Lesson 2.3: Inputs and Conditionals

Student Objectives

Students will be able to...

• Ask for and receive user input in a SNAP program
• Use simple conditional (if and if-else) blocks to alter control flow in a SNAP program

Materials/Preparation

• Lab 2.3 Handout - What Shape is That? (Download in Word) (Link to PDF)

Pacing Guide

Note: This lesson may cover too much material for some classes to handle in one day. Feel free to spill over into a second day, splitting the material however works best for your class.

Instructor's Notes

1. Lecture

   1. Point out that, so far, our programs have had minimal interactivity
      • There was some user interaction in the shape drawing labs, but no back and forth-- just pushing a key to trigger an action
      • This is not normally how computer programs work
      • Ask students for examples of interactive computer programs
      • Ask students for suggestions for making previous labs or activities more interactive
        • There are no right or wrong answers here-- you're just trying to get students thinking about interactivity

   2. Introduce the block

      • Demonstrate that it functions similar to the block but waits for a response from the user
      • The response is stored in the block and can be referenced later
        • Variables will not be introduced until unit 3, so this block will be somewhat magical for now. You should judge your students' preparedness to handle the details and act accordingly.

      • Emphasize that only one input is stored at a time, and that asking a new question deletes the previous answer

        • For example, the following script, intended to draw a bunch of squares where the user specifies both the size and the number of squares, will not work as intended:

          

   3. Introduce conditional statements

      • Define "conditional" - a block used to make a choice between executing two different chunks of code
      • Point out the differences between and
        • Namely, if-else provides a choice between two code paths, whereas if simply chooses between executing code or not
      • Emphasize that only one of the bodies, either the if or the else, will ever be executed
      • Show students the relational operators (<, >, and =)
        • These should be fairly intuitive to most students

2. Activity

   • Direct students to complete the What Shape is That? activity individually or in pairs
     • Help students realize that, although they may seem quite different, parts 2.1 and 2.2 require very similar code

3. Debrief

   • Discuss one or two students solutions
     • Point out differences between the approaches of different students and lead discussion about advantages and disadvantages
     • Place particular emphasis on the choice between if and if-else blocks
       • Explain that, when conditions are mutually exclusive (as in part 2.1), a series of if vs. if-else blocks can be functionally equivalent
       • When the conditions are not mutually exclusive (as in part 2.2), the choice matters more

Accommodation/Differentiation

• Advanced students can be encouraged to add extra functionality, such as attempting to draw the shape the user is specifying or identifying types of triangles (equilateral, isosceles, scalene)

• Students who are struggling can be allowed to skip some parts of the tables in 2.1 and 2.2, focusing on just a few cases

  Lab 2.3 Solution