(Also available in WeScheme)

Students discover that inequalities have an important application in video games: keeping game characters on the screen! Students apply their understanding to edit code so that it will keep Sam the Butterfly safely in view.

Lesson Goals

Students will be able to:

  • apply their understanding of inequalities to keep a game character on the screen

Student-Facing Lesson Goals

  • I can use what I know about inequalities to define the boundaries that will keep a character on the screen.



a mathematical description of the relationship between two variables or quantities, in which they are not necessarily equal

🔗Introducing Sam 15 minutes


Students are introduced to Sam the Butterfly, a simple activity in which they must write simple inequalities to detect when Sam has gone too far in one dimension.


As students complete the worksheet and explore the program, they should notice that Sam’s coordinates are displayed at the top of the screen. When Sam is at (0,0), we only see a part of Sam’s wing because Sam’s position is based on the center of the butterfly image. Students should observe that Sam can go up to, but not beyond, an x of -50. Students can represent this algebraically as x > -50, or (for students who notice that Sam only moves in increments of 10) x ≥ -40.

Every time Sam moves, we want to check and see if Sam is safe.

To further support students, consider asking what three functions are defined in their starter files. Then, ask students what each function should do, when working properly.

  • What should our left-checking function do?

    • Check to see if x is greater than -50.

  • What should our right-checking function do?

    • Check to see if x is less than 690.

  • What should onscreen? do?

    • Answers may vary. Let students drive the discussion, and don’t give away the answer!


  • Complete Left and Right with your partner.

  • Once finished, fix the corresponding functions in your Sam the Butterfly file, and test them out.

Students will notice that fixing is-safe-left keeps Sam from disappearing off the left side, but fixing is-safe-right doesn’t seem to keep Sam from disappearing off the right side! When students encounter this, encourage them to look through the code to try and figure out why.

"False" doesn’t mean "Wrong"!

A lot of students - especially confident ones - may struggle to come up with an example where is-safe-left returns false:

# Students hate writing the second one! examples: is-safe-left​(​189​) is 189 > -50 is-safe-left​(​-65​) is -65 > -50 end

This misconception comes from confusing a statement that is "false" with a program that is "wrong". In the second example, above, the result of is-safe-left​(​-65​) is false, because "65 is greater than -50" is a false statement. Remind your students that you want one example that’s true, and a second that’s false!

Pyret includes some functionality that makes this more explicit, and can help resolve the misconception:

  is-safe-left( 89) is true  because  89 > -50
  is-safe-left(-65) is false because -65 > -50

By writing the answer first ( is-safe-left(-65) is false ), it reduces anxiety about code being "wrong". Students can think of the because as an explanation of why the answer is false.

Emphasize to students that they cannot trust the behavior of a complex system! After looking closely at examples and observing that they all pass, students should suspect that the bug is elsewhere.


  • Does is-safe-left work correctly? How do you know?

  • Does is-safe-right work correctly? How do you know?

🔗Protecting Sam on Both Sides 30 minutes


Students solve a word problem involving compound inequalities, using and to compose the simpler Boundary-checking functions from the previous lesson.


Recruit three student volunteers to roleplay the functions is-safe-left, is-safe-right, and is-onscreen. Give them 1 minute to read the contract and code, as written in the program.

Ask the volunteers what their name, Domain and Range are. Explain that you, the facilitator, will be providing a coordinate input. The functions is-safe-left and is-safe-right will respond with either "true" or "false".

The function is-onscreen, however, will call the is-safe-left function! So the student roleplaying is-onscreen should turn to is-safe-left and give the input to them.

For example:

  • Facilitator: "is-onscreen 70"

  • is-onscreen (turns to is-safe-left): "is-safe-left 70"

  • is-safe-left: "true"

  • is-onscreen (turns back to facilitator): "true"

  • Facilitator: "is-onscreen -100"

  • is-onscreen (turns to is-safe-left): "is-safe-left -100"

  • is-safe-left: "false"

  • is-onscreen (turns back to facilitator): "false"

  • Facilitator: "is-onscreen 900"

  • is-onscreen (turns to is-safe-left): "is-safe-left 900"

  • is-safe-left: "true"

  • is-onscreen (turns back to facilitator): "true"

Hopefully your students will notice that is-safe-right did not participate in this roleplay scenario at all!

  • What is the problem with is-onscreen?

    • It’s only talking to is-safe-left, it’s not checking with is-safe-right

  • How can is-onscreen check with both?

    • It needs to talk to is-safe-left AND is-safe-right.


  • Complete Word Problem: is-onscreen.

  • When this function is entered into the editor, students should now see that Sam is protected on both sides of the screen.

Extension Option What if we wanted to keep Sam safe on the top and bottom edges of the screen as well? What additional functions would we need? What functions would need to change? We recommend that students tackling this challenge define a new function is-onscreen-2.


Bring back the three new student volunteers to roleplay those functions, with the onscreen function now working properly. Make sure students provide correct answers, testing both true and false conditions using coordinates where Sam is onscreen and offscreen.

  • How did it feel when you saw Sam hit both walls?

  • Are there multiple solutions for is-onscreen?

  • Is this Top-Down or Bottom-Up design?

🔗Boundary Detection in the Game 10 minutes


Students identify common patterns between two-dimensional Boundary detection and detecting whether a player is onscreen. They apply the same problem-solving and narrow mathematical concept from the previous lesson to a more general problem.


Have students open their in-progress game file and click "Run". Invite them to analyze the movement of the danger and the target

  • How are the TARGET and DANGER behaving right now?

    • They move across the screen.

  • What do we want to change?

    • We want them to come back after they leave one side of the screen.

  • What happens to an image’s x-coordinate when it moves off the screen?

    • An image is entirely off-screen if its x-coordinate is less than -50 and greater than 690.

  • How can we make the computer understand when an image has moved off the screen?

    • We can teach the computer to compare the image’s coordinates to a boundary on the number line, just like we did with Sam the Butterfly!


Apply what you learned from Sam the Butterly to fix the is-safe-left, is-safe-right, and is-onscreen functions in your own code.

Since the screen dimensions for their game are 640x480, just like Sam, they can use their code from Sam as a starting point.

Common Misconceptions

  • Students will need to test their code with their images to see if the boundaries are correct for them. Students with large images may need to use slightly wider boundaries, or vice versa for small images. In some cases, students may have to go back and rescale their images if they are too large or too small for the game.

  • Students may be surprised that the same code that "traps Sam" also "resets the DANGER and TARGET ". It’s critical to explain that these functions do neither of those things! All they do is test if a coordinate is within a certain range on the x-axis. There is other code (hidden in the teachpack) that determines what to do if the coordinate is offscreen. The ability to re-use function is one of the most powerful features of mathematics - and programming!


  • The same code that "trapped" Sam also "resets" the DANGER and the TARGET. What is actually going on?

These materials were developed partly through support of the National Science Foundation, (awards 1042210, 1535276, 1648684, and 1738598). CCbadge Bootstrap by the Bootstrap Community is licensed under a Creative Commons 4.0 Unported License. This license does not grant permission to run training or professional development. Offering training or professional development with materials substantially derived from Bootstrap must be approved in writing by a Bootstrap Director. Permissions beyond the scope of this license, such as to run training, may be available by contacting