Haani Naz

Jeff Verdegan wrote:

Haani Naz wrote:So i believe the 2nd one (below) will finish. Seeing how n only increments by 1 and i will multiplication of its own value - at some point it will be greater than n. I am right correct?


n = any positive integer i = 1 while (true){ i = i*2 n = n+1 if (i > n){ break } }

Yes, it will finish, but your reasoning applies more to actual integers than to Java ints or longs. (And, like I said, unless you come back and tell me otherwise, I'm going to assume we're dealing with Java primtives here.)

One very simple way to answer this question is to keep in mind that multiplying an int by 2 is the same as doing a bitwise left-shift of 1.

System.out.println(3 * 2); System.out.println(3 << 1);

Think about what happens to the bit pattern when you do that. Better yet, follow Campbell's suggestion and work it out on pencil and paper. Do it with a single 8-bit byte to make your life simpler.

i actually stole this question off a python quiz. Was trying to understand the general implications.

Campbell Ritchie wrote:What about No 2: What if you start with n = 2000000000;? What will happen then?
Go through the execution with pencil and paper for 32 iterations and tell us the values of i and n at that stage. Then work out whether the loop will terminate.

I did one better, i ran the program. At the end:

i was 2147483648
n was 2000000031


So it would finish?

So i believe the 2nd one (below) will finish. Seeing how n only increments by 1 and i will multiplication of its own value - at some point it will be greater than n. I am right correct?


n = any positive integer i = 1 while (true){ i = i*2 n = n+1 if (i > n){ break } }

Jeff Verdegan wrote:

Haani Naz wrote:

Jeff Verdegan wrote:I will say, though, that the last one looks like it might be the Collatz conjecture.

Thanks did a look up on that and i found my answer. Will focus on the 2nd one now!

Be careful though. If you start Collatz with, say 987654321 on paper you will NOT get the same next value as you will in Java.

Also, did you figure out the case where the first one never finishes?

Nah Jeff, i can't seem to figure it out. feel kinda stupid now lol.

Jeff Verdegan wrote:I will say, though, that the last one looks like it might be the Collatz conjecture.

Thanks did a look up on that and i found my answer. Will focus on the 2nd one now!

Hi guys,

Reading up on some the loops to get a firm understanding. Can somebody please clarify the following for me:


In the following examples which do you think best decribes the result:
result: will finish
result: sometimes runs forever
result: unknown



(1).
n = any positive integer i = 0 while (i <= n){ i = i+1 }


The above looks like it will finish.


(2).

n = any positive integer i = 1 while (true){ i = i*2 n = n+1 if (i > n){ break } }




(3).

n = any positive integer while (n != 1){ if (n % 2 == 0){ n = n/2 } else{ n = 3*n + 1 } }



I'm struggling with the last two. Can somebody shed some light on this? thanks.

ok just read Anniruddh Rana's post again.

think its slowly gelling together now, i realize that i need to override the equals method.

will give that a go!

Hi, so i took the advise and am trying to break the problem into smaller chunks.

All i'm trying to do now is check if a person exists or not.

1. if person exists, do nothing
2. if person doesn't exist, add person


here's what i've got so far:

if (option.equals("5")) { MyLibraryHelper helper = new MyLibraryHelper(); System.out.println("Who do you want to lend the book to? "); String name = keyboard.nextLine(); Person p1 = new Person(); p1.setName(name); // if the arrayList is empty, add person if (ml.getPeople().size() == 0) { System.out.println(p1.getName() + " doesn't exist in database. Adding now.."); ml.addPerson(p1); if (ml.getPeople().contains(p1)) { System.out .println("Person has been successfully added"); } } else { // check if person exists helper.checkIfPersonExists(p1, ml); } // print list of people names for (Person aPerson : ml.getPeople()) { System.out.println(aPerson.getName()); }// run through people list }



helper.checkIfPersonExists(p1, ml) makes a call to:


public void checkIfPersonExists(Person p1, MyLibrary ml) { if (ml.getPeople().contains(p1)) { System.out.println(p1.getName() + " already exists in database"); } //if person doesn't exist - add else if (! ml.getPeople().contains(p1)) { System.out.println(p1.getName() + " doesn't exist in database. Adding now.."); ml.addPerson(p1); if (ml.getPeople().contains(p1)) { System.out.println("Person has been successfully added"); } } }

i've also tried changing the if staments above to:

ml.getPeople().contains(p1.getName()) //to check by name

i can't seem to figure out how to NOT add the person if they exist. it doesn't seem to work :s

ml is the MyLibrary object and the getPeople() returns the arrayList of people.

Thanks guys! will test it out and report back when i get a chance.

