Looping

Learning Goals

Explain a loop.
Write a while loop.

Introduction

We want to write a program to print all the numbers from 1 to 5. Here’s a possible solution:

print(1)
print(2)
print(3)
print(4)
print(5)

That is a lot of duplicated code. Imagine if we had to print all the numbers up to 100 or 1000 or a million! How can we make it easier to perform repetitive tasks?

Well, the great news is that programming languages usually have features to make this easier. One of those features is called a loop. A loop is a way to perform operations until a condition is satisfied.

A `while` Loop

Here’s the general structure of a while loop:

while (condition is True):
	# do something

The simplest form of this structure is the following:

# DO NOT RUN THIS CODE
while (True):
	print("Hello")

The above code is telling the computer to display the value "Hello" in the console until the condition is False. There is no logic in the code that instructs the computer to stop.

The `break` Statement

The break statement is used to stop a loop.

while (True):
	print("Hello")
	break

Output:

Hello

The program starts the while loop.
The condition is evaluated to True.
The print function is invoked and displays Hello in the console.
The loop immediately stops once the break statement is executed.

Printing Numbers with a Loop

We can use the while loop and the break statement to write a program to print all the values from a starting value up to an ending value:

current = 1
end = 4

while (True):
  if (current == end):
    break
  print(current)
  current = current + 1

print(current)

A Common Loop Format

You may see the following format being used in a lot of codebases:

i = 0
end = 4

while (i < end):
	print(i)
	i = i + 1

print("The value of i is " + str(i))

Output:

0
1
2
3
The value of i is 4

Use this visualizer to observe how this code is run by Python. The variable i is a common way to represent loop variables, i.e., variables that are only used inside of a loop.

Conclusion

In this lesson, we have learned how to use loops to make writing repetitive code a lot easier. You will be writing loops all the time when you build programs, so be sure to practice writing a bunch of them!

stringtheory92/intro-to-coding-python-looping