I'm a newbie of Java, and while studying it I found out an exercise who killed my brain for almost 7 hours. I tried everything I knew, I read the chapter of the book once again, I've googled for an answer for hours, and even used the Search button here, but nothing helped me. I can't understand what the heck is wrong with line 26:
import java.util.Scanner;
public class Esercizio {
public static void main(String[] args) {
Scanner tastiera = new Scanner(System.in);
System.out.println("");
System.out.println("Scrivi un intero per la lunghezza dell'Array.");
System.out.println("");
int lunghezzaArray = 0;
lunghezzaArray = tastiera.nextInt();
double[] valore = new double[lunghezzaArray];
System.out.println("");
System.out.println("Ora scrivi i numeri che vuoi dentro, possibilmente in ordine crescente.");
System.out.println("");
for (int i = 0; i < valore.length; i++) {
valore[i] = tastiera.nextDouble();}
boolean risultato = true;
risultato = strettamenteMaggiore(valore[]); // Here is the line!
System.out.println("Uscira' True o False? Risultato = " + risultato);
}
public static boolean strettamenteMaggiore(double[] valore) {
boolean risultato = false;
for (int i = 0; i < valore.length; i++) {
if (valore[i] < valore[i + 1]) {
risultato = true;}
else
risultato = false;
}
return risultato;
}
}
The program is in Italian, but I guess it doesn't matter, variables and stuff names are just, names.
Any idea of what's wrong with the code? This is the first time I encounter this error with Arrays and Methods.
Edit:
The exercise asks to write a static method called 'strettamenteCrescente(double[] in)' which returns "true" if every value of the array supplied to the input is greater than the value that precedes it. Else, "false".
Balduz has it right. Just remove the brackets []. In Java, brackets are used to declare the array, as in double[] valore, or to refer to an element of the array, as in valore[i]. To refer to the array object itself, you just use the array name.
You are passing a POSITION of the array to the method, but the method is expecting the whole array.
The solution is:
risultato = strettamenteMaggiore(valore); // Here is the linee
instead of
risultato = strettamenteMaggiore(valore[]); // Here is the line!
Related
Hoping for some help - I've been asked to write a hotel room system using methods for uni. All has been going well until I try to order the array alphabetically.
I have managed to get it to order within the method but it updated the main array (hotel). I want it to keep it within the order method, if that makes sense?
I've included a cut down version below without all the functions.
Currently it will reorder the array hotel so if you view the rooms the array will print like 'e,e,e,etc, George, Peter, Robert' instead of keeping its original form 'e, Robert, Peter, e,e,etc, George'
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
String roomName;
int roomNum = 0;
String[] hotelRef = new String[12];
String[] hotel = new String[12];
initialise(hotel); //initialise
while (roomNum < 13) {
System.out.println("Please select from the menu:");
System.out.println("V : View rooms");
System.out.println("O : Order Guests alphabetically");
String selection = input.next();
switch (selection) {
//There are more switch cases on the original version
case "O":
order(hotel);
break;
default:
System.out.println("");
}
}
}
private static void order(String[] hotelRef) {
int j;
boolean flag = true; //will determine when the sort is finished
String temp;
String[] order = new String[12];
order = hotelRef;
while (flag) {
flag = false;
for (j = 0; j < order.length - 1; j++) {
if (order[j].compareToIgnoreCase(order[j + 1]) > 0) {
//ascending sort
temp = order[j];
order[j] = order[j + 1]; // swapping
order[j + 1] = temp;
flag = true;
}
}
}
for (int y = 0; y < order.length; y++) {
if (!order[y].equals("e")) {
System.out.println("Room " + y + " is occupied by " + order[y]);
}
}
System.out.println("Ordering completed");
}
You should clone the hotelRef instead of assigning the reference like this order = hotelRef;
You could do the following while creating the order array :
String[] order = new String[hotelRef.length]; // to make sure that order has the right size.
and instead of order = hotelRef;
for (int i=0;i<order.length;i++)
order[i]=hotelRef[i]; // thereby cloning
or use System.arraycopy() or any other method to accomplish cloning the array.
