Week 3: Objects, files and exceptions/9a. Data structure - ARRAY

Arrays are static data structures: their SIZE is defined AT INITIALISATION and cannot be changed afterwards. 
It is therefore not possible to add or remove primitives from an array. 
In general, a list is a more useful structure, but some Java methods return an array- so it's a good idea to know how to use them.



// datatype of elements
// empty square brackets []
// variable name
// 'new' operator - define the array object
// datatype of elements
// num_elements

public class Example {
    public static void main(String[] args){
        // Array in which 10 integers
        // can be saved
        int[] numarray = new int[10];

        // Array for strings
        String[] names = new String[20];

        // Array for floating-point numbers
        double[] results = new double[5];
    }
}


At initialisation, fill array with default values
- integer 0
- boolean false
- null



VS


// initialise Array with values
// Array with five integers
int[] numarray = {1, 3, 2, 5, -1};

// Array with strings
String[] names = {"Jack", "Pete", "Jane", "Lisa"};

// Array with floating-point numbers
double[] results = {1.0, 2.5, 0.75};





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import java.util.Arrays;

public class Example {
    public static void main(String[] args){
        int[] numbers = new int[5];
        numbers[0] = 10;             // variableName[idx]
        numbers[1] = 40;
        numbers[4] = -155;

        // When printing an array the method 
        // Arrays.toString is useful.
        System.out.println(Arrays.toString(numbers)); // [10, 40, 0, 0, -155]


        double[] heights = {175.5, 150.75, 201.05};
        System.out.println(heights[0]);
        
        heights[1] = heights[1] + 10;    // variableName[idx]
        System.out.println(heights[1]);  // variableName[idx]
    }
}


Program outputs:
[10, 40, 0, 0, -155]
175.5
160.75








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length





The length of the array (i.e. the number of sub-arrays) is determined by the length attribute. 
The extended for clause can also be used to iterate through an array in the same way as iterating through a list.

import java.util.Arrays;

public class Example {
    public static void main(String[] args){
        int[] numbers = {1,2,3,4,5,6,7};
        System.out.println("Array length: " + numbers.length);

        for (int element : numbers) {
            System.out.println(element);
        }

        for (int i=0; i<numbers.length; i++) {
            numbers[i]++;
        }

        System.out.println(Arrays.toString(numbers));
    }
}

Program outputs:
Array length: 7
1
2
3
4
5
6
7
[2, 3, 4, 5, 6, 7, 8]








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Note that unlike a string, ARRAY LENGTH is an attribute
ARRAY length is not a method - it is not followed by parentheses. 



The example below further illustrates the differences between length detection for a string, an array and a "list":

int[] array = {1,2,3,4,5,6,7};

ArrayList<Integer> list = new ArrayList<>();
list.add(1);
list.add(2);

String str = "Hey";

System.out.println("Array length: " + array.length);
System.out.println("List length: " + list.size());
System.out.println("String length: " + str.length());
 

Program outputs:
Array length: 7
List length: 2
String length: 3