CPSC 110-08: Computing on Mobile Phones
Spring 2012
The Leap Year App

CS Principles

This activity addresses the concept of algorithms, abstraction and programming. It focuses, in part, on the following learning objectives:

Introduction

2012 is a Leap Year. But what exactly is a leap year and what years are leap years?

This activity is a follow-up to the leap year homework assignment. The following rules defines which years are leap years:

Leap Year Definition. A year is a leap year if it is evenly divisible by 4 but not evenly divisible by 100 unless it is also evenly divisible by 400. So 1996 was a leap year. But 1900 was not a leap year because, although it is divisible by 4, it is also divisible by 100 and not by 400. 2000 is a leap year because it is divisible by 400.

Leap Year App

Let's build an app that tells whether any given year is a leap year or not. The user interface (UI) for this app will be as follows (click to enlarge):

The app contains a Label with the text "Leap Year?", a TextBox where the user will enter a year, a Button that the user will click to test whether the year is a leap year, and a Notifier that will display the answer.

For homework we constructed the following blocks (click to enlarge):

  • Functions: As described in the homework, the isLeapYear block is an example of a procedureWithResult block -- also known as a programmer-defined function block. You can find it in the Built-in > Definition drawer.

    The isLeapYear function was only partially completed in the homework assignment, where we had the following challenges:

    1. Translate the if/else pseudocode into a nested App Inventor if/else block.

    2. How do you define is divisible by in App Inventor? HINT: Look in the Math drawer.

    3. Where should you put the nested if/else algorithm?

    The isDivisibleBy Function

    A number a is divisible by b if the result of dividing a by b leaves a remainder of 0. For example, 10 is divisible by 2 because 10/2 gives a quotient of 5 and a remainder of 0. But 10 is not divisible by 3, because 10/3 gives a quotient of 3 and a remainder of 1. We can express this in pseudocode and App Inventor blocks as follows:

    PseudocodeApp Inventor Block
    remainder (a, b) = 0

    Thus, we see that saying a is divisible by b is the same as saying the remainder of dividing a by b is 0.

    Because our leap year definition uses the concept of is divisible with several different values for b, it makes sense to define a function that we can call (just like a built-in function) whenever we want to know whether one number (the year) is divisble by another (4 or 100 or 400).

    The arguments, a and b, serve as place holders into which we can place any integer values we like and the function will return either true or false:

    The Nested if/else Structure

    To finish our leap year app we need to complete the definition of the isLeapYear function. What's needed is a nested if/else block that expresses the leap year definition: 
    # Definition of leap year function
To: isLeapYear (year): 

  # Test the divisibility of year and set isALeapYear to true or false
  if year is divisible by 4:
     then if year is divisible by 100:
        then if year is divisible by 400:
           set isALeapYear to true          # Divisible by 4 and 100 and 400
        else
           set isALeapYear to false         # Divisible by 4 and 100 but not 400
     else
        set isALeapYear to true             # Divisible by 4 but not by 100
  else 
     set isALeapYear to false               # Not divisible by 4

  # Return either true or false
  Return: isALeapYear

    Notice that three 3 individual if/else blocks nested within each other. These express the 3 divisibility tests that are needed to define a leap year.

    To Complete This Lesson

  • After completing the two functions, test that the app works correctly on the inputs: 1900, 1996, 2000, 2012, and 2013.

  • Update your Portfolio page with a new screen shot of your completed blocks..

    Summary of Key Points

    We will be using seeing functions again in subsequent examples. But hopefully this lesson helps you see that functions can: