When to Use Loops
In programming, there are sets of instructions you will need to repeat multiple times. For example, if you want to perform the same task on every item in a list. What if you have a list of campaigns, andyou want to printevery one of them?
When you need to repeat a set of instructions, sometimes you know the number of repetitions in advance; other times, you don't. Thereare also times when the number ofrepetitions does not matter, and you want to repeat the code until a certain condition is met.
For all of these purposes, you use loops.
The for Loop
Theforloop is the core type of looping in Python. A for loop is used to iterate over any sequence. That can be a list, tuple, dictionary, or even a string. With a for loop, you can execute the same code for each element in that sequence.
Python makes it very easy to loop through every element of a sequence. If you want to print every element in a list, it will look like this:
dog_breeds = ["golden retriever", "chihuahua", "terrier", "pug"]for dog in dog_breeds: print(dog)In this code, each element in dog_breeds willbe printed to the terminal. dogis a variable name that updates to be the next element every time the loop repeats. Theoretically,you could change dog to co*ckadoodledoo and it would still perform the same functionality.
You can do the same type of for loop if you want to loop over every character in a string.
To loop through a set of code a certain number of times, you can use the range() function, which returns a list of numbers starting from 0 to the specified end number.
for x in range(5): print(x)This code will print 0, 1, 2, 3, 4 sequentially.
for x in range(100): print(f"{x} bottles of beer on the wall!")The curly brackets {} here will take whatever value is in variablexand put it in its place. So in this example, the code will print:
0 bottles of beer on the wall!1 bottles of beer on the wall!…99 bottles of beer on the wall!The range() function defaults to 0 as a starting value, but you can changeit by adding another integer like so: range(4, 10)!
This range would return values from 4 to 10 (but not including 10).
The while Loop
While theforloop lets you execute code a certain specified number of times, thewhileloopkeeps executing until a certain condition is met.
In the previous chapter,you learned about conditions as statements that evaluate to true or false. The same applies here: code within a while statementwill keep executing until the condition becomes false.
The code snippet below checks the current capacity and increases it by 1 until the maximum capacity is reached (+= 1increases the current value by 1).
maximum_capacity = 10current_capacity = 3while current_capacity <= maximum_capacity: current_capacity += 1Because the current_capacity starts at 3, this code executes 7 times until current_capacity hits 10.
It’s important to be aware of infinite loops. If the condition you set is always true, the loop will continue to run forever!
For example:
x = 0while x != 5: x += 2In this situation, x will never hit 5!
Level-Up:Loop-the-Loop
Context:
Now you are going to write a program that calculates various statistics for a set of numbers provided by the user. This task will allow you to review lists and address the new concept of this chapter: loops.
Instructions:
Ask the user to enter a list of numbers separated by commas (e.g., 1,2,3,4), and display this list.
Calculate and display the sum of the numbers in the list.
Calculate and display the average of the numbers in the list.
Calculate and display the number of numbers in the list that are greater than the average.
Calculate and display the number of numbers in the list that are even.
Once you have completed the exercise, you can run the following command in the VS code terminal pytest tests.py
Let’s Recap!
Loops let you easily repeat tasks or execute code over every element in a list.
A
forloop enables you to repeat code a certain amount of time.A
whileloop lets you repeat code until a certain condition is met.
Loops are great to help you repeat code easily. Next, we’ll dive into functions; another way to help you bundle repeatable tasks.
