Python Programming: Loops
Learning Objectives
After this lesson, you will be able to:
Use a for loop to iterate a list.
Use range() to dynamically generate loops.
Use a while loop to control program flow.
Use break to exit a loop.
Teaching Tips :
Briefly overview today’s learning objectives.
Discussion: A Small List
This situation isn’t so bad:
But what would we do if there were 1,000 items in the list to print?
Teaching Tips :
Try to get students to guess at the idea of putting the print statement in a loop.
Talking Points :
“But, what if we had a list with 283,000 items in it? We can’t write one line for each element. We need a way to write the print statement once and have it run for every element in the list.”
“One of the most powerful things that programming can do for you is automatically perform repetitive tasks. In the tiny scripts we’ve been writing, printing out every variable has not been much of an issue. But with lists in the game now, things can get a bit more challenging. We need a way to automate some tasks so that they repeat for every item in a list.”
The for Loop
The for loop always follows this form:
For example:
Teaching Tips :
Go over the syntax. Point out the indentation - point out that it’s like the if statement they learned.
Talking Points :
“We need what is called a Loop.”
“A common and extremely useful type of loop is the for loop. for loops appear in several computer programming languages and get their name from the fact that they loop (or iterate) for a specific number of times: once for each item in a list.”
This code: * Creates a new list containing some name strings. * Begins the for loop. We loop once for each “name” in the list (names). * Prints the current name inside the loop."
“The for loop is perfect for when you have a specific collection of items — each of which must be processed once — or for when you know that you must execute a set of instructions a specific number of times. Those are the use cases for which it was designed.”
"This is our template. When we write a for loop we do the following:
We replace item with a meaningful name for the items in the collection.
We use this name as the name of the item within our loop.
We replace collection with the name of our list (or other collection).
Indented inside the loop, we write the code to be run for each item.
Python will start with the first item and automatically stop after it loops with the last item."
Knowledge Check: What will this code do?
Think about what the code will do before you actually run it.
Talking Points :
“The statements inside a loop that you want to repeat must be indented like the statements inside an if block. So, if you have three lines of code that you want to execute on each loop iteration, each must be indented one level underneath your for line.”
“Each name is met with "THUNDEROUS APPLAUSE!" because that line, and the two above it, are indented to be in the body of the for loop.”
Repl.it Note : This replit has:
We Do: Writing a Loop
Let’s write a loop to print names of guests.
First, we need a list.
Teaching Tips :
Do this with them! Make sure they’re typing with you, to practice. See if they can give you the syntax you should write.
The code is :
We Do: Write a Loop - Making the Loop
Now, we’ll add the loop.
Skip a line and write the first line of your for loop.
For the variable that holds each item, give it a name that reflects what the item is (e.g. name or person).
Inside your loop, add the code to print "Hello," plus the name.
"Hello, Felicia!"
"Hello, Srinivas!"
Teaching Tips :
Do this with them! Make sure they’re typing with you, to practice. See if they can give you the syntax you should write.
Talking Points :
Remind them that the word collection is a list; this collection is the list variable they have.
The code is :
We Do: Write a loop to greet people on your guest list
Our guests are definitely VIPs! Let’s give them a lavish two-line greeting.
Inside your loop, add the code to print another sentence of greeting:
"Hello, Srinivas!"
"Welcome to the party!"
Teaching Tips :
Do this with them! Make sure they’re typing with you, to practice. See if they can give you the syntax you should write.
Point out the indent.
The code is :
Talking Points :
“Fantastic! Now each guest is greeted by their name and welcomed to the party. Those two print() lines are executed on every iteration because both are indented to be in the for loop’s code block. Think of the indented block as a unit of instructions executed as a group each time the loop runs.”
Discussion: Where Else Could We Use a Loop?
A loop prints everything in a collection of items.
guest_list = ["Fred", "Cho", "Brandi", "Yuna", "Nanda", "Denise"]
What, besides a list, could we use a loop on?
Hint: There are six on this slide!
Teaching Tips :
The answer is a string! We can loop through the characters.
Looping Strings
Loops are collections of strings and numbers.
