6. Conditional Statements - MantsSk/CA_PTUA14 GitHub Wiki

Python `conditions` and `if` statements

When executing code we usually want to make some sort of decisions according to the data we are getting, if statements allows us to that eactly.

How it looks like:

if <test_expression>:
    statement(s)

Flow chart:

For checking of the conditions, python support usual mathematical expressions:

Equals: a == b
Not Equals: a != b
Less than: a < b
Less than or equal to: a <= b
Greater than: a > b
Greater than or equal to: a >= b

Example:

number_one = 500
number_two = 600
if number_one < number_two :
    print("number_one is greater than number_two !")

It is important that the statements after the if clause and colon (:) is indented. Python relies on indentation (whitespace at the beginning of a line) to define scope in the code. Other programming languages often use curly-brackets for this purpose.

`elif`

What if we want to extend out program and print out something else if the statement is not true?

Example:

number_one = 500
number_two = 600
if number_one < number_two:
    print("number_one is greater than number_two!")
elif number_one == number_two:
    print("Numbers are equal !")

Note. There can be as many elif statements as we want

number_one = 500
number_two = 600
if number_one < number_two :
    print("number_one is greater than number_two  !")
elif number_one == number_two :
    print("numbers are equal !")
elif...

`else`

The else keyword catches anything which isn't caught by the preceding conditions:

number_one = 500
number_two = 600
if number_one < number_two :
    print("number_one is greater than number_two  !")
elif number_one == number_two :
    print("numbers are equal !")
else:
    print("number_two is greater than number_one !")

short version of `if ... else`

a = 200
b = 450
print("A") if a > b else print("B")

3 conditions example:

a = 200
b = 200
print("A") if a > b else print("=") if a == b else print("B")

`and`

We have more logical operators that allow us to combine statements into more complex logic. The and keyword is a logical operator, and is used to combine conditional statements. Basically both conditions **must be true for statement to return True **:

a = 200
b = 400
c = 500
if c > a and c > b:
    print("C is the greatest of them all!")

`or`

The or keyword is a logical operator, and is used to combine conditional statements. Basically at least one of the conditions must be true for statement to return True:

a = 200
b = 400
c = 500
if b > a or b > c:
    print("B is at least greater than one of the values !")

Nested `if`

As we have seen with lists being able to carry other lists inside them. There is no exception for if statements.

x = 15

if x > 10:
  print("Above 10")
  if x > 20:
    print("and also above 20!")
  else:
    print("Above 10 but bellow 20!.")

`pass`

If we want to have an empty if statement for some reason we can simply do pass and nothing will happen.

a = 50
b = 80

if b > a:
  pass

`If` with `string` values

We may also do logical operations on strings.

name = "Tom"

if name == "Tom":
    print("Nice to see you Tom")
else:
    print("welcome, user")

Lets do this with lists as well!

user = "Johnny"
privileged_users = ["Tom", "Albert", "Stephen"]
if user in privileged_users:
    print(f"Welcome home {user}")
else:
    print("INTRUDER ALLERT. Silently calling police...")

Or even dictionaries:

my_dict = {"name": "Steven", "born": "1955-02-24", "interests": "Apples"}
if my_dict["name"] == "Steven":
    print("This guy does not like Windows")
else:
    print("No clue who this is")

my_dict = {"name": "Bill", "born": "1955-10-28", "interests": "small software"}
if "Bill" in my_dict.values():
    print("This guy hates apples")
else:
    print("No clue who this is")

Shorter version of `if..`

in Python we love to optimize code. For example, if we want to check that user has entered a name at all . Instead of doing this:

...
if name != "":
  print("Name was not entered")
...

...
if len(name) == 0:
  print("Name was not entered")
...

Do this:

if not name:
  print("Name was not entered")

Same applied to other data structures as well

my_list = []
if not my_list:
  print("oh no, list is empty")

🧠 Exercises:

Let user enter name, surname and age. Print if user is allowed to enter an online casino or not. 21 is the age cap.
Create a database (list of privileged users), print a specific message with a personal greeting if the user is a privileged and just "Welcome"
Allow user to enter two numbers, print out which one is higher than the other, or equal.
Write a small calculator application, that allows user to enter two numbers and a symbol, do the operation and print the answer.
Create a number guessing game from 1-10 with random library. Maximum 3 guessing attempts are allowed.

🌐 Extra reading:

RealPython

6. Conditional Statements - MantsSk/CA_PTUA14 GitHub Wiki

Python conditions and if statements

elif

else

short version of if ... else

and

or

Nested if

pass

If with string values

Shorter version of if..