PT.3 & PT.4: EXPRESSIONS AND ASSIGNMENT STATEMENTS

OBJECTIVES

I can:

• evaluate arithmetic expressions in a program code

• use the modulus operator (%)

• import and initialize a new Scanner to take in user input

• create variables that take the assigned value of the user input.

• choose the correct command that will allow the program to receive the input value that corresponds with its desired data type

• evaluate what is stored in a variable as a result of an expression with an assignment statement

• use a compound assignment operators (+=, −=, *=, /=, %=) in place of the assignment operator

• use an increment operator (++) or decrement operator (−−) to add 1 or subtract 1 from the stored value of a variable

In this lesson, you will learn about assignment statements and expressions that contain math operators and variables.

ASSIGNMENT STATEMENTS

As we have already seen, assignment statements initialize or change the value stored in a variable using the assignment operator =. 

The value of the expression (which can contain math operators and other variables) on the right side of the = sign is stored in the variable on the left. 

In the figure above, score is assigned the value of the expression 10 times points (another variable) plus 5.

Remember, I recommend getting in the habit of saying "gets" or "is assigned" instead of saying "equals" for the = in an assignment statement.

So, we can set one variable's value to a copy of the value of another variable like y = x;

This won't change the value of the variable that you are copying from.

Let's see if we can trace through some code and predict what the output will be:

ADDING 1 TO A VARIABLE

If you have a variable to keep track of a score, you would probably want to increase the score whenever an event occurs in the game that should cause the score to go up.

It is fairly simple to increment (add one to the current value) the variable.

To do this we set the variable to the current value of the variable plus 1:

score = score + 1;

This formula looks kinda crazy if this were a math class.

Remember, the variable on the left is being reassigned to the value of the arithmetic expression on the right.

So, the score variable is set to the previous value of score + 1.

INPUT WITH VARIABLES

Variables are a powerful abstraction in programming because the same algorithm can be used with different input values saved in variables.

When I see the word "abstraction" I think of hiding away the minor details so that you can focus on the big ideas.

Abstraction typically makes things less complex.

An algorithm is a process or set of rules to be followed in calculations or other problem-solving operations, especially by a computer.

The following algorithm will say hello to anyone who types in their name! Behold, the power of variables!

Using the Java System.in and Scanner classes will not be on the exam. However, learning how to get input from a user in Java is still extremely useful.

There are a few steps to using the Scanner class:

1) import java.util.Scanner;

2) Use System.out.print(value); to give some directions to the user on the screen.

3) Create a new Scanner object using the new keyword (I named the scanner "scan").

4) Call the .nextLine(), .nextInt(), .nextDouble(), etc. method to store the value entered by the user in a new variable.

5) Call the .close() method when you are finished using the Scanner.

OPERATORS

Java uses the standard mathematical operators for addition (+), subtraction (-), multiplication (*), and division (/).

Arithmetic expressions can be of type int or double.

An arithmetic operation that uses two int values will evaluate to an int value. 

This is because Java sets aside a certain amount of memory to store an int.

Any arithmetic operations that uses at least one double value will evaluate to a double value.

Java also uses the operator == to test if the value on the left is equal to the value on the right and != to test if the two values are not equal.

Don't get one equal sign = confused with two equal signs ==. 

One equal sign = is used to assign values to a variable, and two equal signs == is used to test for equality.

== and != will return either a true or false when used to test if variables are a certain value.

I recommend using == and != operators with int values and not doubles because double values are an approximation.

In other words, 3.3333 will not equal 3.3334 even though they are very close.

Notice anything surprising above?

That's right, 2/3 resulted in 0...

Again, if you perform operations with two integers, Java will return an int value.

The .666666666666666666 gets truncated (chopped off) unless one of the values used in the operation is a double.

With division, another thing to watch out for is dividing by zero.

An attempt to divide an integer by zero will result in an ArithmeticException error message.

Operators can be used to create compound expressions with more than one operator.

You can either use a literal value which is a fixed value like 2, or variables in these compound expressions.

When compound expressions are evaluated, operator precedence rules (PEMMDAS) are used, so that *, /, and % are done before + and -.

Notice the additional M in PEMMDAS for mod (modulo)!

As usual, anything in parentheses is done first.

It doesn't hurt to put in extra parentheses if you are unsure as to what will be done first (but Mr. C will probably point it out...).

MODULO

There is an operator used in programming that you have probably never learned about in any of your math classes.