Strings are collections of characters!
Teaching Tips :
Don’t spend more than a minute or two on this slide - it’s not an exercise; just a demonstration.
The point is to understand that a string is a collection of characters, and any collection can be looped.
Talking Points :
“You may not realize it, but a string is a collection of characters. Just so you can see that a for loop has the same syntax for any collection, let’s add the following code below what we’ve just written”
Repl.it Note: : This code has:
What about…Looping For a Specific Number of Iterations?
We have:
The loop runs for every item in the list - the length of the collection. Here, it runs 6 times.
What if we don’t know how long guest_list will be?
Or only want to loop some of it?
Talking Points :
“Whenever we have a collection, such as a list or string, and we want to iterate over each item it contains, we should use a for loop. Python has internal logic for determining exactly how many times the loop should run based on the length of the collection.”
“But, what if we want to do something in a loop a specific number of times, but we don’t have a collection to start with? Maybe we are initializing a new collection and we need to add a specific number of items to it or maybe we just want something to run exactly 15 times. In this case, we can have the for loop iterate over a range of numbers.”
Enter: Range
range(x):
Automatically generated.
A list that contains only integers.
Starts at zero.
Stops before the number you input.
Talking Points :
“You can actually feed more parameters into range() to control what number it starts at and how big each step is to the next number, but we will keep it simple for now. For now it is enough to know that if you loop over range(5) then your loop will execute five times. Let’s use this in a loop…”
Looping Over a Range
Let’s look at range in action:
Teaching Tips :
This isn’t an exercise, just a quick demo. Make sure they understand how range is working.
Talking Points :
“We can see that this code prints each of the numbers in our range of 0 through 9 (10 numbers total.) We don’t need to have our loop print anything. This loop could be used to execute any sequence of code 10 times.”
Repl.it Note: This code is:
Looping Over a Range
Looping over names here is really just going through the loop 4 times - at index 0, 1, 2, and 3.
We can instead use range(x) to track the index and loop names: range(4) is [0, 1, 2, 3].
We can then use len(names), which is 4, as our range.
Teaching Tips :
This isn’t an exercise, just a demo. Make sure they understand how range is working. This can be a bit tricky!
Show a regular loop again, if you need to.
Vary the size of range - try range(4) and range(2) - so students can see the differences.
Give a bunch of examples. Range might make sense conceptually, but be hard to remember the syntax for and use.
Talking Points :
“Recall that you can use the len() function to get the length of a list. Since that will always be an integer, we can feed that value into the range() function to generate a range that contains each index in the list.”
“Don’t be alarmed about the function inside the function. That’s fairly common. Let’s break it down: len(names) will return the length of the names list; in this case, 4. The number 4 is then used as the parameter for range() creating a range containing 0, 1, 2, and 3. These happen to all be valid indices for our list so we can use them to modify the values stored at those indices.”
Repl.it Note: This code is:
Range to Modify Collections
Why would you use range on a list, when you could just loop the list?
We can’t do:
But we can do:
Talking Points :
“But there is one special use for this that is vital to know about. When we loop using for item in collection we can’t ever really modify the elements in the list. Whenever we access item we are actually getting a copy of the item in the list. In order to modify the item in the list, we would need the index of that item in the list. And guess what range() gives us…”
Looping Over a Range
Let’s make the list all uppercase:
Teaching Tips :
This is a bit difficult. Make sure they understand that range hits the index versus a regular for loop makes a copy.
Give different examples if need be.
Talking Points :
“Now when we run this we see that the names list has had all the string changed to uppercase. It is necessary to use list indices if you want to modify list elements. If we tried doing the same thing with an item from that list then the changes would only apply to the temporary copy stored in item and would never actually make it into the list.”
Repl.it Note: This code is:
Knowledge Check: Which of the following lines is correct?
Teaching Tips :
Give them time to discuss and come up with an answer!
The answer is B
You Do: Range
Locally, create a new file called range_practice.py.
In it:
Create a list of colors.
Using a for loop, print out the list.
Using range, set each item in the list to be the number of characters in the list.
Print the list.