You can make copy of hotel array in your order method:
String[] hotelCopy = new String[hotelRef.length];
System.arraycopy(hotelRef, 0, hotelCopy, 0, hotelRef.length);
And then just use hotelCopy inside your order method.
The problem lies with the following line
order = hotelRef;
Change it to
order = hotelRef.clone();
Though you are creating a new object, you have assigned the reference to outer object only. So whatever changes you make in the inner object it will be reflected to the outer object.
I'm trying to store 5 different boolean answers (true or false) in 5 different array positions after each loop and then make a method to display the questions which were 'true'.
For example, a test run would go like this:
Question1: Content1 ~ (True or False?) False
Question2: Content2 ~ (True or False?) True
Question3: Content3 ~ (True or False?) False
(loop finished)
Question2: Content2
(exit)
And here is my code so far.
import javax.swing.*;
class booleanTest {
public static void main(String [] params) {
String[] data = {"Test1", "Test2", "Test3", "Test4", "Test5"};
boolean[] user = new boolean[5];
array_input(data, user);
System.out.println(user); // to see if it works atm
System.exit(0);
}
public static String array_input(String[] a, boolean[] b) {
String x = "";
for (int i=0; i<a.length; i++) {
x = JOptionPane.showInputDialog("Data: " + a[i]);
if(x.equals("yes")) {
b[i] = true;
}
else {
b[i] = false;
}
}
return x;
}
//public static String array_print() {
// print the boolean + question here
//}
}
It doesn't work, I understand that the b[i] = true part must be wrong, I should do something else?
If the value at an index of the boolean array is true, print out the value in the String array at that index.
public static void printTrue(boolean[] answers, String[] questions) {
// checking both indices to avoid ArrayIndexOutOfBoundsException
for (int i = 0; i < answers.length && i < questions.length; i++) {
// if the answer is true
if (answers[i]) {
System.out.println(questions[i] + ": " + true);
}
}
}
When you say System.out.println(user); prints something like [Z#3b9187c7, this is because the toString() implementation for Object returns class name + # + hex hashCode().
The Arrays#toString method creates a more readable result:
[false, false, false, true, true]
You only have to return values in a method when you have no other way of accessing the data. If you look at your code you see that you're not even using the returned value, and the last values for x will never be useful anyway. In that kind of case, you can make it a void method. Void methods are used when you want it to perform some kind of operation, but don't need it to return any values. Your code works because an array is an Object and the changes done to it can be seen even outside the method.
Here's more or less how I would implement it. Notice the variable names are a little more descriptive.
import javax.swing.*;
public class Main {
public static void main(String[] args) {
String[] questions = {"Test1", "Test2", "Test3", "Test4", "Test5"};
boolean[] responses = getUserResponses(questions);
}
public static boolean[] getUserResponses(String[] questions) {
boolean[] responses = new boolean[questions.length]; //use the length of the other array. Don't count on it always being 5
for (int i = 0; i< questions.length; i++) {
String x = JOptionPane.showInputDialog("Data: " + questions[i]);
if(x.equals("yes")) {
responses[i] = true;
}
else {
responses[i] = false;
}
}
return responses;
}
}
In general, it's better not to modify parameter Objects and to instead return new ones. Sometimes it is much more useful or necessary to do it that way, but in your case it was not.
It looks like you're on the correct path.
Array creation and element assignment:
Your code for this looks fine. You create the array using the new operator, as you would any object, but must specify an array size, as you did. Element assignment is done using an index, either an integer literal or an integer variable (in the case of your code above). Array indices range from 0 to size-1. It looks like you also got that part right.
