Lesson 8: Arrays

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Array Information:

• Allows us to store multiple values under a single name

  • Multiple values, one variable

• Each value in an array has its own index

  • Starts from 0

• Array lengths are fixed

• Primitive vs object arrays:

  • Primitive arrays store actual primitive value

  • object arrays store refrences to the actual value of the object

• Default values:

  • Numeric arrays == 0

  • Boolean arrays == false

  • Object/String arrays == null

    • null is a keyword meaning no address

• ArrayIndexOutOfBounds errors don’t show up at compile time

Declaring and Creating Arrays:

• Declaring an array:

//Array template
elementType[] identifier;
elementType identifier[];
 
//Examples
double[] weeks;
double weeks[];

• Creating an array:

//Array template
new elementType[length];
//Example
double[] weeks = new double[5] //Creates an array with length 5

Specifying Values:

//Assign values while declaring:
elementType[] identifier = {values}; //template
double[] test = {5.3, 8.1, 0.1, 9};
 
//Assign values one at a time:
identifier[index] = value; //template
double[] test = new double[3];
test[0] = 5.3;
test[1] = 8.1;
test[2] = 0.1;

for-each Statement:

• Use for loop without using indicies/indexes

  • Avoids ArrayIndexOutOfBounds errors

• To edit a value inside the array, use created variable instead of index

//Template
for (arrayType variable : array) {
    bodystatement;
}
 
//Example (sum all values in array)
double test[] = {5.3, 8.1, 0.1, 9};
double sum;
for (double grade : test) {
    sum += grade; 
}

Searching Arrays:

• Can use for loops

Sparce Arrays and Null Checking:

• A null value in an array can cause an error when an object is called on it

  • Can happen when searching through arrays in a for loop

• Can check if a value in an array is null:

if (value != null) {
    statement;
}

CLI Arguments

• Can take CLI arguments from terminal when starting the JVM

  • Java creates the array containing the arguments and makes a refrence to it in args

  • Arguments go into the main ‘args’ parameter

public static void main(String[] args) //String[] args is cli input

Wrapper Classes:

• There is a class for each primitive var that enhances features

  • Ex. Conversion

• Each wrapper class has its own sets of parameters

2D Arrays:

• Arrays of Arrays

  • A “table” of values

• 2 Index values instead of 1

  • First index is row, seccond is column

• Creating 2D Arrays:

//Structure
elementType[][] identifier;
elementType identifier[][];
 
//Examples
double[][] array2d = {{80, 70, 75}, {69, 72, 74}};
double[][] array2d = {{80, 70, 75}, 
                     {69, 72, 74}};
double[][] array2d = new double[2][3];

• Assigning Elements:

//Structure
identifier[row][col] = value;
 
//Examples
array2d[0][1] = 50;
array2d[1][0] = 25;

2D Array Processing (for loops):

• Needs two for loops to go through every value

• Row Major:

  • Row by Row

for (int row = 0; row < array2d.length; row++) {
    for (int col = 0; col < array2d[row].length; col++) {
        bodystatement;
    }
}

• Column Major:

  • Column by Column

for (int col = 0; col < array2d[0].length; col++) {
    for (int row = 0; row < array2d.length; row++) {
        bodystatement;
    }
}

Ragged Arrays:

• 2D Arrays with rows of varying lengths

• Created just like normal arrays

• Important to use .length when traversing instead of a constant valuye

More Dimentions:

• Can create arrays of multiple dimentions:

  • 3D

  • 4D

  • Etc…