if, elif, and else statements¶
In this section we will learn about if, elif, and else statements in Python.
This is where things start to get interesting because now we will have
the ability to use more complex logic and decision making in our
Python code examples and programs.
Create a new Jupyter Notebook called if_elif_else_statements. Follow along and write the code! You are encouraged to tinker with the code, change it, and even break it! It’s the only way to learn!
if statement¶
The syntax in general for the if statement is:
if condition:
statements_to_execute
The condition is any valid python expression that evaluates to True or False.
If the condition evaluates to True, then the statements in the statements_to_execute
are executed. Otherwise the statements_to_execute are skipped all together. Notice also the use of the
colon : after the condition.
This is the first time we are seeing the use of white space and indentation. The use of white space and indentation is very important in Python. An indented white space (tab on keyboard) is used to start a block of code or statements and any code that is indented at that level is executed together as a block of code.
To start practicing with if, elif, and else statements,
we will use the example of a student receiving a final grade
and determining if they pass or not. The logic will begin
very simple but then we continue to add to it.
To start, suppose a grade of 50% is a passing grade.
So if a student gets less than 50% they fail
and if they get over 50% or over they pass.
This is what we want the Python code to do in our first example.
If the final grade is greater than or equal to 50, the code
will print a message with their grade and the fact that they passed.
We will also print a good bye message at the end.
Here is what that would look like using the if statement.
final_grade = 60
if final_grade >= 50:
print(f'Your final grade was {final_grade} so you passed!')
print('Good Bye, see you next year.')
Your final grade was 60 so you passed!
Good Bye, see you next year.
Since the expression final_grade >= 50 is True, the indented statement block (the line with the print)
is executed. The alternative would be if
final_grade >= 50 is False. In that case the indented statement block
would be skipped all together and not executed. Here is an example where final_grade is less than 50
so the indented statement block is skipped and it goes straight to the good bye message.
final_grade = 49
if final_grade >= 50:
print(f'Your final grade was {final_grade} so you passed!')
print('Good Bye, see you next year.')
Good Bye, see you next year.
Before continuing to the next example, play around with the above example.
Try changing the final_grade to different values and seeing
when the if block of code gets executed.
else statement¶
In the previous example we saw that if the condition (final_grade >= 50) was
True then the next indented code block was executed. If the conditon was False
then it was skipped. There are times when we want to
execute some alternative logic when the condition is False.
This is what the else statement is for. The general syntax is:
if condition:
# executed if condition is True
statements_to_execute
else:
# executed if condition is False
alternative_statements_to_execute
Lets add to our example by using the else statement.
Now, if the student has a failing grade (less than 50) than
it prints a message saying the student failed.
final_grade = 49
if final_grade >= 50:
print(f'Your final grade was {final_grade} so you passed!')
else:
print(f'Your final grade was {final_grade}. You failed. Try harder next time.')
print('Good Bye, see you next year.')
Your final grade was 49. You failed. Try harder next time.
Good Bye, see you next year.
So just to say it one more time. If the condition is True then the first indented
block of statements is executed. Otherwise, the statements indented after the else
statement are executed. Never will it be the case that both blocks are executed. It’s one or the other only.
Check it out in the above example by changing the final_grade to 50 or greater.
elif statements¶
The elif statement is short for else if. It is used when we want to check for multiple conditions.
If the first condition for if is False, then next condition for the elif block will be checked.
If the elif condition is True than the indented statements for that elif block are executed.
If the elif condition if False than the code in the else block is executed.
There can be multiple elif statements. Among the if statements, elif statements,
and else statements, only one block of statements will be executed.
The if block can have only one else block but can have multiple elif blocks.
The general syntax is:
if first_condition:
# executed if first_condition is True
first_statements_to_execute
elif second_condition:
# executed if first_condition is False and second_condition is True
second_statements_to_execute
elif third_condition:
# executed if first_condition is False and second_condition is False and third_condition is True
third_statements_to_execute
.
.
.
else:
alternative_statements_to_execute # executed if all conditions above are False
It’s really important to remember that only one of the indented block
statements will get executed (not multiple blocks). It all depends on what
condition is True. If all the conditions are False than the else
block gets executed.
