07 Prepare: Preparation Material
Overview
Many complex problems can be solved by repeating simple tasks over and over again. For example, image or movie editing software might repeat the same procedure for each frame of the movie or for each pixel of an image. Similarly, programs that model a company's financial data or perform scientific analysis need to examine each data point, one after another.
In this lesson you learn the tools you need to make your programs perform steps many times, and you'll practice this by creating an interactive word puzzle.
Loops
An important feature of programs is the ability for the computer to repeat certain steps over and over again. This concept of a loop is the topic of this week.
As you might expect, there are many variations of loops depending on how many times or under which conditions the program should repeat certain pieces. In Python, we tell the program to loop by using either the for or the while keyword, depending on our specific situation.
Preparation Material
There are two types of loops in Python, while loops and for loops. This lesson will focus on while loops and the next lesson will focus on for loops.
Watch the following videos:
Introduction to Loops (11 mins)
A while loop continues while something is still true, or as long as it's true, or stated another way, "until it's no longer true."
For example, you might keep asking the user for a number as long as, or while they keep typing numbers under 10, and stop as soon as they enter one that is larger than that:
number = 0
# Keep looping as long as the number is less than 10
while number < 10:
number = int(input("What is the number? "))
print("Finished with the loop")
The output of this program could be something like the following:
What is the number? 3
What is the number? 7
What is the number? 2
What is the number? 24
Finished with the loop
This also works when the computer is updating a value, rather than getting input from the user. The following program counts up to 5 and then stops:
# Start with the number 1
number = 1
# Keep looping as long as the number is less than or equal to 5
while number <= 5:
# Display the number
print(f"The number is: {number}")
# Update the number to be one more than it was
number = number + 1
print("Finished with the loop")
The output of this program could be something like the following:
The number is: 1
The number is: 2
The number is: 3
The number is: 4
The number is: 5
Finished with the loop
Declaring variables before they are used
Just like with if statements, any variables that will be used in the while expression, need to be declared and assigned values before they are referenced in that condition.
In the following example, the program will cause an error because it tries to check of number is less than 10, but the variable does not exist yet.
# BAD EXAMPLE: This code does not define a value for the number before it is used
while number < 10: # ERROR, number is not defined yet
number = int(input("What is the number? "))
print("Finished with the loop")
The following example corrects the mistake by declaring the variable and setting it equal to a value prior to the loop.
# GOOD EXAMPLE: This code correctly sets the variable to a value before it is used
number = 0
while number < 10:
number = int(input("What is the number? "))
print("Finished with the loop")
In an example such as these, what value should you initialize the variable to? The answer is that you can pick any value that allows the program to enter the loop. In the case above, the loop will run as long as number is less than 10. So if we started it at something more than 10, it would never run:
# BAD EXAMPLE: This code sets the variable to a number that prevents
# the code from ever entering the loop.
number = 25 # ERROR: This number is greater than 10, so the loop will not run
while number < 10: # This body of this loop will NEVER run
number = int(input("What is the number? "))
print("Finished with the loop")
In the following two examples, the mistake is corrected by assigning the variable to a value that allows the loop to run.
# OK EXAMPLE: This code sets the variable to a number that allows the loop to run
# But it is not great, because it sets it to a non-standard value of 6.
number = 6 # This number is less than 10, so the loop will run, but it is not standard
while number < 10: # This body of this loop will run just fine
number = int(input("What is the number? "))
print("Finished with the loop")
# GOOD EXAMPLE: This code sets the variable to a number that allows the loop to run
# It uses a standard initialization value of 0.
number = 0 # This number is less than 10, and is a standard value
while number < 10: # This body of this loop will run just fine
number = int(input("What is the number? "))
print("Finished with the loop")
Notice that in the previous two examples, the variable is set to a value less than 10, so the loop will run. Technically, any number less than 10 would work (including negative numbers), but it is common practice to initialize unused integer variables to 0 or -1, to initialize strings to "", and to initialize boolean variables to False.
Variable Scope
Just as you need to declare a variable before the loop if you want to use it in the condition statement, variables that are first declared inside the body of a loop (or an if statement) should not be used after the loop. Sometimes programming languages like Python will allow this to work, but it is not considered good practice, because it can cause bugs to arise in your code that are difficult to track down.
# BAD EXAMPLE: This code uses the variable "name" outside the loop where
# it was declared
number = 0
while number < 10:
number = int(input("What is the number? "))
name = input("What is your name?")
print(f"Your name is: {name}")
# GOOD EXAMPLE: This code first declares the variable "name" before the loop so
# that it can be used afterward.
number = 0
name = ""
while number < 10:
number = int(input("What is the number? "))
name = input("What is your name?")
print(f"Your name is: {name}")