Skip to main content

Math, String Operations and Comparisons

In the last lesson we have learned how to output data to our screen, how to declare a variable and what different core data types exists in Python. In this lesson we will learn how math works in Python.

Lesson Overview​

At the end of the following lesson you will be confident at:

  • Doing math with numbers in Python
  • Doing math with strings in Python
  • Comparing results to see if they are the same or one is bigger than the other

Performing Operations on Numbers​

Number data types, int and float are very useful when we must do some math.

Addition​

Adding two numbers together can be done with + operator.

Python Sandbox


Subtraction​


Subtraction of two numbers is done with - operator.


Python Sandbox


Multiplication​


Multiplication of two numbers is done with * operator.


Python Sandbox


Division​


Division of two numbers is done with / operator.


Python Sandbox


info
Division always returns float
You must be careful when dividing two numbers as the second number must not be equal to 0. If the divisor (second number) is 0, Python will raise ZeroDivisionError.


Floor Division​


Sometimes we may need to only get whole number from division and we dont care about the remainder. In that case we can use something called floor (integer) division.


Floor division is done with // operator.


Python Sandbox


Modulo Operator​


Sometimes we may need to know remainder of division but we do not care about the whole number, only the remainder. In that case we use modulo operator.


Modulo in done with % operator.


Python Sandbox




Result of this operation is 1 because 3 goes into 10 three times and whats left is 1.




Exponentiation​


Raises the left value to the power of the right value.


Exponentiation is done with ** operator.


Python Sandbox


Performing Operations on Strings​


We can perform various operations on strings in Python. For example, we can:


Add Strings Together​


Python Sandbox


Multiply Strings​


Python Sandbox


Get a Single Letter​


Python Sandbox


In programming we count from 0
Python always starts counting at 0. This is called an index.


Convert Case​


Python Sandbox


Strings Cannot Be Modified in Place​


Strings in Python are immutable.
This means that they cannot be modified in place and every operation on strings results in a new string.


We demonstrate this in the following example


Python Sandbox


String Formatting​


Modern Python recommends usage of f-string for string formatting. Only difference is that we use letter f in front of the string to mark it as f-string.
Then we can use { } syntax to inject variables directly into strings.


Python Sandbox


Comparisons​


We can compare values


In Python we use these comparison operators:


    

  • Equal to (==)
    • 

  • Not equal to (!=)
    • 

  • Greater than (>)
    • 

  • Less than (<)
    • 

  • Greater than or equal to (>=)
    • 

  • Less than or equal to (<=)
    • 



All of these return a boolean value. True or False


Python Sandbox


Logical Operators​


In Python we have 3 logical operators. These are:


    

  • and
    • 

  • or
    • 

  • not
    • 



and Logical Operator​


and operator is used to check if both conditions (on left and right side) are True. If so the whole expression is evaluated to True.


or Logical Operator​


or operator is used when we need to check if any of the conditions are True. If so, whole expression is evaluated to True


not Logical Operator​


not operator is used for inverting logic. If something is True it will invert it to False


Python Sandbox


Excercise​


Try to solve excercise-3 for better understanding.


Assignment​


In the last assignment we have declared our shop variables and printed the inventory. In this one you are tasked with:


    

  1. Open your code editor in your last project directory simple-python-shop
    2. 

  2. Open main.py file we have been working on
    3. 

  3. Standardize your strings:

      

    • Convert shop_name to uppercase (e.g., MERLIN SHOP)
      • 

    • Convert item name to titlecase (e.g., Excalibur)
      • 

    

    4. 

  4. Calculate the checkout price:

      

    • Create variable total by multiplying item_price and item_quantity
      • 

    

    5. 

  5. Comparison checks:

      

    • Create a boolean is_large_order that evaluates whether item_quantity is greater then or equal to 3.
      • 

    

    6. 

  6. Update your output to use f-string :

    Welcome to MERLIN SHOP
    --------------------------------
    Item: Excalibur
    Purchased: 3 x 67.20
    Total: $201.60
    Large Order: True
    --------------------------------
    Thank you for your purchase !
    

    7. 

  7. Commit your changes with git and push to Github
    8. 



Deepen Your Knowledge​


    

  1. Learn more about Basic math in Python from article in Stanford University, covering all the topics in this lesson but in a different style and a bit more.
    2. 

  2. Broaden your knowledge about Strings from this Google article and read Common String Operations from official Python documentation.
    3. 

  3. Read about Comparisons and Boolean Operators - and, or and not in official Python documentation.
    4. 



What's Next​


Now that we have learned how to do calculations and comparisons, its time for our program to change behavior based on those calculations or comparisons. Lets dive in conditionals.