Lesson 2.04: Conditionals

Learning Objectives

Students will be able to...

• Define and identify: if, else, elif, conditionals, flow of control
• Create chaining if statements
• Understand how conditional statements alter the flow of control of a program

Materials/Preparation

• Do Now
• Lab - Game Show
• Solution
• Associated Reading - section 2.3 of Book
• Read through the do now, lesson, and lab so that you are familiar with the requirements and can assist students

Pacing Guide

Instructor's Notes

1. Do Now

   • Project the Do Now on the board, circulate around the class to check that students are working and understand the instructions.
   • Students should quickly realize that they do not have all the tools necessary to complete the task.

2. Lesson

   • Ask students what they felt like they needed that they had in Snap!

   • Explain that in order to write useful programs, we almost always need the ability to check conditions and change the behavior of the program accordingly.

     • Conditional statements give us this ability to affect the flow of control.
       • The simplest form is the if statement. The boolean expression after if is called the condition. If it is true, then the indented statement gets executed. If not, nothing happens.

     if x > 0: 
       print("x is positive")
     

     • Write out the syntax of the if statement on the board. Point out the Boolean expression(condition), the colon, and the indentation.
     • Ask students if they recall what else went along with the if statement when they used it in Snap!
   • else is used when there are two possibilities and the condition determines which one gets executed.
   • Demonstrate the syntax of else
     • Describe the elif statement
   • Sometimes there are more than two possibilities and we need more than two branches. One way to express a computation like that is a chained conditional:
     • Demonstrate the syntax of elif

3. Lab

   • Students convert the triangle program written in Snap! into Python.
   • Students must also write a program that simulates a list index using if statements.
4. Debrief
   • Check student progress and completion of the lab, wrap up by taking any final questions.

Accommodation/Differentiation

• Convert and finish the following SNAP Vending Machine program in Python.

If students are moving quickly, this lesson can move onto lists.