Module 3: Variables and Integers

Objectives

By the end of this module, for simple HelloWorld-like programs, you will be able to:

Create variable declarations.
Assign values to variables by simple assignment, and print them out.
Distinguish between integers in strings versus actual integers.
Demonstrate ability to perform operations on integers for a desired output.
Simplify expressions with constants to single value.
Evaluate expressions with variables in them.
Convert English descriptions of operations into expressions.
Mentally trace execution with expressions and calculations.
Identify new syntactic elements related to the above.

First, an analogy

Suppose we have boxes. Consider the following rules about "boxes":

Each box can store only one item.
The possible things that can be stored inside are called values.
Thus, at any given moment, a box's value is whatever's inside it.
Each box is "typed"
=> What this means:
- A box meant for shoes is allowed to store only shoes.
- A box meant for caps is allowed to store only caps.
Each box has a unique name:
There is a cloning process that works like this:
- The value inside one box is cloned.
- The cloned value is placed inside another.
There is a strange shortcut notation to specify cloning:
- Here, the = (equals sign) does not mean "equals."
- It has been repurposed to mean "clone", "copy," or, in programming-language jargon "assign".
Usage: "x is assigned the value in y".

In-Class Exercise 1: Suppose z is a box that stores caps. What will the statement z = y result in? Draw a picture.

Integer variables

Consider the following program:

In-Class Exercise 2: Edit, compile and execute the above program.

Now let's examine key parts of this program:

First, i is the name of a "box" (of sorts).
The term used for "box" is variable.
=> i is a variable.
Variables must be "typed"
=> i can only store integers.
To create and name a variable, we use a variable declaration:
Here, int is a reserved word in Java.
To put something in a variable, we use assignment
=> with the repurposed = (equals) sign.
When we print a variable, what gets printed is its value.
=> Thus, the number 5 gets printed

Next, let's look at assignment between variables:

This is the analogue of cloning.
Consider this addition to the above program:
We say, in short, "i is assigned to j".

In-Class Exercise 3: What happens if you don't place a value in a variable? Find out by editing and compiling this program:

In-Class Exercise 4: Identify the output of this program just by reading (mental execution).

In-Class Exercise 5: Identify the output of this program just by reading

A rule: a variable must be declared before it is used (assigned a value or have a value copied from it).

In-Class Exercise 6: What is the compiler error you get for this program?

Variations

The following variations are worth knowing:

Comma-separated declarations:
Note: variables need not be declared in the order they are used in the program.
Declaration and assignment can be combined:
Another example:

In-Class Exercise 7: Consider the following partially-complete program:

Insert code where indicated to have the program first print 6, then 5. Do this only by using assignments between variables.

Note writing style:

A space on either side of the equals sign for assignment.
The same when you combine declaration and assignment for a single variable.
It's customary to remove the space for multiple declarations.

Integer operators

Let's examine the familiar binary operators +, -, *, /

Addition: +
Subtraction: -
Multiplication: *
Division: /
Consider this example with addition:
What happens during execution:
- The values in i and j are added.
- The resulting value goes into variable k.
A long-ish way of saying this aloud:
=> "k is assigned the sum of the values of i and j"
A shorter way:
=> "k is assigned i plus j"
Here's an example with multiplication and division:

In-Class Exercise 8: What does it print?

Next, let's consider some unary operators:

These are operators that apply to a single variable.
For example:
The unary operators here are the ++ (increment) and -- (decrement):
Note writing style: no space between variable and unary operator, but this is also acceptable:

In-Class Exercise 9: Mentally execute this program - what does it print?

Expressions and operator-precedence

Consider the following program:

In-Class Exercise 10: What does it print?

About expressions:

An expression combines constants (like 1, above), and variables using operators.
Example:
i*j - (i+1)*(j-1).
The above expression is really equivalent to:
(i*j) - ((i+1) * (j-1)).
Here, we added some clarifying parentheses.
Operator precedence allows us to reduce the number of clarifying parentheses.
Java precedence follows standard predence in math: /, *, +, -.
The above expression is NOT the same as:
i*j - i+1*j-1.

In-Class Exercise 11: Add parentheses to clarify the expression i*j - i+1*j-1. What does it evaluate to when i=7 and j=3?

More about expressions and assignment

Here are two more operators, one unary (negative sign) and one binary (remainder or mod operator), that are commonly used:

In-Class Exercise 12: What does it print?

Now we'll look at a strange (initially) but very useful type of assignment:

Consider this program:
Prior to evaluating the expression, i has value 7.
On the right side, the current value of i is used to evaluate the expression.
=> Thus, the expression evaluates to (7 + 7/2) = 10.
This evaluated value then goes into variable i.
=> After the assignment, i has the value 10.

When things go wrong

As you might imagine, there are many ways to inadvertently create errors.

Let's start with compilers errors.

In-Class Exercise 13: Try to identify the compiler error (or errors), by reading each program below.
Program 1:

Program 2:

Program 3:

Program 4: