Interaction Design WikiProgramming

Variables (en)

deutsche Version

A variable is a placeholder for a value. Variables need to be defined with a data-type, to define its storage space requirements. The name of the variable must start with a letter, but it may contain numbers within the name. In processing, variable names are case-sensitive.

Variables also have a lifetime. The computer manages the storage of variables, and when the life span is over, the memory is released again (Garbage-Collector). The range (Scope) in which a variable is accessible or active is defined by brackets ({}) (see also classes and functions). Variables that are at the top level are called global variables as they are accessible all over the program. Variables declared within brackets are local variables. If a local variable has the same name as a global variable, the local variable overrides it within its scope.


// variable Declaration 
int x1 = 15; // an integer i.e a whole number 
float valF = 0.323; // a floating point number i.e a numbers with a decimal point
boolean bFlag = false; // a true or false value i.e a single bit value
String message = "hallo 1"; // a collection of characters 
char character = 'g'; // a sigle character. Note the different quotation marks
 
String msg1 = new String("hallo 2");
 
println(x1);
println(valF);
println(bFlag);
println(message);
println(character);
println(msg1);


We can also make copies or combine variables. 

int x1 = 10;
int x2 = 5;
println(x1);
println(x2);
 
int x3 = x1 + x2; // 15
x2 = x3 - x1; // 5
println(x3);
println(x2);


Example

Almost the same as the first exercise, but now with variables.

int length1 = 150;
int length2 = 200;
 
size(300,300); // define window size 
background(0); // define background colour
stroke(255,255,255); // define line colour
 
line(100,10,100,length1); // draw a line 
line(150,10,150,length2);
line(200,10,200,250);
 
fill(0,0,0); // fill colour
strokeWeight(5); // line thickness 
ellipse(100,length1,50,50); // draw an ellipse
ellipse(150,length2,50,50); // draw an ellipse
noFill(); // turn off fill
ellipse(200,250,50,50); // draw an ellipse


Primitive Data Types and Memory

Each kind of variable (it's data type) has an allocated amount of memory, which is enough to store a particular range of values. 

TypeBytesValue range
byte1 byte-128 to 127
short2 byte-32,768 to 32,767
int4 byte-2^31 to 2^31-1
long8 byte-2^63 to 2^63-1
float4 byte-3.4e38 to 3.4e38
double8 byte-1.7e308 to 1.7e308
boolean1 bit true or false
char2 byte

'\u0000' to '\uffff'


Operators 

An operator is a symbol that tells the computer to perform a particular math or logic operation. Some of the most common operators are:

+ Addition

/ Division

- Subtraction

* Multiplication 

These you already know but there are some very useful arithmetic operators you may not have seen in the following example:

int x1 = 10;

x1++; // increment value by one 
x1--; // decrement value by one
 
x1 += 2; // Add assign, this is the same as writing x1 = x1+2; 
x1 -= 2; // Subtract assign, this is the same as writing x1 = x1-2; 
x1 /= 2; // Divide assign (less common), this is the same as writing x1 = x1/2; 

Exercise 2:

Take your results from exercise 1 and modify your code to use variables. Create variable X and variable Y that can be used to position your graphic on the screen.