I'm having a hard time thinking about how should I implement the checking for duplicates while the string array with length of 5 is initially empty. Before adding an element in the array, I have to check first if it already exists in the array but because the array is initially empty (which means the five elements are null) it prompts an error, I think that is because I'm trying to compare the element (that I'm trying to add in the array) to null.
What I want to do is check if the the length of the array is less than the limit, check if the element that I want to add has no duplicate in the array. If it doesn't have a duplicate, then I'll add it in array, if it has a duplicate then I won't add it then I'll print a prompt message.
I am working on a project with multiple classes, here's the snippet of my code:
public class Collections {
Guardian[] guardians;
int count;
final static int MAX_GUARDIANS = 5;
public Collection () {
guardians = new Guardian[Collection.MAX_GUARDIANS];
}
public void addGuardians (Guardian guardian) {
if (this.count < MAX_GUARDIANS) {
for (int i = 0; i < guardians.length; i++) {
if (guardians[i].equals(guardian)) {
System.out.println("The guardian is already in the list!\n");
} else {
this.guardians[this.count++] = guardian;
System.out.println("Guardian "+guardian.getName()+" was added to the list!");
}
}
} else {
System.out.println("Maximum number of guardians in the list has been reached!\n");
}
}
}
Is it possible to compare the element that I'm planning to add to null?
So when you want to search for a duplicate, you have to search the whole array first. Then if there's no duplicates, add an element after the for loop.
for (int i = 0; i < count; i++) {
if (guardians[i].equals(guardian)) {
System.out.println("The guardian is already in the list!\n");
return; // <-- add this to EXIT when find a match
}
}
// now that you've searched the whole list,
// you can add a new element
guardians[count++] = guardian;
System.out.println("Guardian "+guardian.getName()+" was added to the list!");
You can try using a HashSet<String> to keep track of duplicates, and keep track of unique strings in an array.
Declare a hashset:
HashSet<String> set = new HashSet<String>();
// Or:
// Set<String> set = new HashSet<String>();
Check if a string is in a set with:
if(set.contains("Hello")) {
// String is in the set
}
Add a string to a set with:
set.add("Hello");
Use a list instead of an array to be able to check if it contains that guardian already.
public class Collections {
List<Guardian> guardians;
int count;
final static int MAX_GUARDIANS = 5;
public Collections () {
guardians = new LinkedList<>();
}
public void addGuardians (Guardian guardian) {
if (guardians.size() >= MAX_GUARDIANS) {
System.out.println("Maximum number of guardians in the list has been reached!\n");
return;
}
if (guardians.contains(guardian)) {
System.out.println("The guardian is already in the list!\n");
} else {
guardians.add(guardian);
System.out.println("Guardian "+guardian.getName()+" was added to the list!");
}
}
}
Related
Over the passed couple of hours I have been working on an assigment with no luck in figuring it out. For reference I am going to post the instructions below and then explain what I have done.
Write a Java class that implements the StringSet interface (see
attached text document). One of your instance variables must be an
array of Strings that holds the data; you may determine what, if any
other instance variables you need. You will also need to implement the
required methods, one or more constructors, and any other methods you
deem necessary. I have also provided a tester class that you should be
able to run your code with.
So far I have created an implementation of the interface named MyStringSet. I have put all of the methods from the interface into my implementation and have written the code to what I think will work. My main problem is that I don't know how to put the data from the main method that is called into an array. The user types in a file and and then it is supposed to return word count and other methods. Since the file is being called from the tester class, I need to store that data into an array or an array list which I have already created. Below I have listed my current implementation and the tester class that I use. Any help is greatly appreciated!