Printing the results:
In your scenario, you only want to some of the results, where the array value is true. A simple loop with an if(user[i]) would do the trick.
for (int i = 0; i < user.length; i++) {
if (user[i]) {
System.out.println(data[i] + " = " + true);
}
}
This is a modified example from the book, Head First Java. It's a kind of Battleship game where a 3 element array is being used as the battleship. The user has to guess these 3 locations. Currently, I've hard-coded the values of the ship location to 2,3,4. When the user guesses the correct location "Hit" is printed. If not then "Miss" is printed. If a user guesses all 3 locations then "Kill" is printed. But I have a problem. Currently if the user enters the same location multiple times, it still gives a hit. I tried to fix this by changing the value of a variable that has already been hit (int cell) to "-1". But for some reason this didn't fix it too. Please tell me what I am doing wrong.
public class Game {
public static void main(String[] args) {
// TODO Auto-generated method stub
int [] location = {2,3,4};
SimpleDotCom firstGame = new SimpleDotCom();
firstGame.setLocation(location);
firstGame.checkYourself("2");
firstGame.checkYourself("2");
//firstGame.checkYourself("2");
}
}
public class SimpleDotCom {
int [] loc = null;
int numOfHits = 0;
void setLocation (int [] cellLocation){
loc = cellLocation;
}
void checkYourself(String userGuess){
int guess = Integer.parseInt(userGuess);
String result = "Miss";
for(int cell:loc){
if (guess == cell){
result = "Hit";
numOfHits++;
cell = -1;
break;
}
if (numOfHits==loc.length){
result = "Kill";
}
}
System.out.print("Result: " + result);
System.out.println(" ** Num of Hits: " + numOfHits);
}
}
When you loop over loc, you get an int cell for each location. The problem is that that variable doesn't have any connection to the array, it's only a copy. If you change it, nothing's going to happen to the original array. I suggest looping over loc with a traditional for(;;) and using the current array index within the loop's logic to set the right "cells" to -1.
because you are assigning -1 to local variable. not updating in array actually
for(int cell:loc){ // cell is local copy of element in array is you have array of primitive int
if (guess == cell){
result = "Hit";
numOfHits++;
cell = -1;
break;
}
if (numOfHits==loc.length){
result = "Kill";
}
}
You can use traditional for loop for this or use List which has methods for adding removing elements.
You need to update the array at the correct index, not simply change the value of the cell variable, which only references the array element at the current iteration state.
You should probably use a traditionnal for loop for that, since you cannot get the index for free from an enhanced for loop.
for (int i = 0; i < loc.length; i++) {
//code...
loc[i] = -1; //instead of cell = -1;
}
Here is the program that I am trying to run:
/**
* Write a description of class mainGame here.
*
* #author Anthony Parsch
* #version 0.1.1
*/
//Import what I need to.
import java.io.*;
public class mainGame
{
/**
* Constructor for objects of class mainGame
*/
public static void main(String[] args)
{
// initialise instance variables
int xCoord = 10; //The map's max x coordinate +1
int yCoord = 10; //The map's max y coordinate +1
int playerX = 0; //The player's current x coordinate
int playerY = 0; //The player's current y coordinate
//Declare the arrays
String[][] map; //[x][y]The map
String[][] direc; //[x][y]Which directions that you can go
String[][] items; //[x][y]Where items are at
String[] inv; // Holds your inventory.
int[][] helpInt; //[x][y] All the other stuff in the
//Initalize the arrays
//---The player arrays
inv = new String[10]; //The player's inventory
inv[0] = "0";
inv = addItem(inv, "Blarg");//GET RID OF THIS LATER
//---The map arrays
map = new String[xCoord][yCoord]; //Descriptions
direc = new String[xCoord][yCoord]; //north y++,west x--,south y--,east x++
items = new String[xCoord][yCoord]; //The items within the world
//Declare the values of map
map[0][0] = "You wake up with the feel of cold metal on your back. The only other thing in this room is the door.";
map[0][1] = "You are now in a hallway with a door behind you and one either side. Forward is a continuation of the hallway.com";
//Declare the values of direc
direc[0][0] = "north";
direc[0][1] = "north, south, east, west";
print(inv[0]); //Check that the functions work
print(findDirec(direc, 0, 0));
}
/**
* Finds and displays the avaliable exits for a coordinate.
