Using var As A Data Type in Programs

Overview

In this lesson, students will begin to develop an understanding of when it is appropriate to use var and when it is not appropriate.

For AP CSA teachers, the use of var is outside the scope of the AP CSA course and exam.

Learning Objectives

• 1.4.A.3 Use var to create variables of inferred types.

Skills

• S1.C Explain the impact design has on data storage.
• S2.A Write program code and implement algorithms.

Student Outcomes

Students will be able to:

• determine when var can be used instead of a data type.

Duration: 1 class period

Resources

• Learn - Local Variable (var)
• Practice - Exploring var with the Java Playground

Warm-up / Motivate

The use of var for a data type works because the compiler can intuitively determine what type of data is being stored based on what is assigned to the variable. It is a short-cut that can be used. Students should have a sound understanding of data types before beginning to use var instead of explicitly defining the data type.

Learn

Either as a group or on their own, have students review the tutorial: Learn - Local Variable (var)

Practice

Have students complete practice exercises: Practice - Exploring var with the Java Playground

Apply

Incorporate var into a prior program.

1. Have students revisit a prior program and include var in all places that are unambiguous.
2. Identify places in the program where the use of var cannot be used.
3. Identify places in the program where the use of var can be used, but shouldn't be used.

Wrap-up / Extension

Using var on AP CSA Free Response Questions

How does this look on the exam? Now that we have taken a chance to understand var outside of the AP CSA exam. Let's look at an example of how students might use this on the AP Computer Science A (AP CSA) exam.

The following are examples of how students could use var on the AP CSA Free Response Exam. You can find all of the questions here

Free Response Question 1 - Bird Feeder

This question involved a simulation of a bird feeder and requires students to do the following:

• generate random values
• use if statement
• use loops

Part A - Example Canonical Solution

There are three variable declarations in the sample solution above:

1. bearPresent – is assigned to the Boolean false. The type is unambiguous with the assignment.
2. bear – is assigned to a double from Math.random(). You could use var, but it can be ambiguous to use with a method return.
3. birdConsumed – even though there is a method call, the typecast to an int makes the type clear that the value being assigned is an int.

A sample solution using var is as follows:

Part B - Example Canonical Solution

There are two variables declared in the sample solution above:

• count - Since the variable is being assigned to 1, it is unambiguous and is a good place to use var.
• days - The for loop counter days can be declared using var.

This sample solution using var is as follows:

Free Response Question 2 - Scoreboard

In this free response question, students are asked to write a class for a scoreboard with two teams. Students will need to:

• declare instance variables
• write constructors
• write methods
• use if statements

The following is a sample solution:

In this solution, we have many data types where students might be tempted to use var. However, you cannot use var for the following:

• instance variables
• constructor or method parameters
• method return types

If a student were to use var in these places, they will likely lose points.

There is only one place in the solution above where a student could use var if they wanted to do so. In the getScore method the String variable, retVal, is declared and assigned the concatenation of team scores and a string hyphen. Since the concatenation of strings with int values creates a String, it is unambiguous what type we are using and var can be used appropriately.

A sample solution using var is as follows:

NOTE: In this solution we have used the ternary conditional operator. This operator is not a part of the AP Java Subset. You do not need to teach this operator.

Learn more on the Ternary Conditional Operator.

Free Response Question 3 - WordList

In this free response students will need to:

• loop over a list
• access adjacent elements of a list
• use string methods
• add values to a list
• create a new list

Part A - Sample Canonical Solution

In part A of this free response question, students are asked to determine if the previous word is a substring of the next word.

In this solution, there are three variables being declared:

• index - the for loop counter index can be declared with var
• nextWord and currentWord - students could use var to declare these variables, but this would be considered less readable, since the value these are being assigned comes from a method call. In this case, we are using common method calls and will likely know that wordlist stores String values, but it is still considered less than desirable to use it in these two cases.

A sample solution using var is as follows:

Part B - Sample Canonical Solution

In part B, students need to make a list of all word that have the target word at the beginning, but with the beginning of the word is removed.

In this solution, var could be used in two places:

• retList - a new ArrayList that is able to store string values
• word - used in the enhanced-for loop to store the values in wordList, which are strings

A sample solution using var is as follows:

Free Response Question 4 - GridPath

In this free response students will need to:

• access elements in a 2D array
• use a provided class
• use conditionals and loops

Part A - Sample Canonical Solution

In this part, students need to return the smaller of two values, either below or to the right if they exist.

In this solution, we have declared four variables:

• numRows and numCols - The value assigned to numRows and numCols is from the constant length. Since this is really common, it is most likely readable to use var in these cases. However, if there is any doubt about the readability, you will want to use the type and be clear.
• below and right - The values assigned to below and right are what is being stored in grid and is not ambiguous.

A sample solution with var is as follows:

NOTE: In this solution we have used the ternary conditional operator. This operator is not a part of the AP Java Subset. You do not need to teach this operator.

Learn more on the Ternary Conditional Operator

Part B - Sample Canonical Solution

In this solution, students are summing the values along a path using the return value from getNextLocation.

In this solution, we have two variables that are being declared and assigned values:

• total - which is unambiguous being assigned to 0.
• next - assigned a returned type from a method call. This method is a local method and would be easy to find, however, it is still considered less readable to use var in this case.

A sample solution with var is as follows:

Final Thoughts on Using var with AP CSA

Use of var and the AP CSA Free Response Questions

• Using var is OUTSIDE the AP CSA Java Subset
• Students can write correct solutions using var
• Using var can be used in any FRQ. Students need to be careful using it with FRQ 2, since they cannot use it with instance variables, parameters, and return types.

Scoring and Teaching

• Student solutions that use correct Java syntax can earn full credit for their solution
• Questions are written with an in AP Java subset solution in mind
• Students should be encouraged to write solutions within the AP Java Subset
• Students could lose points if they use var and do not assign the variable to a value that is not unambiguous.