I want to add a new element that has several arguments in an array. I know how to add with just one argument, but with more i don't know.
My code is:
private Calculate[] calculation;
public Element(int numElements) {
calculation = new Calculate[numElements];
}
public void addElement(Date elementDate, double price, ElementType type) {
int numElements = elements.length;
int i = 0;
if (i < numElements) {
Calculate[i] = calculation.elementDate;
Calculate[i] = calculation.price;
Calculate[i] = calculation.type;
i++;
}
}
Calculate[i] = calculation.elementDate;
Calculate[i] = calculation.price;
Calculate[i] = calculation.type;
You shouldn't assign to the same array index 3 times. You're overriding what you've just set.
Try this (Calculate should have a constructor):
Calculate[i] = new Calculate(elementDate, price, type);
You're also maintaining an index i, but you're not looping over anything. i is just incremented from from zero to one and is not really used (apart from an almost useless conditional check).
I suggest you read over a beginners Java tutorial. You seem to be missing a lot of the fundamentals, and Stack Overflow is not a place where we should have to show you how to write a for-loop. It's well-documented and demonstrated in a tonne of tutorials already.
I assume Calculate is a class defined else where with a constructor
There is a couple of issues in this piece of code:
You want to update the array, but specify array. Calculate[] is the array type. The name if the array is calculation. The other thing is that you are trying to access calculation.elementDate etc. but since that is an array, it does not have the field elementDate. I assume your Calculate class has that field. Also, you are not applying a loop. So currently your code will only update the array on index 0.
My code:
public void addElement(Date elementDate, double price, ElementType type) {
for(int i = 0; i < elements.length; i++) { // for loop over all elements in your array
Calculate calculate = new Calculate(elementDate, price, type) // I assume this constructor is available to initialize the Calculate object
calculation[i] = calculate;
}
Hope this helps.
Related
I am trying to add Books in my code. Let's say someone wants to add 30 books, the iteration goes from 0 to 30 which is fine. What if he wants to add 10 more books later, then it will simply do nothing useful, since I need them to start from 30 to 40. How can I fix this?
int currentBooks = 0;
do {
System.out.print("How many books would you like to add? ");
int nbBooks = sc.nextInt();
// Add nbBooks amount to inventory array
if (inventory.length-currentBooks >= nbBooks) {
for (int w = 0; w < inventory.length; w++) {
inventory[currentBooks] = new Book();
currentBooks = w;
}
valid = true;
break password;
}
else {
System.out.print("You can only add " + inventory.length + " books.\n");
add = true;
}
} while(add);
The disadvantage of a plain array (Book[] in your case) is that its length cannot be changed. You should use a List (despite the fact that you're not allowed to, for some strange reason).
With the List interface
Therefore, you are better off using the List interface (and an implementation of it, for instance, ArrayList), which uses an array internally, but it automatically extends its internal array if needed, so you don't have to worry about it:
// List is an interface, so we need a certain implementation of that interface
// to use. ArrayList is a good candidate:
List<Book> books = new ArrayList<>();
Now we have created an ArrayList with the initial length of 0. The length can be obtained using the size() method, as opposed to an array's length property.
int nbBooks = sc.nextInt();
for (int i = 0; i < nbBooks; i++) {
books.add(new Book());
}
Without the List interface
However, if you cannot or may not use the List interface, you have a few options, depending on what exacly you want.
One of the options is to create a class which holds your array with Books, and a length as a property, because you have to store the length somewhere:
class BookList {
private Book[] books = new Book[100]; // Or some maximum length
private int size;
public void add(Book book) {
this.books[this.size] = book;
this.size++;
// You could optionally 'extend' the array with System.arraycopy
// when the internal array exceeds 100, but I'll leave that to
// you
}
}
Note that this is virtually a kind of homebrew version of the ArrayList class.
In your case you have defined inventory somewhere. You'll need to introduce inventorySize or something, and each time you add a book, you also increment the inventorySize variable.
Book[] inventory;
int inventorySize;
and your method:
...