*
* #param map[][] The map array from which this method pulls the directions from.
* #param x The x value of the map
* #param y The y value of the map
* #return The string value of which way you can go
*/
static String findDirec(String[][] map, int x, int y){
//Pulls the directions
String match = map[x][y];
//Checks Directions
boolean nor = match.matches("(.*)north(.*)");
boolean sou = match.matches("(.*)south(.*)");
boolean wes = match.matches("(.*)west(.*)");
boolean eas = match.matches("(.*)east(.*)");
//Displays directions
String placeHolder = "You can go ";
if (nor == true){
placeHolder = placeHolder + "north, ";
} else if(sou == true) {
placeHolder = placeHolder + "south, ";
} else if(wes == true) {
placeHolder = placeHolder + "west, ";
} else if(eas == true) {
placeHolder = placeHolder + "east";
}
//---Checks if east is in the string, if not it removes the space and comma
if (eas == false){
StringBuffer soo = new StringBuffer(placeHolder);
soo.delete((placeHolder.length()-3), (placeHolder.length()-1));
placeHolder = soo.toString();
}
//Adds the ending period
placeHolder = placeHolder + ".";
//Returns the string
return placeHolder;
}
//Adds an item to an inventory
static String[] addItem(String inv[], String item){
int i; //Counter for the for loop, and where to add the item at.
boolean stop = false;
for(i=0; stop = true; i++)
{
if(inv[i].equals("0"))
{
stop = true;
}
}
inv[i] = item;
return inv;
}
static void print(String entry){
System.out.print(entry);
}
}
And when I try and run it through the Command Prompt, I get this error:
Exception in thread "main" java.lang.NullPointerExcpetion
at mainGame.addItem(mainGame.java:113)
at mainGame.main(mainGame.java:38)
When I paste this in to a text editor, line 113 is simply a closing brace }.
However, one line before that is a logic flaw which I presume is really line 113 for you.
for(i=0; stop = true; i++)
{
if(inv[i].equals("0"))
{
stop = true;
}
}
Each iteration of the loop assigns true to stop and then tests if true equals true, which it does. Your condition to exit the loop is when true equals false, which is never the case, therefore your loop goes forever until an error occurs. Also, don't you want to iterate while stop is false? I think you have it backwards.
The next problem is your if statement, which is probably where your NullPointerException is coming from:
if(inv[i].equals("0"))
{
stop = true;
}
You assume that inv[i] refers to an object. You need a null check.
Three recommendations:
Never use = for a comparison. Use ==. Since this is a boolean, you can even simplify this to stop.
Check the length in your for loop.
Compare "0" to inv[i] instead of the other way around to avoid null pointer dereferencing.
Try this:
boolean stop = false;
for (int i = 0; i < inv.length && !stop; i++)
{
if("0".equals(inv[i])
{
stop = true;
}
}
Another option, and this is a matter of form, is to remove the looping variable and just break out of the loop explicitly.
for (int i = 0; i < inv.length; i++)
{
if("0".equals(inv[i])
{
break;
}
}
inv = new String[10]; //The player's inventory
inv[0] = "0";
inv = addItem(inv, "Blarg");//GET RID OF THIS LATER
So you only initialize one index of your array but here:
for(i=0; stop = true; i++)
{
if(inv[i].equals("0"))
{
stop = true;
}
}
.. you loop through all of them. just kidding, didn't read the full problem. You should still read the rest of my answer, but the reason why you get the NPE is because your loop condition is broken. (by the way your for loop condition is broken, it should test for equivalence using the == operator, not the assignment = operator.)
So what you actually are doing with that code is this :
inv = new String[10];
At this point you have a new String array of capacity 10, with no values inside, something like this:
[null][null][null][null][null][null][null][null][null][null]
inv[0] = "0";
Now you set [0] to "0", so:
["0"][null][null][null][null][null][null][null][null][null]
Then your loop attempts to access all of those null references, that'll probably be why you have a null reference exception.