My Implementation:
public class MyStringSet implements StringSet {
String[] myArray = new String [] {};
List<String> myList = Arrays.asList(myArray);
//default constructor
public MyStringSet(){
resize(5);
}
// precondition: larger is larger than current Set size
// postcondition: enlarges Set
public void resize(int larger) {
myArray = Arrays.copyOf(myArray, myArray.length + larger);
}
// postcondition: entry is inserted in Set if identical String
// not already present; if identical entry exists, takes no
// action. Calls resize if necessary
public void insert(String entry) {
Set<String> myArray = new HashSet<String>();
Collections.addAll(myArray, entry);
}
// postcondition: removes target value from Set if target is
// present; takes no action otherwise
public void remove(String target) {
if(target != null){
int n = 0;
int index = n;
for(int i = index; i < myArray.length - 1; i++) {
myArray[i] = myArray[i+1];
}
}
}
// precondition: Set is not empty
// postcondition: A random String is retrieved and removed from
// the Set
public String getRandomItem () {
String s = "String is Empty";
if (myArray != null) {
int rnd = new Random().nextInt(myArray.length);
return myArray[rnd];
}
else {
return s ;
}
}
// precondition: Set is not empty
// postcondition: the first item in the Set is retrieved and
// removed from the Set
public String getFirstItem () {
String firstItem = myList.get(0);
return firstItem;
}
// postcondition: returns true if target is present, false
// if not
public boolean contains(String target) {
if (target == null) {
return false;
}
else {
return true;
}
}
// postcondition: returns true if Set is empty, false if not
public boolean is_empty( ) {
if(myArray == null){
return true;
}
else {
return false;
}
}
// postcondition: returns total number of Strings currently in set
public int inventory() {
int total = myList.size();
return total;
}
// postcondition: returns total size of Set (used & unused portions)
public int getCapacity( ) {
int capacity = myArray.length;
return capacity;
}
}
Tester class:
public class SetTester
{
public static void main(String [] args) {
StringSet words = new MyStringSet();
Scanner file = null;
FileInputStream fs = null;
String input;
Scanner kb = new Scanner(System.in);
int wordCt = 0;
boolean ok = false;
while (!ok)
{
System.out.print("Enter name of input file: ");
input = kb.nextLine();
try
{
fs = new FileInputStream(input);
ok = true;
}
catch (FileNotFoundException e)
{
System.out.println(input + " is not a valid file. Try again.");
}
}
file = new Scanner(fs);
while (file.hasNext())
{
input = file.next();
words.insert(input);
System.out.println("Current capacity: " + words.getCapacity());
wordCt++;
}
System.out.println("There were " + wordCt + " words in the file");
System.out.println("There are " + words.inventory() + " elements in the set");
System.out.println("Enter a value to remove from the set: ");
input = kb.nextLine();
while (!words.contains(input))
{
System.out.println(input + " is not in the set");
System.out.println("Enter a value to remove from the set: ");
input = kb.nextLine();
}
words.remove(input);
System.out.println("There are now " + words.inventory() + " elements in the set");
System.out.println("The first 10 words in the set are: ");
for (int x=0; x<10; x++)
System.out.println(words.getFirstItem());
System.out.println("There are now " + words.inventory() + " elements in the set");
System.out.println("5 random words from the set are: ");
for (int x=0; x<5; x++)
System.out.println(words.getRandomItem());
System.out.println("There are now " + words.inventory() + " elements in the set");
}
}
main problem is that I don't know how to put the data from the main method that is called into an array
You're doing that correctly already, following this simple example
StringSet words = new MyStringSet();
words.insert("something");
However, the contents of the insert method seem incorrect 1) you never check for identical values 2) never resizing 3) Collections.addAll doesn't do what you want, most likely
So, with those in mind, and keeping an array, you should loop over it, and check where the next non-null, non-identical value would be placed
// postcondition: entry is inserted in Set if identical String
// not already present; if identical entry exists, takes no
// action. Calls resize if necessary
public void insert(String entry) {
int i = 0;
while (i < myArray.length && myArray[i] != null) {
if (myArray[i].equals(entry)) return; // end function because found matching entry
i++;
}
if (i >= myArray.length) {
// TODO: resize()
insert(entry); // retry inserting same entry into larger array
}
// updates the next non-null array position
myArray[i] = entry;
}
As far as displaying the MyStringSet class goes, to see the contents of the array, you'll want to add a toString method
I'm trying to iterate over an ArrayList of ArrayLists - but somehow everything fails on me and I don't understand the error message.
The error is:
Exception in thread "main" java.lang.ClassCastException: java.lang.Integer cannot be cast to java.util.ArrayList
I've tried using a regular for(int i; i < lists.length; i++) but get the same error. All I want to do is check if any of the ArrayLists in "lists" contains the integer "v".
public static boolean listsContains(ArrayList<ArrayList<Integer>> lists, int v) {
boolean b = false;
for (ArrayList<Integer> list : lists) {
if (list.contains(v)) {
b = true;
} else {
b = false;
}
}
return b;
}
The actual line that causes the error is the "for (ArrayList list"...