System.out.print("How many books would you like to add? ");
int nbBooks = sc.nextInt();
for (int i = 0; i < nbBooks; i++) {
this.inventory[this.inventorySize + i] = new Book();
}
this.inventorySize += nbBooks;
You can also check for the last non-null element (or the first null element) and consider that the length of the array, but that would be very bad code, because, for instance, you have to walk over the array to calculate its length, which might be pretty expensive in performance.
I'm quite new to arrays and methods, and I've been seeing this error recurring through several programs: error '[' expected.
In each occasion, it seems to correct itself as I adjust something else, but in this particular case, I am completely stumped.
By the way, I am using several methods and arrays to create a quiz (before you ask, yes, this is an assignment and I agree, a list is a better way to handle this data - but that is not an option).
It is possible that I am not passing the arrays correctly between methods, as I'm a little muddy on that process. From my understanding, in order to send/receive (i.e. import/export) an array or other variable between methods, I must declare that variable/array in the method header parameters.
import java.util.Scanner;
public class H7pseudo
{
public static void main(String[] args)
{
//call getAnswerkey method
getAnswerkey(answerkey[i]);
//call getAnswers method
getAnswers(answers[i]);
//call passed method? necessary or no?
boolean passed = passed(answerkey[i], answers[i], qMissed[i], points);
//Print results of grading
if (passed)
{
System.out.println("Congratulations! You passed.");
}
else
{
System.out.println("Try again, sucka. You FAILED.");
}
//call totalPoints
totalIncorrect(points);
//call questionsMissed
questionsMissed(qMissed[i]);
}
//get answer key (create answerkey array & export)
public static void getAnswerkey(answerkey[i])
{
//create answerkey array here
char[] answerkey;
//determine number of questions (indices)
answerkey = new char[20];
//input values (correct answers) for each index
//for our purposes today, the answer is always 'c'.
for (int i = 0; i <=20; i++)
{
answerkey[i] = 'c';
}
}
//get student answers (create answers array & export)
public static void getAnswers(answers[i])
{
//initialize scanner for user input
Scanner scan = new Scanner(System.in);
//create answer array here
char[] answers;
//determine number of questions (indices)
answers = new char[20];
//prompt for user input as values of each index
for (int i = 0; i <= 20; i++) {
answers[i] = scan.nextChar();
}
}
//grade student answers (import & compare index values of arrays:answers&answerkey
//create & export qMissed array
public static boolean passed(answerkey[i], answers[i], qMissed[i], points)
{
int points = 0;
//create new array: qMissed
boolean[] qMissed;
//determine number of questions to be graded
qMissed = new boolean[20];
//initialize values for array
for (int i = 0; i <= 20; i++) {
qMissed[i] = false;
}
//cycle through indices of answerkey[i] & answers[i];
for (int i = 0; i =< 20; i++)
{
if (answers[i] == answerkey[i])
{
correct = true;
points = points+1;
qMissed[i] = true;
}
else {
qMissed[i] = false;
}
}
//evaluate whether or not the student passed (15+ correct answers)
if (points >= 15)
{
passed = true;
}
else
{
passed = false;
}
return passed;
}
public static void totalIncorrect(points)
{
int missed = 20 - points;
System.out.println("You missed " + missed + " questions.");
}
public static void questionsMissed(qMissed[i])
{
// for each index of the array qMissed...
for (int i = 0; i < qMissed.length; i++)
{
//...print correct and false answers.
system.out.println(i + ": " + qMissed[i] + "\n");
}
}
}
You can't define array size in the method signature, in Java.
public static void getAnswerkey(answerkey[i])
You can't put anything inside the [] in a method declaration. Also, you have to mention the type:
public static void getAnswerKey(char[] answerkey)
This is not the only reason your code won't work as intended, but I'll leave the rest as part of the exercise.
Look at your method definitions:
public static void questionsMissed(qMissed[i])
This is wrong. You should define the type of the variable and it should not contain [i] like an element of an array. It should be something like this:
public static void questionsMissed(int qMissed)
Or if you want to pass the array, write it like this:
public static void questionsMissed(int[] qMissed)
Apart of this, there are other several errors in your code:
getAnswerkey(answerkey[i]); //answerkey is not defined
getAnswers(answers[i]); //answers is not defined
It would be better if you start reading a Java tutorial first.
