Square

Now we'll go back to elementary school and try to write a program that calculates the perimeter and the area of a square.

The perimeter of a square with a side length of a can be computed by the P = 4a formula, and the area formula is S = a². So let's say that our square has a side length of a = 356 cm.

Print the result with print(). Save the program into the file ~/pyladies/02/square.py and run it; this is what it should print:

The perimeter of a square with a side of 356 cm is 1424 cm.
The area of a square with a side of 356 cm is 126736 cm2

The result should be computed by Python so don't write the numbers 1424 and 126736 into your code.
If you don't know what to do, look into your program printing.py from the lesson about print, where one of the lines does a similar thing.

print('The perimeter of a square with a side of 356 cm is', 4 * 356, 'cm')
print('The area of a square with a side of 356 cm is', 356 * 356, 'cm2')

Smaller square

If everything works, try to change the program so it computes the perimeter and the area of a square with a side of 123 cm.

print('The perimeter of a square with a side of 123 cm is', 4 * 123, 'cm')
print('The area of a square with a side of 123 cm is', 123 * 123, 'cm2')

Variables

Could you make it even for a side of 3945 cm, 832 cm, 956 cm? Do you enjoy rewriting numbers? If the program were longer (few pages) how would you make sure that you didn't forget to rewrite one of the numbers?

There is a way how to write a program without rewriting all the numbers every time: You name the side of the square and then you just use that name. In Python, variables are used to name values. They are being used this way:

side = 123
print("The perimeter of a square with a side of", side,"cm is ", side * 4,"cm.")
print("The area of a square with a side of", side,"cm is", side * side, "cm2.")

So you write the name, then = and after that the expression whose value will be assigned to that variable. Whenever you write the name of the variable, Python will use just its value.

Which leads us to one of the core principles of programming: Don't repeat yourself, DRY. When there is a value, an expression or the same piece of code repeatedly, a good programmer will name that part, and then they use the name several times. It often happens that the program needs to be changed - either there is a mistake or the task has changed. And then, it is easier to make that change only in one place.

On top of that, clear names makes reading the program much easier: 4 * side (maybe squareSide would be clearer) doesn't need any comment, but with 4 * 183, it's not clear what the numbers mean.

Comments

Now we will make our code clearer with comments. In Python, the comment begins with a Hash (Pound) sign "#", after which you can write anything until the end of the line. Everything following this sign is ignored by Python code execution.

Comments are important! Programs are not read only by computers, but also by other humans. In your comments, you can include statements like: what the whole program does, explain how a more complicated part works and clarify something that is not clear enough.

Whenever you write a program, try to get into the role of someone who will read it, and all that may be unclear should be specified in the comments. Help yourself. After a few months you will not remember what the code is about.

# This program computes the perimeter and the area of a square

side = 123
print("The perimeter of a square with a side of", side,"cm is ", side * 4,"cm.")
print("The area of a square with a side of", side,"cm is", side * side, "cm2.")

There are many opinionated suggestions how often and where to use comments. These suggestions also depend on level of experience of programmers contributing to the project.

Easiest way how to decide in the beginnings where/when to place comments and what should they contain is:

Comment often and whenever you are not 100% sure about certain part of code that your future YOU in a few months will understand what it does and why.

Input

Finally, we will learn how to improve the program so that we don't have to write the number in the program - and users can (in)put their own number.

Just like you used print, we will now use a different function to capture user input: We will explain the details later, for now, just remember these:

• If you want to retrieve a text(string) from the user, use:

  variable = input('Enter some text: ')
  

• If you want to retrieve a whole number, use:

  variable = int(input('Enter some whole number: '))
  

• If you want to retrieve a decimal, use:

  variable = float(input('Enter some decimal: '))
  

  The text inside the parentheses can be tailored according to your needs. It serves as the text prompt for the user, so use it to ask for any needed info.

And now try to put it all together: Ask user for a side of a square and use the variable as an input to the computation.

# This program computes the perimeter and the area of a square

side = float(input('Enter the side of a square in centimeters: '))
print("The perimeter of a square with a side of", side,"cm is ", side * 4,"cm.")
print("The area of a square with a side of", side,"cm is", side * side, "cm2.")