Edited: For clarity I edited in the code with more declarative generics (which works just as little as the first code I posted unfortunately).
Edit2: Ok so it's somehow not the method itself that causes the problem so upon request here's the rest of the code that populates these lists. The code is not done but I got caught with this problem while finishing it.
public static void main(String[] args) {
Graph g = DataSource.load();
ArrayList<ArrayList<Integer>> lists = new ArrayList<ArrayList<Integer>>();
for(int i = 0; i < g.numberOfVertices(); i++) {
if(!(listsContains(lists, i))) { // add list if node is unlisted (since after first iteration one entire network is found)
listsCreate(lists, i);
}
Iterator it = g.adj(i).iterator(); // create iterator for current node's edges
if (!(it.hasNext())) { // node has no edges
listsCreate(lists, i);
} else { // node has edges, iterate through them
while(it.hasNext()) {
Edge current = (Edge) it.next();
if(!(listsContains(lists, current.to))) { // unlisted node
int index = listsIndexOf(lists, current.from);
findNetwork(g, lists.get(index), current.to);
} else {
continue; // node already listed
}
}
}
}
System.out.println("Number of connected graphs: " + lists.size());
} // Main
You did not specify the inner ArrayList's components' type. And from your log I can tell that it contains Integers:
public static boolean listsContains(ArrayList<ArrayList<Integer>> lists, int v) {
for (ArrayList<Integer> list : lists) {
if (list.contains(v))
return true;
}
return false; // No inner arrayList contains 'v'
}
EDIT:
or using Java 8 :
public static boolean listsContains(ArrayList<ArrayList<Integer>> lists, int v) {
return lists.stream().anyMatch(list -> list.contains(v));
}
Because you test:
list.contains(v)
list is of type ArrayList without inside type
v is int
replace your ArrayList by ArrayList< Integer >
So I have this code:
public class SortedIntList extends IntList
{
private int[] newlist;
public SortedIntList(int size)
{
super(size);
newlist = new int[size];
}
public void add(int value)
{
for(int i = 0; i < list.length; i++)
{
int count = 0,
current = list[i];
if(current < value)
{
newlist[count] = current;
count++;
}
else
{
newlist[count] = value;
count++;
}
}
}
}
Yet, when I run the test, nothing prints out. I have the system.out.print in another class in the same source.
Where am I going wrong?
EDIT: Print code from comment:
public class ListTest
{
public static void main(String[] args)
{
SortedIntList myList = new SortedIntList(10);
myList.add(100);
myList.add(50);
myList.add(200);
myList.add(25);
System.out.println(myList);
}
}
EDIT2: Superclass from comment below
public class IntList
{
protected int[] list;
protected int numElements = 0;
public IntList(int size)
{
list = new int[size];
}
public void add(int value)
{
if (numElements == list.length)
System.out.println("Can't add, list is full");
else {
list[numElements] = value; numElements++;
}
}
public String toString()
{
String returnString = "";
for (int i=0; i<numElements; i++)
returnString += i + ": " + list[i] + "\n";
return returnString;
}
}
Let's walk through the logic of how you want it to work here:
first you make a new sorted list passing 10 to the constructor, which make an integer array of size 10.
now you call your add method passing 100 into it. the method sets position 0 to 100
now you add 50, the method sets 50 in position 0 and 100 in position 1
now you add 200, which gets placed at position 2
and you add 25. which gets set to position 0, and everything else gets shuffled on down
then your method will print out everything in this list.
So here are your problems:
For the first add, you compare current, which is initialized at 0, to 50. 0 will always be less than 50, so 50 never gets set into the array. This is true for all elements.