I want to vote up Luiggi's answer, but I don't have enough reputation to do that :)
Congrats, cordivia, on getting started with Java!
Here is how an array is declared:
type[] arrayName = new type[numberOfElements]
For example, you did this right in your method definition for getAnswerkey():
char[] answerkey;
answerkey = new char[20];
The part in the method definition inside the parentheses defines the kind of data the method is willing to accept from the outside. So if you don't need to put something into the method to get something out of it, you don't need to put anything in the parentheses. Define the method like this:
getAnswerkey() {
...But that's not the whole story. If you want to get something out of the method, it needs to have a return type as well. A return type is what you're gonna get out of the method when the method's done doing it's magic. For example, if you want to get an int array out of a method you would do something like this:
public static int getTheInteger() {
Since you want an array of chars from the method, you'll want to do something like this:
public static char[] getAnswerkey() {
So that's how you get a method to give you something back. If don't want anything back, you put void:
public static void noMarshmallows() {
Now, when you use the method, you're gonna need to do something with what it gives you, or it did all that work for nothing. So you need to store the return value in a variable when you call the array (calling methods is what you've been doing in main). You know how to store something in a variable. You use the '=' operator:
int myVeryFavoriteNumber;
myVeryFavoriteNumber = 5;
So, you do the same thing when you're getting something out of an array. You assign the return value to a variable. If you want to do this with an array, do this:
int[] myFavs;
myFavs = getMyFavoriteNumbers();
Same with chars:
char[] answerKey;
answerKey = getAnswerKey();
Voila! Your answer key is now right out in the open for the rest of main to see :)
Now, if you want to put something into a method and have it do something with what you put in, you define a parameter. You know how this works. It's just like declaring a variable, and that's exactly what it is. Parameters go in the parentheses and only the method using the parameter sees that variable name (it's local). Something like this:
public static void plusOneToAll (int[] numbers) {
for (int i = 0; i < numbers.length; i++) {
numbers[i] = numbers[i] + 1;
}
}
Notice int[] numbers in the parentheses. The type being passed in is int[] or integer array. numbers is just the parameter name. It functions just like a variable, but it is declared locally (inside the parentheses) and use locally (inside the method). So, if you wanted to compare the answers from two arrays and return the number of matches (like a total score for instance), you would do something like this:
public static int getTotalScore (char[] correctAnswers, char[] userAnswers) {
int totalScore = 0;
for (int i = 0; i < correctAnswers.length; i++) {
if (userAnswers[i] == correctAnswers[i]) {
totalScore = totalScore + 1;
}
}
return totalScore;
}
Notice the return type: int (written before the method name). Inside the array I'm using the variable totalScore to keep track of the number of times the answers match. The method takes two char arrays from the outside and uses them on the inside. Finally, I return an int: totalScore. That kicks the value of totalScore back out to whatever called it (main).
If I might add one last thing: Do yourself a favor and go pick up a copy of Head First Java. It's hard to find a good tutorial online and Java textbooks are just plain boring. The Head First series are kind of like coloring books for adults.
Best of luck!
Okay I have tried to write a simple Java code in BlueJ, that finds and prints the product of all the entries in data such as if data is {1,2,3,4} then the result will be 24.
And my code is below:
public class Product {
public static int[] product(int[] a) {
int [] s = new int[a.length];
for (int i =0; i< a.length; i++)
s[i] = a[i]*a[i];
return s; //the definition of your method...
}
public static void main(String[] args) {
//calling the method to seek if compiles
int[] results = Product.product(new int[] { 1,2,3,4 });
//printing the results
System.out.println(java.util.Arrays.toString(results));
}
}
The above code is giving me the square of each number, which is not what I want to have, somehow I have modify the code that the result will be 24 but I couldn't figure it out, anyone knows how to do it?
First of all, if you are first writing Java it is important to know that variable, function and class names are quite important. Please note that having Product.product() is not a good idea, since the function name is almost the same as the class name. Anyway, regarding your code. Your code is indeed returning the square of your input, what you would want is the following:
public class Product {
public static int getProduct(int[] input) {
int total = 1;
for (int v : input) {
total *= v;
}
return total;
}
}
This will return an integer value with the product of your input array. This also uses a for-each loop instead of a regular for-loop for readability. Also you don't need the index in this case. Good luck with it!