For example:
5-10 minutes
Teaching Tips :
If students seem a bit lost when you get here, run this as a partner exercise.
This is tough! Give them a bit before going over the answer - bring up your own file or a repl.it and write it, talking through each step.
Quick Review: For Loops and Range
for loops:
# On a list (a collection of strings) guest_list = ["Fred" , "Cho" , "Brandi" , "Yuna" , "Nanda" , "Denise" ]
for guest in guest_list: print ("Hello, " + guest + "!" )
# On a string (a collection of characters) my_string = "Hello, world!"
for character in my_string: print (character)
##### Range ##### range (4 ) # => [0, 1, 2, 3] # Using Range as an Index Counter names = ["Flint" , "John Cho" , "Billy Bones" , "Nanda Yuna" ]
for each_name in range (4 ): print (names[each_name])
# OR for each_name in range (len (names)): print (names[each_name])
# Using Range to Change a List: guest_list = ["Fred" , "Cho" , "Brandi" , "Yuna" , "Nanda" , "Denise" ]
for guest in range (len (guest_list)): guest_list[guest] = "A new name"
Teaching Tips :
Quickly review key takeaways.
Ask if there are questions. That was a lot!
The While Loop
What about “While the bread isn’t brown, keep cooking”?
Python provides two loop types.
for:
You just learned!
Loops over collections a finite number of times.
while:
You’re about to learn!
When your loop could run an indeterminate number of times.
Checks if something is True (the bread isn’t brown yet) and runs until it’s set to False (now the bread is brown, so stop) .
Talking Points :
“Now that we’ve learned how to write for loops to to iterate lists, and how to use range to dynamically generate loops, we’re going to transition to while loops. I’m going to show you how to write while loops, and then you’re going to practice while loops with your partner.”
While Loop Syntax
Teaching Tips :
Call out that it won’t print 10.
Call out the indents!
Talking Points :
“We won’t always have the luxury of a collection of discrete data items for controlling our loop. Frequently, we will need to write a loop that will run an unknown number of times. This is what the while loop is for. It’s another loop construct that will continue to iterate while a given condition is true . These loops are quite useful for data sets of unknown sizes, or for when we need to loop until some value changes.”
“Outside of our while loop, we create the variable a, which we’ll use as our conditional. We then start our loop. We say to loop”while a is less than 10." Then, in the loop, we print a and add one to its value. Once the value of a reaches 10, the loop condition evaluates to false and the loop finishes."
While Loop: Be Careful!
Don’t ever do:
And don’t ever do:
Your program will run forever!
If your program ever doesn’t leave a loop, hit control-c.
Teaching Tips :
We’re telling them this in advance of practicing, in case they do it!
Make it clear why this runs forever.
Talking Points :
“While loops present a potential”gotcha" in programming: the infinite loop. Because the while loop only terminates when a condition turns to false, it’s possible to write the loop in such a way that it never terminates. This creates a serious bug in your code where the loop never, ever returns control to the app, and it will freeze indefinitely. The way to avoid this is to always remember to update your conditional variable inside your loop block."
We Do: Filling a Glass of Water
Create a new local file, practicing_while.py.
In it, we’ll create:
A variable for our current glass content.
Another variable for the total capacity of the glass.
Let’s start with this:
Can you start the while loop?
Teaching Tips :
Make sure they’re typing this with you and not just copying and pasting the code.
Before you go to the next slide (adding the loop), see if they can guess the syntax.
Talking Points :
“Can you think of the mental process you follow when pouring water into a glass? You start with an empty glass and begin adding water to it, right? While you are adding the water, you must constantly check to see if the glass has reached its maximum capacity. If it has, you then stop pouring. Otherwise, you continue adding water. Let’s see how that would work as a while loop…”
“We a variable for our current glass content and we need one for the total capacity of the glass.”
“What we want to do is add water to the glass one unit at a time until the glass reaches capacity. Said another way, while the glass contains less than its capacity, add another unit of water. Can you start the loop code before we move to the next slide?”
We Do: Filling a Glass of Water
Add the loop:
That’s it!