Let’s evolve our example with displaying the students grade.
Now we will display a slightly different message based on the students final_grade.
final_grade = 90
if final_grade >= 90:
print(f'Your final grade was {final_grade}. You did amazing work! You passed!')
elif final_grade >= 80:
print(f'Your final grade was {final_grade}. You did really good work! You passed!')
elif final_grade >= 70:
print(f'Your final grade was {final_grade}. You did good work! You passed!')
elif final_grade >= 50:
print(f'Your final grade was {final_grade}. You did okay work. You passed.')
else:
print(f'Your final grade was {final_grade}. You failed. Try harder next time.')
print('Good Bye, see you next year.')
Your final grade was 90. You did amazing work! You passed!
Good Bye, see you next year.
In the above example we added multiple elif statements to show how they work.
Take some time to play around with the code by changing the final_grade to different values such as
95, 90, 80, 75, 70, 50, and 45. Execute the code for these different values
and take note of what print statement gets executed.
In closing, when using if, elif, and else statements, you always start with an if statement.
Both the elif and else statements are optional. There can be multiple elif statements and only
one else. Remember to use the colon : after each condition as well as the indented white space.
Here are a few simple examples that you can play around with.
Example 1: if¶
x = 'HELLO' # change to 'hello' to make the print statement execute
if x == 'hello':
print('hello world')
Example 2: if else¶
x = 6 # change to different values to see what blocks below get executed.
if x + 5 > 10:
print(f'{x+5} is strictly larger than 10.')
else:
print(f'{x+5} is not strictly larger than 10.')
11 is strictly larger than 10.
Example 3: if elif else¶
x = 0.1 # change to different values to see what blocks below get executed.
if x > 0:
print(f'{x} is positive.')
elif x < 0:
print(f'{x} is negative.')
else:
print(f'{x} is zero.')
0.1 is positive.
Example 4: multiple True conditions¶
Remember that even if multiple conditions are True, only
one of the blocks will be executed (the first condition for which it is True).
x = 101
if x >= 100:
print(f'{x} is big.')
elif x > 0:
print(f'{x} is positive.')
elif x < 0:
print(f'{x} is negative.')
else:
print(f'{x} is zero')
101 is big.
Example 5: else is optional¶
Remember that elif and else are optional.
x = 0
if x >= 100:
print(f'{x} is big.')
elif x > 0:
print(f'{x} is positive.')
elif x < 0:
print(f'{x} is negative.')
Example 6:¶
Here we will look at another example to practice
with the if, elif, and else statements. Suppose an employee
has to work on Monday, Wednesday, and Friday and has to wear
a different color uniform on each of those days. Red on Monday,
blue on Wednesday, and green on Friday. Also suppose they have to remember
to bring lunch on Monday and Wednesday but not on Fridays. We will write a simple
program that prints what color to wear and if the employee needs to bring
food, based on the day of the week.
day = 'Friday' # can change to any day of the week. Capitalize only the first letter.
if day == 'Monday':
print('You need to wear red today.')
print('You also need to bring food.')
elif day == 'Wednesday':
print('You need to wear blue today.')
print('You also need to bring food.')
elif day == 'Friday':
print('You need to wear green today.')
print('You do not need to bring food.')
else:
print('You do not work today. Have a good day off.')
You need to wear green today.
You do not need to bring food.
Try changing the day to different days of the week
and executing the above code to see how it all works.
Nested if statements¶
When you are working with indented code blocks, you can write any valid
Python code within those indented code blocks. This means for example that
you can nest if, elif, and else statements within one another.
When you have an if statement inside
another if statement and so on, this is called nesting.
Here is a simple example.
x = 15 # run the code with x as 15, 7, 4, and -2 to see the different blocks executed
if x > 5:
print(f'{x} is larger than 5')
if x > 10:
print('and is also larger than 10')
else:
print('but is not larger than 10')
else:
print(f'{x} is smaller than 5')
if x < 0:
print('and is also negative')
15 is larger than 5
and is also larger than 10
Before going onto the next section spend some time
playing around with if, elif, and else statements.
Keep it simple and try to get the syntax down and memorized.