First, your product method needs to return an int rather than an int [].
You need to maintain the product as a variable. You can set it to 1 initially, and then multiply it by each element of the a array in turn; then you just return this value.
Basically, I have an ArrayList of BudgetItem objects (BudgetItem being a class I made). Each BudgetItem has three instance variables, price, name, and quantity with setters and getters for each. I am adding each BudgetItem to the ArrayList one at a time, each with different information in the price variable. When I go to print out any price element of the ArrayList, using the get method, the console will always print the last price that was entered. Below, I will paste some sample code to help:
public class Register {
private ArrayList<BudgetItem> register = new ArrayList<BudgetItem>();
public Register(double[] price) {
//Creates a register ArrayList with specified number of
//elements and populates it with BudgetItems that contain
//the data in the price array entered in the declaration.
BudgetItem bi = new BudgetItem();
for(int i = 0; i<price.length; i++) {
bi.setPrice(price[i]);
register.add(bi);
if(i=0) { //This if statement is for debugging.
System.out.println(register.get(i).getPrice());
}
}
//This is also for debugging.
double i = register.get(0).getPrice();
System.out.println(i);
}
}
Through my debugging efforts I found that the problem is with the get method in ArrayList. No matter what I do it returns the last instance of price that was entered. My question is why won't the get method return the specified element?
Well the problem is that you always modify the same BudgetItem object.
Try :
for(int i = 0; i<price.length; i++) {
BudgetItem bi = new BudgetItem(); <-- move it inside the for loop
bi.setPrice(price[i]);
register.add(bi);
}
You are only adding a single BudgetItem and setting and resetting its price. You need to add new BudgetItems and set their prices accordingly.
Add BudgetItem bi = new BudgetItem(); inside for loop
I have a loop in which I calculate a value and add it it a list. So, I do something like that:
x = getValue()
values.add(x)
while (true) {
x = getValue();
values.add(x)
}
I found out that this approach does not work since I add the same instance to the list. In more details, in every cycle of the loop I re-assign a new value to the x and doing so I change values of all elements that were already added to the list (so in the end I get a list of identical elements).
To solve this problem I did the following:
x = getValue();
Integer[] valueToAdd = new Integer[n];
for (int i=0; i<n; i++) {
valueToAdd[i] = x[i];
}
while (true) {
x = getValue();
y = new Integer[n];
for (int i=0; i<n; i++) {
valueToAdd[i] = x[i];
}
values.add(valueToAdd)
}
In this way I wanted to create a new instance every time want to add a value to the list. But it does not work since I get a duplicate local variable error.
It is also strange to me that I do not have this error if I declare the same variable many times in the loop. The problem appears only if I first declare a new variable outside the loop and then also in the loop.
Is there a way in Java to re-use the same name for different instances?
ADDED
I need to clarify some issues. I did not show all the code. I have the break command in the loop (when a new value cannot be generate, I exit the loop). x and value have Integer[] type.
ADDED 2
Since it was mentioned that the problem can be in the getValue() I need to in more details here. Actually I do not have getValue() in my code (I used getValue() here to make my example shorter). In my code I had:
Integer[] x = new x[n];
while (true) {
for (int i=0; i<n; i++) {
x[i] = y[i];
}
values.add(x)
}
And it did not work since in my values list I had identical elements (and I know that in the loop on every cycle x had a new value).
ADDED 3
Why all elements of my list seems to be the same?
Your problem is not what you think it is. For example take a look at this simple program:
String x = null;
List<String> l = new ArrayList<String>();
for (int i = 0; i < 10; i ++) {
x = String.valueOf(i);
l.add(x);
}
System.out.println(l);
It prints the numbers from 0 to 9. This is because java is pass-by-value (check here). You are not passing the reference to x, you are passing the value of x (in the add method).
So the problem lies in the getValue() method, which returns the same object.
Update: Now the question makes more sense. You are working with the same object x everytime, and just changing its state. In order to put different values just move the declaration inside the loop:
while (true) {
Integer[] x = new x[n];
...
}
If you need it outside the loop, well, simply use another variable there. It does not have to be named x. Since you won't be using it inside the loop anyway.