EDIT: Seeing the super class this is how you should look to fix your code:
public class SortedIntList extends IntList
{
private int[] newlist;
private int listSize;
public SortedIntList(int size)
{
super(size);
// I removed the newList bit becuase the superclass has a list we are using
listSize = 0; // this keeps track of the number of elements in the list
}
public void add(int value)
{
int placeholder;
if (listSize == 0)
{
list[0] = value; // sets first element eqal to the value
listSize++; // incriments the size, since we added a value
return; // breaks out of the method
}
for(int i = 0; i < listSize; i++)
{
if (list[i] > value) // checks if the current place is greater than value
{
placeholder = list[i]; // these three lines swap the value with the value in the array, and then sets up a comparison to continue
list[i] = value;
value = placeholder;
}
}
list[i] = value; // we are done checking the existing array, so the remaining value gets added to the end
listSize++; // we added an element so this needs to increase;
}
public String toString()
{
String returnString = "";
for (int i=0; i<listSize; i++)
returnString += i + ": " + list[i] + "\n";
return returnString;
}
}
Now that I see the superclass, the reason why it never prints anything is clear. numElements is always zero. You never increment it because you never call the superclass version of the add method.
This means that the loop in the toString method is not iterated at all, and toString always just returns empty string.
Note
This is superseded by my later answer. I have left it here, rather than deleting it, in case the information in it is useful to you.
Two problems.
(1) You define list in the superclass, and presumably that's what you print out; but you add elements to newList, which is a different field.
(2) You only add as many elements to your new list as there are in your old list. So you'll always have one element too few. In particular, when you first try to add an element, your list has zero elements both before and after the add.
You should probably have just a single list, not two of them. Also, you should break that for loop into two separate loops - one loop to add the elements before the value that you're inserting, and a second loop to add the elements after it.
we have sequence of 4 characters (A,B,C and D)that map to numbers form 1 to n.
we define components to be:
Component(k) :
A {cell[k]}
if Color(left_k) = Color(k)
then
A <-- A U Component(left_k)
if Color(right_k) = Color(k)
then
A <-- A U Component(left_k)
return A
there is 3 types of operations(the numbers in list indicate the input):
by giving index it should remove the component in that index(the numbers mapping to characters are fixed)
example : AABBBDA is the string. if index is 3 it should return AADA
by giving index it should rotate the string based on the component on that index(the numbers mapping to characters are fixed)
example : AABBBDA is the string. if index is 3 it should return DABBBAA
it should print the string.
inputs are like:
1 2 --> first operation with index=2
2 3 --> second operation with index=3
3 --> third operation
It's an assignment, happy to get help.
this is what i've tried so far:
public static void main(String[] args)
{
int numberOfOps;
String[] print = new String[30];
List list = new List();
Scanner input = new Scanner(System.in);
int count = input.nextInt();
String colors = new String();
colors = input.next();
for(int i = 0; i < count; i++)
{
list.add(colors.charAt(i));
}
numberOfOps = input.nextInt();
list.printElement();
for (int i = 0; i < numberOfOps; i++)
{
int op = input.nextInt();
if(op == 1)
{
int index = input.nextInt();
char c = list.item[index];
int temp = index;
int prevIndex = index;
int nexIndex = index;
if(index != 0)
{
while (list.item[--index] == c)
{
prevIndex--;
}
while (list.item[++temp] == c)
{
nexIndex++;
}
list.setNext(prevIndex-1, nexIndex+1);
}
else
{
while (list.item[++temp] == c)
{
nexIndex++;
}
list.setNext(prevIndex, nexIndex+1);
}
}
if(op == 2)
{
int index = input.nextInt();
}
if(op == 3)
{
print[i] = list.printElement();
}
}
}
here is my List class:
public class List {
// reference to linked list of items
public static final int MAX_LIST = 20;
public static final int NULL = -1;
public char item[] = new char[MAX_LIST]; // data
public int avail;
public int next[] = new int[MAX_LIST]; // pointer to next item
private int numItems; // number of items in list
public List()
{
int index;
for (index = 0; index < MAX_LIST-1; index++)
next[index] = index + 1;
next[MAX_LIST-1] = NULL;
numItems = 0;
avail = 0;
} // end default constructor
public void add(char e)
{
item[avail] = e;
avail = next[avail];
numItems++;
}
public String printElement()
{
String temp = null;
int index = 0;
while(index<avail)
{
temp += item[index];
System.out.println(item[index]);
index = next[index];
}
return temp;
}
public int size()
{
return numItems;
}
public void setNext(int i, int value)
{
next[i] = value;
}
}
if you test it you'll get, it has lots of problems, such as, I have no idea to do the rotate operation, and it has problem with connecting two components when the middle component has been removed.