Hi guys,

i'm working on a library lending system..

the logic i'm using is:

1. get name of person you would like to lend the book to
2. if arraylist is empty, add person/set name
3. loop through peroson arraylist
3.1 compare person name
3.1.1 if person name matches then lend book
3.1.2 if person name doesn't match add person to arraylist
[go back to step 3]




Here's my incomplete code:

if (option.equals("4")){ String name = ""; boolean contd = true; int personExists = 0; while (contd) { if (name.equals("")) { System.out .print("Who would you like to lend the book to? "); name = keyboard.nextLine(); p1.setName(name); }// if name is empty if (ml.getPeople().size() == 0) { System.out .println("Database is empty. Will add person now"); //Person p1 = new Person(); //p1.setName(name); ml.addPerson(p1); System.out.println("Done."); }// if database is empty, add person to person list // iterate through objects in array for (int i = 0; i < ml.getPeople().size(); i++) { Person aPerson = ml.getPeople().get(i); System.out.println("Person in database: " + aPerson.getName()); if (aPerson.getName().equals(name)) { // working to come later System.out .println("Book checked out to person successfully"); // set name to an empty string so book can be lent // to another person name = ""; }// if person name matches name entered else if (!aPerson.getName().equals(name)) { System.out .println("Person doesn't exist in the database. will add now.."); Person p2 = new Person(); p2.setName(name); ml.addPerson(p2); System.out.println("Done."); }// if person name doesn't match string entered }// for loop to iterate through list of people // print people in listed in array for (Person aPerson : ml.getPeople()) { System.out.println("Person in list:" + aPerson.getName()); } } //while contd is true

The problem with my code despite being able to add the first person and second person successfully is when i add the third person what happens is, it goes through each item in the arrayList and subsequently adds the new person 2 more times.

so for instance:
check if person name in the arrayList is the same
if not add person to the arrayList

[when the arrayList has 2 people, it will add the third person 3 times]





here's the output so that what i'm talking about makes sense:

Who would you like to lend the book to? Harry Database is empty. Will add person now Done. Person in database: Harry Book checked out to person successfully Person in list:Harry Who would you like to lend the book to? Sue Person in database: Harry Person doesn't exist in the database. will add now.. Done. Person in database: Sue Book checked out to person successfully Person in list:Harry Person in list:Sue Who would you like to lend the book to? Dan Person in database: Harry Person doesn't exist in the database. will add now.. Done. Person in database: Sue Person doesn't exist in the database. will add now.. Done. Person in database: Dan Book checked out to person successfully Person in database: Dan Person doesn't exist in the database. will add now.. Done. Person in database: Book checked out to person successfully Person in list:Harry Person in list:Sue Person in list:Dan Person in list:Dan Person in list: Who would you like to lend the book to?



i've made the assumption that names are unique.

can somebody pleas advise/help me in getting this to work? i'm new to java and i can't seem to think of a solution to implement. how can i do search of the entire arrayList by the person name and add the new person if it doesn't exist..?

this is me trying to learn java as a hobby and its doing my head in lol.


THANKS!

Hi there,

I have the following piece of code which fails at the point it needs to accept input and i can't seem to figure out why.


public static void main(String[] args) { Scanner keyboard = new Scanner(System.in); MyLibrary ml = new MyLibrary("Haani's Library"); //this won't print anything cos you haven't added any books ml.toString(); System.out.println("You have connected to " + ml.getName()+"..\n"); while(true){ //Main Menu System.out.println(); System.out.print("What would you like to do? \n" + "\n"+ "1. Add Books \n" + "2. Lend a book to a person \n" + "3. Check which Books are available \n" + "4. Exit \n" + "Enter choice [1 | 2 | 3 | 4]: "); String option = keyboard.nextLine(); if (option.equals("1")){ boolean cont=true; while(cont){ System.out.print("Enter name of Book: "); String bookName= keyboard.nextLine(); System.out.print("Enter name of the Author for " + bookName + ":"); String bookAuthor=keyboard.nextLine(); Book b1 = new Book(bookName); b1.setAuthor(bookAuthor); ml.addBook(b1); System.out.println("Would you like to add another book? [y/n]"); String choice = keyboard.nextLine(); if (choice.equals("y")){ cont=true; } else if (choice.equals("n")) { cont=false; } }//end-while }//end-if if(option.equals("2")){ boolean contd = true; //Print available books ArrayList<Book> availableBooks = ml.getAvailableBooks(); String input; //while (contd){ Person p1 = new Person(); System.out.print("Name of person lending book to: "); input = keyboard.nextLine(); p1.setName(input); System.out.print("What is the maximum number of books to lend: "); int maxBooks = keyboard.nextInt(); p1.setMaximumBoks(maxBooks); //add person to library ml.addPerson(p1); /* System.out.println("The available books are.."); for (Book book : availableBooks) { System.out.println(book); }*/ System.out.println("Which book would you like to check out to " + p1.getName()); input = keyboard.nextLine(); System.out.println(input); /******* //check if book name entered is in the database //if it is check it out - either successful or failed for (Book book : availableBooks) { if (book.title.equalsIgnoreCase(checkoutBook)){ boolean result =ml.checkOut(book, p1); if(result==true){ System.out.println("Check out: successful."); } else{ System.out.println("Check out failed: Book has already been checked out or " + p1.getName() + " has reached Maximum book limit"); } }//end-if: (book.title.equals("bookName")) else if (!(book.title.equalsIgnoreCase(checkoutBook))){ System.out.println("Book doesn't exist"); } }//end-for System.out.println("Would you like to check out another book? [y/n]"); String choice = keyboard.nextLine(); if (choice.equals("y")){ contd=true; } else if (choice.equals("n")) { contd=false; } }//end-while ******* */ }//end-if



The issue is at line 115.


