2.3 Conditions and if statements

2.3 Conditions and if statements#

In this Section you will learn how to control the flow of your code — process data differently based on some conditions. For that you will learn a construction called the if statement.

`if` keyword#

The if statement in Python is similar to how we use it in English. ”If I have apples, I will make an apple pie” — clearly states that an apple pie will exist under the condition of you having apples. Otherwise, no pie.

Well, it is the same in Python:

amount_of_apples = 0

if amount_of_apples > 0:
    print("You have apples!\nLet's make a pie!")

print('End of the cell block...')

As you can see - nothing is printed besides ’End of the cell block…’.

But we can clearly see that there is another print statement! Why it is not printed? Because we have no apples… thus no pie for you.

Let’s acquire some fruit and see whether something will change…

# adding 5 apples to our supply

amount_of_apples += 5

if amount_of_apples > 0:
    print("You have apples!\nLet's make a pie!") 

print('End of the cell block...')

Now you can see that the same if statement prints text. It happened because our statement amount_of_apples > 0 is now True.

That’s how an if statement works — you type the if keyword, a statement and a colon. Beneath it, with an indentation of 4 spaces (1 tab), you place any code you want to run in case that if statement is True. This indentation is the same as described in Notebook 1 when defining a function.

If the result of the conditional expression is False, then the code inside of the if statement block will not run. Here’s another example:

my_age = 25

if my_age >= 18 and my_age <= 65:
    print("I'm an adult, I have to work right now :(")

print('End of the cell block...')

Slightly different setting but still the same construction. As you can see in this case, the condition of the if statement is more complicated than the previous one. It combines two smaller conditions by using the keyword and. Only if both conditions are True the final result is True (otherwise it would be False).

Attention

The condition can be as long and as complicated as you want it to be, just make sure that it is readable.

`elif` keyword#

Now, let’s add a bit more logic to our last example:

my_age = 25

if my_age >= 18 and my_age <= 65:
    print("I'm an adult, I have to work right now :(")
elif my_age > 65:
    print("I can finally retire!")

print('End of the cell block...')

Still the same output, but what if we change our age…

my_age = 66

if my_age >= 18 and my_age <= 65:
    print("I'm an adult, I have to work right now :(") # msg #1
elif my_age > 65:
    print("I can finally retire!") # msg #2

print('End of the cell block...')

See.. we have a different output. Changing the value of our variable my_age changed the output of the if statement. Furthermore, the elif keyword helped us to add more logic to our code.

elif is short for else if. Now, we have three different output scenarios:

print message #\(1\) if my_age is within the \([18, 65]\) range;
print message #\(2\) if my_age is bigger than \(65\); and,
print none of them if my_age doesn’t comply with none of the conditions (as shown below).

my_age = 15

if my_age >= 18 and my_age <= 65:
    print("I'm an adult, I have to work right now :(") # msg #1
elif my_age > 65:
    print("I can finally retire!") # msg #2

print('End of the cell block...')

One can also substitute an elif block by a different if block, however it is preferred to use elif instead for two reasons:

to ”keep the condition together” and to reduce code size.
in case two or more of the if/elif statements are met, only the first one that is met will run (when using only if, all statements that are met will run). We will see an example of how this is useful later on in this notebook.

It is important to know that there should be only one if block and any number of elif blocks within it.

A last example below:

# Setting my_age to run the first elif block
my_age = 88

if my_age >= 18 and my_age <= 65:
    print("I'm an adult, I have to work right now :(")
elif my_age > 65:
    print("I can finally retire!")
elif my_age < 10:
    print("I'm really, really young")

# Setting my_age to run the second elif block
my_age = 7

if my_age >= 18 and my_age <= 65:
    print("I'm an adult, I have to work right now :(")
elif my_age > 65:
    print("I can finally retire!")
elif my_age < 10:
    print("I'm really really young")

print('End of the cell block...')

`else` keyword#

We can go even further and add an additional scenario to our if statement with the else keyword. It runs the code inside of it only when none of the if and elif conditions are True:

my_age = 13

if my_age >= 18 and my_age <= 65:
    print("I'm an adult, I have to work right now :(")
elif my_age > 65:
    print("I can finally retire!")
elif my_age < 10:
    print("I'm really really young")
else:
    print("I'm just young")

print('End of the cell block...')

On the previous example, since my_age is not between \([18,65]\), nor bigger than \(65\), nor smaller than \(10\), the else block is run.

Below, a final example:

# Setting age to run the if block
my_age = 27

if my_age >= 18 and my_age <= 65:
    print("I'm an adult, I have to work right now :(")
elif my_age > 65:
    print("I can finally retire!")
elif my_age < 10:
    print("I'm really really young")
else:
    print("I'm just young")

print('End of the cell block...')
print('------------------------')

# Setting age to run the first elif block
my_age = 71

if my_age >= 18 and my_age <= 65:
    print("I'm an adult, I have to work right now :(")
elif my_age > 65: # first elif block
    print("I can finally retire!")
elif my_age < 10:
    print("I'm really really young")
else:
    print("I'm just young")

print('End of the cell block...')
print('------------------------')

# Setting age to run the second elif block
my_age = 9

if my_age >= 18 and my_age <= 65:
    print("I'm an adult, I have to work right now :(")
elif my_age > 65:
    print("I can finally retire!")
elif my_age < 10: # second elif block
    print("I'm really really young")
else:
    print("I'm just young")

print('End of the cell block...')
print('------------------------')

# Setting age to run the else block
my_age = 13

if my_age >= 18 and my_age <= 65:
    print("I'm an adult, I have to work right now :(")
elif my_age > 65:
    print("I can finally retire!")
elif my_age < 10:
    print("I'm really really young")
else: # else block
    print("I'm just young")

print('End of the cell block...')
print('------------------------')

That’s almost everything you have to know about if statements! The last two things are:

It goes from top to bottom. When the first condition to be True runs, it skips all conditions after it — as shown below:

random_number = 17

if random_number > 35:
    print('Condition #1')
elif random_number > 25:
    print('Condition #2')
elif random_number > 15:
    print('Condition #3')
elif random_number > 5:
    print('Condition #4')
else:
    print('Condition #5')

You can put almost everything inside each condition block and you can define variables within each block:

my_income = 150
my_degree = 'BSc'

if my_degree == 'BSc':
    x = 5
    if my_income > 300:
        b = 2
        print('I am a rich BSc student')
    else:
        print('I am a poor BSc student')

elif my_degree == 'MSc':

    if my_income > 300:
        print('I am a rich MSc student')
    else:
        print('I am a poor MSc student')

print('x =', x)
print('b =', b)

As you can see, we can make it as complicated as we want in terms of conditional branching.

Additionally, you can see that only variables within the blocks which run were created, while other variables were not. Thus, we have a NameError that we tried to access a variable (b) that was not defined.

Comparison operators#

For your easy reference, please see below the comparison operators used for comparing numbers.

Comparison operator	Description
`a == b`	a is equal to b
`a > b`	a is larger than b
`a < b`	a is smaller than b
`a >= b`	a is larger than or equal to b
`a <= b`	a is smaller than or equal to b
`a != b`	a is not equal to b

Attention

There is a major difference between = and ==. a = 1 assigns the number 1 to variable a. a == 1 tests if the variable a is equal to 1, and will return True or False (for use in if statements).

Additional study material

Official Python Documentation - https://docs.python.org/3/tutorial/controlflow.html
Think Python (2nd ed.) - Chapter 5

After this Notebook you should be able to:#

know how to load Python modules
use functions from the math module
find available functions from any module
understand conditions with if, elif and, else

2.3 Conditions and if statements

Contents