When you're writing code in any programming language, you need to able to make decision based on the current state of the program. You'll want to be able to compare values to decide what path you want to take next. This is called logic.
To use logic in our program, we'll need to learn about the comparison operates that Python offers.
The most basic comparison operators we have is the equality operator. This simply checks if two values are equivalent or not. If they are the same, the result will be
true, and if they are not equal, the result will be
To use the equality operator, simply use two equal signs.
number1 = 3 number2 = 3 print(number1 == number2)
number1 = 3 number2 = 6 print(number1 == number2)
The inequality operator functions exactly like the equality operator we just saw but in reverse. If the two values are the same, it will result in
false, and if they are different, it will result in
To use this operator, put an exclamation point in front of the equal sign, like so:
number1 = 3 number2 = 3 print(number1 != number2)
number1 = 3 number2 = 6 print(number1 != number2)
Greater Than and Less Than Operators
You can compare the values of two variables using the greater than and less than operators. Here is how to use them:
number1 = 3 number2 = 6 print(number1 > number2) # greater than
number1 = 3 number2 = 6 print(number1 < number2) # less than
Or Equal To Operators
In the case where you need to check if a value is either less than/greater than or equal to another value, Python also has an operator for that. It looks like a combination of the operators we saw before:
print(3 <= 5) print(3 >= 5) print(7 <= 8) print(7 >= 9)
True False True False
Now that are able to compare two values, we can now do something with that result. Using conditionals we can now take different paths of code depending on if the value inside it is
Using the if keyword is very straightforward. If whatever you are checking is
true, then whatever is inside the next block of code will run.
Let's see an example of this:
burritos = 6 if (burritos > 5): print("You ordered too many burritos.")
You ordered too many burritos.
Now let's order a correct number of burritos and see what happens:
burritos = 3 if (burritos > 5): print("You ordered too many burritos.")
Nothing happened! The code block did not execute because the value of
burrito was not greater than
You're probably thinking, well, I want to output something even if the correct number of burritos were ordered. Well, that's where else comes into play.
We can define another block of code to run whenever the
if conditional was false.
burritos = 3 if (burritos > 5): print("You ordered too many burritos.") else: print("You ordered a correct number of burritos!")
You ordered a correct number of burritos!
That's cool, but what if we wanted more than 2 code paths? That is where else if comes into play, or elif.
burritos = 2 if (burritos > 5): print("You ordered too many burritos.") elif (burritos == 2): print("You ordered the PERFECT number of burritos!") else: print("You ordered a correct number of burritos!")
You ordered the PERFECT number of burritos!
Since code executes from top to bottom, it will first do the first conditional. Since the value was not greater than
5, it moved on the second conditional. Since the conditional there was
true, that code block was executed instead of the one below it.
So far we've seen how logic works when we only have a single conditional to work with. Eventually, you will need to use multiple conditionals to finally make the decision about what path of code you want to take. When you want to use multiple conditionals at once, you use a logical operator.
By using the and operator, the block of code will only execute if both sides of the operator are true:
isHungry = True foodAvailable = True if (isHungry and foodAvailable): print("Since I am hungry and there is food, I shall eat.")
Since I am hungry and there is food, I shall eat.
Since in this case both variables were
true, the resulting code block was ran, and the print statement was displayed.
While using the
and operator, both conditionals needed to be
true, but with the
or operator, only a single one needs to be
Consider this example:
sunny = False bored = True if (sunny or bored): print("Since it is either sunny outside or I'm bored, I will go play basketball.")
Since it is either sunny outside or I'm bored, I will go play basketball.
Even though it is not sunny outside, because you are bored, you decided to play basketball anyway.
In our final logical operator, you can invert the results of a conditional entirely but using the not operator.
Let's look at an example:
temperature = 50 hot = temperature > 70 if not (hot): print("Since it isn't hot, I will wear boots today!")
Since it isn't hot, I will wear boots today!
If it isn't hot, which we defined as greater than
70 degrees, you will wear boots, which you did!