Here is the output:
What would you like to do? 1. Add Books 2. Lend a book to a person 3. Check which Books are available 4. Exit Enter choice [1 | 2 | 3 | 4]: 1 Enter name of Book: War and Peace Enter name of the Author for War and Peace:Daffy Duck Would you like to add another book? [y/n] n What would you like to do? 1. Add Books 2. Lend a book to a person 3. Check which Books are available 4. Exit Enter choice [1 | 2 | 3 | 4]: 2 Name of person lending book to: Haani What is the maximum number of books to lend: 4 Which book would you like to check out to Haani What would you like to do? 1. Add Books 2. Lend a book to a person 3. Check which Books are available 4. Exit Enter choice [1 | 2 | 3 | 4]:


As you can see instead accepting input it skips it.

can somebody please tell me why?

thanks in advance!

Hi Guys,

i'm currently learning java off the book head first java. its a great book but i'd like to be able to apply my knowledge by doing some pracs.

anybody know of any websites where they offer tutorials, labs etc. so i can practice my skills?

thanks.

Hi,

been reading up head first java. there's a bit about creating a battleship game and i just jumped ahead and tried doing it myself.

so far i managed to create a grid which populates ships denoted by 'x' in 3 cells. i'm having trouble getting the checks right if it tries to populate it in the same cells.


here is my code. please assist in giving me advice is getting it to work, i'd really appreciate it.


import java.util.*; import java.math.*; public class Array2DTest { public static void main(String[] args) { //create the grid final int rowWidth = 5; final int colHeight = 5; Random rand = new Random(); String [][] board = new String [rowWidth][colHeight]; //fill the grid for (int row = 0; row < board.length; row++) { for (int col = 0; col < board[row].length; col++) { board[row][col] = "-"; //rand.nextInt(10); } } // System.out.println("Board length: " + board[1].length); //to randomly decide if the ships are assigned to the grid via row or column int toss; int row_pos; int column_pos; int count = 0; // run through the grid 3 times and create 3 ships while (count < 3){ //i'm forcing it only use rows at this point. toss = 1; //rand.nextInt(2) if (toss == 1){ row_pos=rand.nextInt(5); //random row is assigned System.out.println("row position: " + row_pos); count++; int temp; int rowSpace; int i=0; int col=0; //creating a ship. 3 'x' markers on the board. loop 3 times. while( i < 3){ //if row already has a 'x'. move to next column while ( board[row_pos][col] == "x"){ col++; System.out.println("col increment: " + col); } //System.out.println("board length marker 1: " + board[row_pos].length); if ((rowSpace=( 1 + board.length) - col) >= 3 ) { System.out.println("board length marker 2: " + rowSpace ); board[row_pos][col]= "x"; col++; i++; } else { //if no more cells available in row, generate a new row position while ( (temp = rand.nextInt(5)) != row_pos) { row_pos = temp; } } }//end-i < 3 } /* else if (toss == 0) { column_pos=rand.nextInt(5); System.out.println("column position: " + column_pos); count++; int i=0; int row=0; while( i < 3){ board[row][column_pos]= "x"; row++; i++; } }*/ }//end-count < 3 System.out.println("Printing starts here.."); //display output for(int i = 0; i < board.length; i++) { for(int j = 0; j < board[i].length; j++) { System.out.print(board[i][j] + " "); //System.out.println(); } System.out.println(); } } //end of main }




here's a working output for me:


row position: 2
board length marker 2: 6
board length marker 2: 5
board length marker 2: 4
row position: 0
board length marker 2: 6
board length marker 2: 5
board length marker 2: 4
row position: 4
board length marker 2: 6
board length marker 2: 5
board length marker 2: 4
Printing starts here..
x x x - -
- - - - -
x x x - -
- - - - -
x x x - -


non-working output:

row position: 2
board length marker 2: 6
board length marker 2: 5
board length marker 2: 4
row position: 1
board length marker 2: 6
board length marker 2: 5
board length marker 2: 4
row position: 1
col increment: 1
col increment: 2
col increment: 3
board length marker 2: 3



As you can see as soon as its the same row it fails to work. it also fails to print the grid : /

Thanks in advance.

Ernest Friedman-Hill wrote:Look at these three lines very carefully:

button1 = new JButton("shock me"); button1 = new JButton("bliss"); button1 = new JButton("huh?");

Notice anything wrong?

gosh now i just feel stupid lol

thanks man.

Jesper de Jong wrote:

Haani Naz wrote:This fails to work - i get run time errors.

Please tell us exactly what those errors are, that would make it a lot easier to help you find the cause.

Sure. this is what i get:

Exception in thread "main" java.lang.NullPointerException at java.awt.Container.addImpl(Container.java:1045) at java.awt.Container.add(Container.java:365) at org.javaguitutorial.gui.FirstGUI.<init>(FirstGUI.java:47) at org.javaguitutorial.gui.FirstGUI_driver.main(FirstGUI_driver.java:11)