Talking Points :
“We declare our glass variable and set it to 0 water, currently. Then, we declare our glass’ capacity and set it to 12 units. Next, we set up our while loop. We want to loop while the glass has a value less than glass_capacity. Inside of our loop, we add 1 unit of water to our glass. Each time the loop runs, it checks the value of glass to see if it has reached the same value as glass_capacity. The loop stops once glass reaches 12, conveniently before we spill.”
You Do: A Guessing Game
Now, get with a partner! Let’s write the the game.
Decide who will be driver and who will be navigator. Add this to your existing file.
Set a variable, answer to "5" (yes, a string!).
Prompt the user for a guess and save it in a new variable, guess.
Create a while loop, ending when guess is equal to answer.
In the while loop, prompt the user for a new guess.
After the while loop, print “You did it!”
Discuss with your partner: Why do we need to make an initial variable before the loop?
5 MINUTES
Teaching Tips :
Run this as a partner exercise. Set student up in pairs, then walk around to be sure they’re all working and no one is stuck.
Talking Points :
“Now that you know how to accept input from the keyboard, make a game where the player must correctly guess a number. The game will continually prompt the user to enter a guess if they did not guess correctly. Once they correctly guess the number, the program congratulates them and exits. Can you think of how this would be written?”
“You need a variable to hold the correct answer. You need another variable to hold each subsequent guess. We will need to compare the guess to the answer and if it is wrong, ask the user again for a guess. This sounds like we will be repeatedly asking for a guess while the previous guess was incorrect.”
“One thing to keep in mind is that input() returns a string so if the user types 5 it will result in the string "5". You cannot compare numbers to strings in Python. To work around this for a number guessing game, set your correct answer variable to be the string of the number (e.g.”4" or “9”) instead of the number itself. This way when you do your loop comparison, you’ll be comparing the same types."
You Do: A Guessing Game (Solution)
How’d you do? Questions?
Teaching Tips :
Walk through the answer here. Show it working. Check for understanding.
Exiting a Loop
There are times when you may want to exit a loop before the final condition has been met. Perhaps the input from another part of the program has satisfied another, separate condition that makes the rest of the loop unnecessary. Enter the break statement.
Continuing a Loop
There are times when you may want to to continue a loop without running code beneath the continue statement . The continue allows you to do just that! After the continue statement is triggered, the loop continues the next iteration of the loop without executing any code beneath it on that iteration .
Prints:
Passing within a Loop
The pass statement is like a placebo in a loop: it allows a loop to execute without any interruption. This example may seem odd, and we’ll cover the more common use case in the next example.
Prints:
Passing within a Function or Class
The most common use case for pass is to act as a placeholder for a function that has yet to be written. Developers will often do this if they’re creating the architecture for a program but haven’t gotten to actually building the logic yet.
What happens if we don’t put the pass statement in the code and attempt to execute the function definition?
Teaching Tip :
The students should be able to guess that without pass, execution will not proceed properly, resulting in our architectural scaffold breaking.
Throwing Exceptions within a Function or Class
Note that the previous example will allow the function to be called, but the function won’t do anything. If the programmer wishes to alert the user, they may also use raise to interrupt the program execution. The following is common to see in larger applications that are in the process of being built by a dev team:
What happens when we call this function? Hint: look at the type of error that is returned!
Talking Point :
The raise statement will halt the program and verbosely tell the programmer that the function is not implemented!
Summary + Q&A
Loops:
Common, powerful control structures that let us efficiently deal with repetitive tasks.
for loops:
Used to iterate a set number of times over a collection (e.g. list, string, or using range).
range use indices, not duplicates, so it lets you modify the collection.
while loops:
Run until a condition is false.
Used when you don’t know how many times you need to iterate.
That was a tough lesson! Any questions?
Teaching Tips :
Briefly recap.
Check for understanding! Bring up an interpreter if needed, or if you have extra time. The more they see this, the better!
Then, go over next steps.
Encourage students to check out all the additional resources at the end of each presentation.
Additional Reading
Teaching Tips :
Encourage students to go through these in their spare time, especially if they need more help.
