Python Lesson #2:
Math, Operators, and If Statements


Type the following into your IDLE editor:

print(1 + 1)
print("1 + 1")

Do you see a difference between the two results you receive?

The first line prints out “2”, while the second line literally prints out “1 + 1”. In the first line, Python can automatically recognize the numbers as integers and the + sign as an operator. It “evaluates” this operation because it knows that is what it is supposed to do with these special characters.

The second line is printed literally since it’s surrounded by quotation marks, which turns it into a string. Python does not consider doing anything with what’s inside the string because the string is the value itself, and special characters within it are not seen as special characters.

Using these symbols (known as “operators”), Python can do math operations on integers/floats (numbers):

Python can also compare two separate values using the following symbols:

If statements

If statements check to see whether a condition is met. It returns (internally tells the computer) the result as either of the Boolean values True or False. If it returns True, Python will proceed to the next line. If it returns False, Python will skip over the block of code within the if statement (the part after the if statement which has been indented).

An example:

result = 1
if result == 1:
    print("The result is 1")

In the first line, a variable named result is made. In the second, Python calls to the variable result to see if the value in it is equal to 1. Since they are equal, it executes the indented line beneath, printing “The result is 1”. If result were not 1, it simply would not print anything. You can test this by changing the result to 2 and seeing what happens.

The most important thing to remember here is the double equals sign. While a single equals sign declares that one thing is equivalent to another, a double equals sign checks to see if two things are equal. To use the paint bucket metaphor again: The single equals sign is like filling a bucket of paint with a certain color. On the other hand, the double equals sign is like looking into one bucket of paint and then comparing its color to the paint inside another bucket. If they are the same color, the statement would be True. If not, it would be False. It’s helpful to read a single equals sign as “is”, and a double equals sign as “is equal to”. You can also read a single equals sign like a statement of fact, and a double equals sign like a question you want the answer to.

Note the syntax: There is a colon after 1 and an indent in the line after. The colon indicates that the if statement condition is fully completed (like a period when writing a sentence in English), and the indent is necessary to tell the computer that the print() is within the if statement (so if the result is True, the that’s the next step it will move on to). It’s common to forget these when starting out, but in time it will come naturally.

There are additional things we can add to if statements: elif (else if) and else. They can execute code in case the initial if statement returns False instead of True.

An example:

result = 2
if result == 1:
    print("The result is 1")
elif result == 2:
    print("The result is 2")
    print("The result is not 1 or 2")

In the first line, a variable named result is created with the value 2 stored in it. The second line has an if statement checking to see whether the variable result is equal to 1. Since it is not, it returns False, and does not execute what is underneath it. It then goes down to the elif line. elif basically does the same thing as an if statement but can only work after something above has returned False, so you cannot start with it. It checks to see if the variable result is equal to 2. Since it is, it executes the print statement beneath it. The else statement is never reached here. else statements run if everything above it has returned False. Try changing the value of result to a different number and seeing what happens.

Try it!

With the following variables in mind, translate the commented lines into python code:

my_favorite_color = "(INSERT YOUR FAVORITE COLOR)"
erics_favorite_color = "Orange"

If Eric's favorite color matches mine,
print we have the same favorite color;
if it does not match,
print we don't have the same favorite color.

Writing out the logic for what you want to program with comments is also known as pseudocoding. It’s very helpful to work out the general logic of the code before figuring out the proper syntax for it!


my_favorite_color = "Blue"
erics_favorite_color = "Orange"
if my_favorite_color == erics_favorite_color:
    print("We have the same favorite color!")
    print("We don't have the same favorite color.")

We likely have very similar lines, but in the future your answer may look completely different even though it gives the same result. That’s okay! The beauty of coding is that there are so many ways of approaching problems. They may vary in efficiency, conciseness, and readability, but in time you’ll learn new ways to optimize these aspects.