Variables and Operators

1. Variables and Operators

2. What is a Variable? • Variables allow a program to store data at one point and refer back to it later • A variable is container for data with an associated type and name, or identifier

3. Type • “A number of … things having in common … characteristics that distinguish them as a … class.” • Free Dictionary definition • For example: • Characters • Strings • Integers • Colors • Any Java class

4. Creating a Variable • Before a variable can be used in a program it must be declared • Variables are declared by first stating the type of the data to be stored, followed by the variable’s name int count; • This declares a variable named “count” that stores an integer (a whole number)

5. Variables and Computer Memory • All data that a program uses must be stored somewhere in the computer’s memory • Each piece of data is stored in a specific location, referred to as the “address” • A variable is a name assigned to the address that contains the data

6. Data Types • We can store different types of data • In Java, variables can hold primitive data types or references to objects • Primitive data types include types for true/false data, characters, whole numbers and real numbers • For now, we’re just going to look at primitive data types, we’ll look at objects later

7. Primitive Data Types • boolean — truth values — true or false • char — characters — ’a’, ’G’, ’#’, ’3’ • Integer values – whole numbers • byte (-128 to 127) • short (-32768 to 32767) • int (-2147483648 to 2147483647) • long(-9223372036854775808 to 9223372036854775807) • Floating values -- decimals • float (1.401298e-45 to 3.402823e+38) • double(4.94065645841246e-324 to 1.79769313486231e+308)

8. Strongly-Typed Languages • Java is a strongly typed language, we have to define what kind of data we want to hold in a variable before it can be used • The advantage of a strongly typed language is that we can get the compiler to check that we are using a variable correctly before we run the program

9. Weakly Typed Languages • JavaScript and ActionScript are weakly typed languages, they do not require variables have their data type defined in advance • The advantage of weakly typed languages is that we can program quickly without having to worry about declaring variables first

10. Naming Variables • There are some restrictions on variable names: • Variable names can only include a limited range of characters: a-z, A-Z, 0-9, _ • Variable names cannot contain spaces, the underscore character “_” is often used instead of a space • Variable names must not start with a number

11. Variable Naming Conventions • In Java programs, variable names typically begin with a lowercase letter and use a capital letter to start each new word • e.g. width, rect4, fillColour, isFilled • Variables that refer to constant data typically use all uppercase letters and underscores between each word • e.g. PI, CENTER, MAX_VALUE

12. Initializing Variables • A variable can be declared with no value and then assigned a value later // declare an int named y1 but do not initialize int y1; // assign the value 5 to the variable y1 y1 = 5; • A variable can be initialized at the time it is declared // declare an int named y1 initialized to 5 int y1 = 5;

13. Initializing Variables • A variable can be initialized with a value, the value of another variable,or by evaluating an expression // declare a char named letter initialized to ‘a’ char letter = ‘a’; // declare a double named d1 initialized to 132.32 double d1 = 132.32; // declare a double named d2 initialized to d1 double d2 = d1; // declare a float name z initialized to x * y + 15 float z = x*y + 15.0f;

14. Arithmetic Operators = assignment operator x=9 + addition 3 + 4 - subtraction 5 - 7 * multiplication 5 * 5 / division 14 / 7 % modulo 20 % 7 ++ Increment operator; increments a value by 1 -- Decrement operator; decrements a value by 1 Compound Assignment Operators += -= *= /= %=

15. Integer Operations +, -, *, /, and % 5 / 2 yields an integer 2. 5.0 / 2 yields a double value 2.5 5 % 2 yields 1 (the remainder of the division)

16. Operator Precedence • Here’s another problem. What’s the answer to this? x = 7 + 3 * 6; • Two Options (depending on the order of operations): • Perform addition first: 7 + 3 = 10  10 * 6 = 60 • Perform multiplication first: 3*6 =18  7+18 = 25 • Which option is correct?

17. Operator Precedence Operator precedence represent rules for evaluating mathematical expressions. Every programming language has similar rules.

18. Built-In Variables • Processing has some built-in variables that are quite useful. Since they are built-in, these variables should not be declared, initialized or assigned; they should just be read. • width / height :: The dimensions of the window. • mouseX / mouseY:: The current coordinates of the mouse. • frameCount :: The number of frames that have been drawn since the program started.

19. Color Models • colorMode(RGB, 255); // processing’s default color model // used almost exclusively in // computer science applications • colorMode(HSB, 360, 100, 100); // hue, saturation, value(brightness) // used predominately in art, available // in most graphics and animation packages

20. Examples // 1 colorful rectangle centered in the canvas size(300,300); // colorMode(HSB,255,255,255); background(20,20,255); fill(20,255,20); stroke(255,20,20); rectMode(CENTER); /* CENTER is a built-in variable that can’t be changed—a constant */ rect(width/2,height/2,50,50);

21. More Examples // 5 small rectangles across // the top of the canvas size(300,300); int xIncrement=width/7; rect(xIncrement, 10, 20, 20); rect(xIncrement*2, 10, 20, 20); rect(xIncrement*3, 10, 20, 20); rect(xIncrement*4, 10, 20, 20); rect(xIncrement*5, 10, 20, 20);

22. More Examples // the mystery program that changes the display as the size changes size(950,800); int boxSize = width/3 ; int xPos ; xPos=boxSize*0 ; fill(xPos*17%255, xPos*11%255, xPos*4%255); // note we are in rectMode(CORNER) by default rect(xPos, 0, boxSize, height) ; xPos=boxSize*1 ; fill(xPos*17%255, xPos*11%255, xPos*4%255); rect(xPos, 0, boxSize, height) ; xPos=boxSize*2 ; fill(xPos*17%255, xPos*11%255, xPos*4%255); rect(xPos, 0, boxSize, height) ;

23. In-class Lab • Create a pattern of six objects of the same shape and color (such as lines or rectangles). • Revise your code to set the positions of the shapes with functions of the width and height variables. • Test your code with several different window sizes to make sure the pattern remains consistent. • Keeping a monochromatic color palette using the HSB color model, modify your code so that each of your shapes is a different brightness while the hue and saturation remain constant. • Use a variable to set hue and saturation and test with a several values for each.