The percent sign (%) is the mod (modulo) or remainder operator.

The mod operator returns the remainder after you divide the first number by the second.

So, 5 % 2 = 1 because 2 goes into 5 twice with a remainder of 1.

Let's see how many we can get correct!

1) What will System.out.print(11 % 10); return in the console?

2) What will System.out.print(8 % 2); return in the console?

3) What will System.out.print(9 % 2); return in the console?

4) What will System.out.print(9 % 5); return in the console?

5) What will System.out.print(3 % 4); return in the console?

Answers:

1) 1

2) 0

3) 1

4) 4

5) 3

The tricky one is #5. 4 doesn't go into 3 so modulus will return a remainder of 3. It's all remainder if the first number is smaller than the second!

A useful application of modulus is to determine if a value is even or odd. ALL even values % 2 = 0 and all odd values % 2 = 1.

PT.4: COMPOUND ASSIGNMENT OPERATORS

Compound assignment operators are shortcuts that do a math operation and assignment in one step.

For example, x += 1 adds 1 to x and assigns the sum to x.

This is exactly the same a x = x + 1.

This pattern is possible with any operator that is put in front of the = sign, as seen below.

The most common shortcut operator ++, the plus-plus or increment operator, is used to add 1 to the current value.

In other words, x++; is the same as x += 1; and the same as x = x + 1;.

The increment operator is used a lot in loops which we will see later.

Have you heard about the programming language C++? Its name is actually an inside joke... The programming language C was improved to create C++.

The decrement operator is used to subtract 1 from the current value.

In other words, y-- is the same as y -= 1, and the same as y = y - 1.

These are the only two double operators; this shortcut pattern does not exist with other operators.

CODE TRACING

Code tracing is a technique used to simulate a dry run through the code line by line by hand as if you are the computer executing the code.

Tracing can be helpful for debugging or proving that your program runs correctly or for figuring out what the code actually does.

Trace tables can be used to track the values of variables as they change throughout a program.

To trace through code, write down a variable in each column or row in a table and keep track of its value throughout the program.

Some trace tables also keep track of the output and the line number you are currently tracing.

On the exam, you might find it helpful to make a quick trace table to determine the values of the variables at the end.

Can we trace through the following code?

SUMMARY

• Arithmetic expressions include expressions of type int and double. 

• The arithmetic operators consist of +, -, *, /, and % (modulo for the remainder in division. 

• An arithmetic operation that uses two int values will evaluate to an int value. With integer division, any decimal part in the result will be truncated (thrown away). 

• An arithmetic operation that uses as least one double value will evaluate to a double value. 

• Operators can be used to construct compound expressions. 

• During evaluation, operands are associated with operators according to operator precedence to determine how they are grouped. (*, /, % have precedence over + and -, unless parentheses are used to group those.)

• An attempt to divide an integer by zero will result in an ArithmeticException to occur. 

• The assignment operator ( = ) allows a program to initialize or change the value stored in a variable. The value of the expression on the right is stored in the variable on the left. 

• During execution, expressions are evaluated to produce a single value. 

• The value of an expression has a type based on the evaluation of the expression. 

• Compound assignment operators (+=, -=, *=, /=, %=) can be used in place of the assignment operator. 

• The increment operator ( ++ ) and decrement operator ( - - ) are used to add 1 or subtract 1 from the stored value of the variable. The new value is assigned to the variable. 

EVIDENCE

1) Complete the following Google Forms. These forms must be 100% correct and includes released AP practice questions. To stop working and return later, hit submit! You can "edit your response" and continue where you left off.

PT.3 Evidence 

PT.4 Evidence 

2) CREATE a calculator program using the Scanner class AND mathematical operators shown in this lesson. This should be a calculator that takes values from the user to calculate something useful! There are a lot of science and mathematical equations that you could use; for example, geometric formulas, solving the quadratic equation, Celsius to Fahrenheit (and vice versa), density = mass / volume, E = mc^2, F = ma, PV = nRT, etc. I don't want everyone to do the same type of calculator, so once you know what you want to create make sure to get it approved! 

Pair Programming is optional but encouraged! 

Once you finish your program, come and present it to me. I will ask questions about it and give you formatting suggestions.

3) After finishing the PT.4 Evidence form, find a partner to play the Operator Maze Game with! Mr. C has hard copies of the game board for you. If we have an odd number of students in the class someone will need to play twice (please and thank you).