Conditional statements

Now that we know the basics of print statements, variables, and reading in input from the user – we can combine all these awesome skills with conditional statements. Conditional statements allow the computer to make decisions based off the value of an expression. In the real world, we use conditional statements all the time to make decisions! For example, if it is raining outside, then I use an umbrella. In this example, the I evaluated the weather outside and because it was raining, I made the sequential decision to use an umbrella.

What conditional statements can you think of that you use in everyday life?

If statements

We’ll start off with an if statement in its simplest form! Looking at the real world example, you can see certain words used and those apply to programming statements as well. If it is raining outside, then I use an umbrella. if tells us that we need to make a decision. Then we have the decision to make. Is it raining? That question is called the expression. Then, if it is raining, I use an umbrella. That is a statement that applies when the expression is truthful.

if (expr):
statement


Shown above we have three different parts:

1. If – this tells the computer that we are going to be deciding based off the expression inside the parenthesis
2. Expr – this represents the expression that we are evaluating. If the expression is “truthful” then the computer will enter the if statement and execute the statement.
3. Statement – what the computer will complete if the expression is “truthful”
x = 0
y = 5

if (x<y):       #truthful
print('yes')
if (y<x):       #falsy
print('yes')
if (x == 10):   #falsy
print('yes')


Let’s break down the example above and decide why some statements are “truthful” or “falsy”.

We are given two variables x and y are they are each assigned a value. In the first if statement we are testing if the value of x is less than the value of y. Since 0 is less than 5, this is true! So, the first if- statement will print yes.

However, we can see that y is not less than x, so it evaluates to false. The inside of the if statement will not be executed then. The same applies for the if statement evaluating if x is equal to 10. We see that x is equal to 0, so this expression is false, and the inside of the if statement won’t be executed.

Else statements

Now that we know how to use if statements to conditionally execute a single statement or multiple statements, let’s see what else we can do!

Sometimes, you need to evaluate a condition in order to act accordingly if it is true, but if it false we act differently. Here is the simplest form of the else statement.

if (expr):
statement1
else:
statement2


This is really like the if statement! However here, if the if statement is false (not truthful), the program will automatically execute the statement2 in the else statement.

A real-life example is if I’m hungry, I’ll eat. Else (in other words, I’m not hungry), I won’t eat. Let’s take a look in python code!

if (hungry):
eat
else:
dont_eat


Challenge

Let’s put together everything we’ve learned so far! Let’s see if we can create a program that prompts the user for their name. The user can write their name into the console. Then the computer can decide if their name is equal to your name and print out a response. Else, print out a different response!

Hint: Try talking out the different paths a computer can take! Make sure to identify what the if and else conditions are.