Q
T	F	T
F	T	T
T	T	T
F	F	F

Example


                computers = ['toshiba', 'thinkpad', 'lenovo', 'hp', 'alienware', 'dell']
      
                for computer in computers:
                  if computer == 'thinkpad' or computer == 'alienware':
                    print(f'A {computer} is a very nice computer!')
                  else:
                    print(f'The {computer} is a great computer!')
                print('We love computers!')


                The toshiba is a great computer!
                A thinkpad is a very nice computer!
                The lenovo is a great computer!
                The HP is a great computer!
                A alienware is a very nice computer!
                The dell is a great computer!
                We love computers!