Conditionals

Code sample link: https://replit.com/@jjoco/python-conditionals

Conditionals are used to decide whether to run different codeblocks given some condition(s). In Python, this can be accomplished using if-else statements. For those familiar with other languages, Python does not support switch-case, unfortunately.

If statement

If-statements in Python are slightly different from other language syntax. Take the following example:

if condition:
    #Do stuff

If the condition is satisifed (ie is True), then its code block is executed.

There are no parentheses surrounding the condition statement, and there are no curly braces wrapping the conditional code block. Code statements that belong to the if code block must be indented once more with respect to the if statement. The following is a valid if-statement:

count = 6
if count > 4:
    print("Above 4")
'''
Prints out:
Above 4
'''

In the following example, since count is greater than 4, the program goes into the if code block to execute the print statement. However, in the following example:

count = 3
if count > 4:
    print("Above 4")

'''
Nothing is printed to standard output
'''

the program will simply skip the if statement since the if condition was not satisfied.

Elif

Python does not use else if; it uses elif, instead.

.
.
.
elif condition:
    #Do stuff

An elif block must be after an if block, or else there will be runtime errors. Here's an example if-elif block:

count = 3
if count > 4:
    print("Above 4")
elif count == 3:
    print("Equal to 3")

'''
Prints out:
"Equal to 3"
'''

Here, count does not satisfy the initial if condition; fortunately, the elif condition is satisfied, and its respective code block is executed. Of course, if neither the if nor elif conditions are satisfied, any code within those blocks will be skipped.

Else

The else block the block executed in a bigger if-else block in case neither the initial if condition nor the subsequent elif conditions are met. The syntax for else is the following:

.
.
.
else:
    #Do stuff

Like elif, an else block must be after an if block, or else there will be runtime errors. Here's an example if-else block:

count = 3
if count > 4:
    print("Above 4")
else:
    print("Not above 4")
'''
Prints out:
"Not above 4"
'''

Since the if condition was not satisfied, the else block is executed.

Combined if-else

if, elif and else can be combined like so:

if condition_1:
.
.
.
elif condition_2:
.
.
.
else condition_n:
    #Do stuff

Here's an example of a combined if-elif-else block:

count = 3
if count > 4:
    print("Above 4")
elif count == 4:
    print("Equal to 4")
else:
    print("Less than 4")

Since neither the if nor the elif conditions are satisfied, the else block is executed.

Note that elif and else blocks cannot exist on their own without belonging to a bigger if block.

Keep in mind that you can chain several elif blocks:

if condition_1:
.
.
.
elif condition_2:
.
.
.
elif condition_3:
.
.
.
elif condition_4:
.
.
.
else condition_n:
    #Do stuff

Here's an example with multiple elif's choosing what letter grade to print out given a test grade:

test_grade = 70
if test_grade >= 90:
    print("A")
elif test_grade >= 80:
    print("B")
elif test_grade >= 70:
    print("C")
elif test_grade >= 60:
    print("D")
else:
    print("F")
'''
Prints out:
C
'''

Multiple condition checking

In Python, you can check multiple conditions in an if or elif before deciding to execute its respective code block. Important keywords are logical operators and, or, and not.

and

If you want two conditions to be met before executing a block, use the and keyword (&& in other languages).

Syntax:

if condition_1 and condition_2:
    #Do stuff

Let's say we wanted if I should order takeout based on my energy levels and the amount of food I already have in my fridge:

energy_levels = 20
fridge_content = 10
if energy_levels < 30 and fridge_content < 30 :
    print("Order delivery")

'''
Prints out:
Order delivery
'''

Since both conditions are satisfied, the if block is executed. Because I'm too low energy and don't have enough stuff in my fridge, I decide to order takeout.

or

If you want either of two conditions to meet before executing a code block, use or (|| in other languages).

Syntax:

if condition_1 or condition_2:
    #Do stuff

In the following example, I decide if I should rest depending on the period of the day and my energy levels.

energy_levels = 5
period_of_day = "afternoon"
if period_of_day == "midnight" or energy_levels < 10 :
    print("Go rest")
'''
Prints out:
Go rest
'''

In the above example, even though only one of the conditions was satisfied, the if block is executed. Even though it's the afternoon, my energy levels are low enough, such that I decide to rest.

not

If you don't want a condition to be met before executing a code block, use not (! in other languages).

Syntax:

if not condition:
    #Do stuff

To translate the decision to go outside only if the weather condition is not raining, we can do the following:

weather = "sunny"
if not weather == "raining":
    print("Go outside")
'''
Prints out:
Go outside
'''

Obviously, if I initially set weather = "raining", then that code block would not execute.

Mixing multiple logical operators

You can have multiple logical operators in the same conditional line; use parentheses to denote which logical comparisons should take priority over others; an abstract example can look like:

if (condition_1 or condition_2) and (condition_3 or condition_4):
    #Do stuff

Or look like

if (condition_1 and condition_2) or (condition_3 or condition_4):
    #Do stuff

In both of these examples, the result of the parentheses are calculated first, then the results are compared:

if condition_1_2 or condition_3_4:
    #Do stuff

This example can continue ad infintum.