7.1. Iteration

When repeating the same action over and over again, any human will eventually make a mistake. Computers, on the other hand, possess the incredible ability to carry out repetitive tasks without making mistakes.

Suppose you want to print out the integers 0 through 50. With the Python you currently know, your program would look like this:

print(0)
print(1)
print(2)
print(3)
print(4)
# and so on...

Typing out this code is repetitive, boring, and also error-prone. Even using copy-paste, the large amount of code makes it likely that we will make a simple mistake, such as skipping a number or misspelling print.

The code is also hard to modify. If we want to make a simple change—such as printing all the way to 100, or only printing even numbers—then we are forced to update a large number of lines.

Programming languages provide tools that allow us to repeat a sequence of statements in a much simpler way.

Repeating the same set of statements again and again is called iteration. This chapter explores two ways Python makes iteration simple and flexible—the for and while loops.

To give you a taste of what’s to come, here is how we could write the program above using a for loop.

for num in range(51):
   print(num)

We will explore the details soon, but take a moment to marvel how simple this program is compared to the one above!

7.1.1. Don’t Repeat Yourself

Learning about iteration and loops gives us the chance to introduce one of the most popular phrases in programming: Don’t Repeat Yourself, or DRY. A common piece of advice from instructors and experienced programmers is that you should “Keep your code DRY.”

Anytime you find yourself typing the same set of statements over and over again in a program, little alarm bells should start going off in your head. Instead of all that typing, consider how you could structure the process so you only need to write the statements once.