To fix it, simply every index position in your array to anything that is not-null:
Arrays.fill(inv, "");
I use Arrays.fill() because it's shorter.
In your for loop condition, which one do you prefer?
stop = true or stop == true
for(i=0; i<inv.length && !stop; i++)
{
if(inv[i]!=null && inv[i].equals("0"))
{
stop = true;
}
}
Can someone could be kind and help me out here. Thanks in advance...
My code below outputs the string as duplicates. I don't want to use Sets or ArrayList. I am using java.util.Random. I am trying to write a code that checks if string has already been randomly outputted and if it does, then it won't display. Where I am going wrong and how do I fix this.
public class Worldcountries
{
private static Random nums = new Random();
private static String[] countries =
{
"America", "Candada", "Chile", "Argentina"
};
public static int Dice()
{
return (generator.nums.nextInt(6) + 1);
}
public String randomCounties()
{
String aTemp = " ";
int numOfTimes = Dice();
int dup = 0;
for(int i=0 ; i<numOfTimes; i++)
{
// I think it's in the if statement where I am going wrong.
if (!countries[i].equals(countries[i]))
{
i = i + 1;
}
else
{
dup--;
}
// and maybe here
aTemp = aTemp + countries[nums.nextInt(countries.length)];
aTemp = aTemp + ",";
}
return aTemp;
}
}
So the output I am getting (randomly) is, "America, America, Chile" when it should be "America, Chile".
When do you expect this to be false?
countries[i].equals(countries[i])
Edit:
Here's a skeleton solution. I'll leave filling in the helper methods to you.
public String[] countries;
public boolean contains(String[] arr, String value) {
//return true if value is already in arr, false otherwise
}
public String chooseRandomCountry() {
//chooses a random country from countries
}
//...
int diceRoll = rollDice();
String[] selection = new String[diceRoll];
for ( int i = 0; i < selection.length; i++ ) {
while (true) {
String randomCountry = chooseRandomCountry();
if ( !contains(selection, randomCountry ) {
selection[i] = randomCountry;
break;
}
}
}
//...then build the string here
This doesn't check important things like the number of unique countries.
You need a data structure which allows you to answer the question "does it already contain item X?"
Try the collection API, for example. In your case, a good candidate is either HashSet() or LinkedHashSet() (the latter preserves the insert order).
You'd probably be better of using another structure where you save the strings you have printed. Since you don't want to use a set you could use an array instead. Something like
/*
...
*/
bool[] printed = new bool[countries.length];
for(int i=0 ; i<numOfTimes ; /*noop*/ )
{
int r = nums.nextInt(countries.length);
if (printed[r] == false)
{
i = i + 1;
printed[r] = true;
aTemp = aTemp + countries[r];
aTemp = aTemp + ",";
}
}
return aTemp;
Consider what you're comparing it to:
if (!countries[i].equals(countries[i]))
are you comparing c[i] to c[i]? or c[i] to c[i-1]? Or do you need to check the whole array for a particular string? Perhaps you need a list of countries that get output.
make list uniqueCountries
for each string called country in countries
if country is not in uniqueCountries
add country to uniqueCountries
print each country in uniqueCountries
When you do this, watch out for index out of bounds, and adjust accordingly
Much faster way to do it then using HashSets and other creepy stuff. Takes less code too:
public String randomCounties() {
List<String> results = Arrays.asList(countries);
Collections.shuffle(results);
int numOfTimes = Dice();
String result = " ";
for(int i=0 ; i<numOfTimes; i++) {
result = result + countries[i] + ", ";
}
return result;
}
If you want to avoid outputting duplicate values, you need to record what values have already been listed or remove values from the pool of possibilities when they get selected.
You mention that you do not want to use Sets or ArrayList (I assume you mean Lists in general), I assume that is a requirement of the assignment. If so, you can accomplish this by building arrays and copying data between them the same way that an ArrayList would.
one note, your current implementation chooses between 1 and 6 entries from and array of 4 entries. If you force the selections to be unique you need to decide how to handle the case when you have no more unique selections.