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Arrays and Arrays and

Arrays and - PowerPoint Presentation

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Uploaded On 2018-01-21

Arrays and - PPT Presentation

ArrayLists Array Before we have only looked at single variables An array can hold a number of variables int anIntArray declares an array of integers anIntArray new int 10 allocates memory for 10 ID: 625567

elements arraylist element index arraylist elements index element array stringlist list method int position size data collection add arrays set anintarray length

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Presentation Transcript

Slide1

Arrays and ArrayListsSlide2

Array

Before we have only looked at single variables.

An array can hold a number of variables.

int

[]

anIntArray

; // declares an array of integers

anIntArray

=

new

int

[10]; // allocates memory for 10

integers

Can hold primitive data types and objects. Slide3

Length of an array

To the

the

“size” of an array use .length

Is this a method?

Useful when iterating through an array

for(int i = 0; i < anIntArray.length; i++) {

System.

out.println

("Element at index "+

i

+": "

+

anIntArray

[

i

]);

}

Common error “Out of bounds exception”. Slide4

Arrays – benefits

Arrays are good when you know how many elements you will have to deal with.

However, if the number of elements is not known, or may change we have to manage an array.

Deleting elements in an array and shuffling elements can be tricky and error prone.

Sorted/Unsorted Arrays. Slide5

ArrayList

An

A

rrayList

can be thought of as a variable length array.

Elements/items can be added and deleted, and inserted at a given position.

import

java.util.ArrayList

;Slide6

Declare an arrayList

ArrayList

<String>

stringList

=

new

ArrayList

<String

>();

//note that we must state the data type of the variables we are going to store.

In this case the data type is StringSlide7

Adding an element

Here we add two strings

stringList.add

("John");

stringList.add

("David");

Note that we cannot add something of a different data type – we will get a compile time errorSlide8

add(int index, E element) 

Inserts

the specified element at the specified position in this list

.

It also shifts all elements up by one position

And of course the size of the

ArrayList

has increased by one. Slide9

Contains

We can ask if the

arrayList

stringList

contains a given String

The method will return true or false

E.g.

stringList.contains

("John")Slide10

Is it Empty

We could check if an

arrayList

is empty like this

if

(

stringList.size

() == 0){

   

System

.out.println

("

ArrayList

 is empty");

}

isEmpty

() method returns true

if this

 

ArrayList

 contains no elements.Slide11

Removing an Item at an Index

stringList.remove

(0); 

This will remove the item at position/index 0

It will also shuffle all the elements down by one position.

So the

arrayList

is now shorter by one elements

Check this with the size() method. Slide12

Replacing an element 

use

set (

int

index, E element) method of java 

ArrayList

to replace any element from a particular index.

Below

code will replace first element of

stringList

with “AAA".

stringList.set

(0,“AAA");

Set index 0 to “AAA”Slide13

Clearing an ArrayList

To “reset” an

ArrayList

just call the method

c

lear()

This will remove all the data, and set the size to zero

Confirm by calling method size(). Slide14

indexOf and lastIndexOf

indexOf

Returns

the index of the first occurrence of the specified element in this list,

or

-1 if this list does not contain the element

.

lastIndexOf

Returns the index of the last occurrence of the specified element in this list,

or -1 if this list does not contain the element.Slide15

addAll

So far we have only added/deleted single elements

addAll

(Collection<? extends E> c)

Appends all of the elements in the specified collection to the end of this list, in the order that they are returned by the specified collection's Iterator.

addAll

(

int

index, Collection<? extends E> c)

Inserts all of the elements in the specified collection into this list, starting at the specified position.Slide16

removeRange

removeRange

(

int

fromIndex

,

int

toIndex

)

Removes from this list all of the elements whose index is between

fromIndex

,

inclusive

, and

toIndex

,

exclusive

.

BE CAREFUL WITH “off by one errors”!!!

We (computer scientists) start counting at zero (most computer languages), which means the last elements of

ArrayList

size n is n-1, and the first (1st) has index zero.