This is a difficult question to answer, because the requirements are not properly stated.
For example the first bunch of pseudo-code does not make it clear whether A is a set, a multi-set or a list. The notation (use of curly brackets, and U (union?)) seems to say set ... but the output seems to be a list. Or maybe it is supposed to be a schema for a data structure??
And even the inputs are not clearly described.
But putting that on one side, there is still room for some (hopefully) helpful advice.
Make sure that >>you<< understand the requirements. (I imagine that the real requirements for the assignment are better stated than this, and the details have been "lost in translation".)
I would actually use an array list (or a StringBuilder) rather than a linked list for this. (But a properly implemented linked list ... implementing the List API ... would work.)
But whatever data structure you chose, there is no point in implementing it from scratch ... unless you are specifically required to do that. There are perfectly good list classes in the Java standard libraries. You should reuse them ... rather than attempting to reinvent the wheel (and doing a bad job).
If you are required to implement your own data structure type, then your current attempt is a mess. It looks like a hybrid between an array list and a linked list ... and doesn't succeed in being either. (For example, a decent array list implementation does not need a MAX_LIST, and doesn't have next pointers / indexes. And a linked list does not have any arrays inside it.)
I'm doing a program where user input five numbers and in the end the numbers are printed out which is working fine. What I can't get to work is a boolean function to check for duplicates. It should check for duplicates as the user write them in, so e.g. if number one is 5 and the second numbers is also 5, you should get an error until you write in a different number. Meaning if the user input a duplicate it should NOT be saved in the array. This is obviously an assignment, so I'm just asking for a hint or two.
This program is written based on pseudo-code given to me, and therefore I have to use a boolean to check for duplicates with the public boolean duplicate( int number ) class.
I've tried getting my head around it and tried something by myself, but obviously I'm doing a stupid mistake. E.g.:
if(int i != myNumbers[i])
checkDuplicates = false
else
checkDuplicates = true;
return checkDuplicates;
DuplicatesTest class:
public class DuplicatesTest {
public final static int AMOUNT = 5;
public static void main(String[] args) {
Duplicates d = new Duplicates(AMOUNT);
d.inputNumber();
d.duplicate(AMOUNT);
d.printInputNumbers();
}
}
Duplicates class:
public class Duplicates {
private int amount;
private int[] myNumbers;
private boolean checkDuplicates;
public Duplicates(int a) {
amount = a;
myNumbers = new int[amount];
}
public void inputNumber() {
for(int i = 0; i < amount; i++ ) {
int input = Integer.parseInt(JOptionPane.showInputDialog("Input 5 numbers"));
myNumbers[i] = input;
}
}
public boolean duplicate( int number ) {
<BOOLEAN TO CHECK FOR DUPLICATES, RETURN FALSE OR TRUE>
}
public void printInputNumbers() {
JTextArea output = new JTextArea();
output.setText("Your numbers are:" + "\n");
for(int i = 0; i < myNumbers.length; i++) {
if (i % 5 == 0) {
output.append("\n");
}
output.append(myNumbers[i] + "\t");
}
JOptionPane.showMessageDialog(null, output, "Numbers", JOptionPane.PLAIN_MESSAGE);
}
}
Sorry if the code tag is messy, I had some trouble with white fields in between and such. I'm new here.
Don't store the numbers in an array. Use a Set<Integer> instead. And then do a Set#contains() operation. It's O(1) operation which is actually far better than iterating over the array to search for duplicates.
Ok, if it's a compulsion to use an array, then you should modify your current approach, to return true as soon as you find a duplicate, instead of iterating over the array again. In your current approach, since you are setting the boolean variable to false in the else block, your method will return false if the last element of the array is not the same as what you are checking. So, just modify your approach to:
// loop over the array
if (number == myNumbers[i])
return true;
// outside the loop, if you reach, return false
return false;
Note that your current if statement will not compile. You are declaring an int variable there, which you can't do.
if (int i == myNumbers[i]) // this is not a valid Java code.
int nums[] = new int[5];
int count = 0;
public boolean duplicate(int number)
{
boolean isDup = false;
for (int i = 0; i <= count; i++)
{
if (number == nums[i])
{
isDup = true;
break;
}
}
if (!isDup)
{
count++;
nums[count] = number;
}
return isDup;
}