This is one of the first programs I am writing by myself. I want to make a physics calculator where many objects can interact with each other and give the user an option to add more objects. My idea is to have a for loop that runs through each object pulling on each other like this.
for(int n=1; n<=totalObjs; n++){
objName = "object"+n;
for(int i=1; i<n; i++){
obj2Name = "object"+i
objName.getMass();
//getting mass and position from both
//calculations here}
for(int x=n+1; x<=totalObjs; x++){
//same stuff as in the previous for loop}
}
I know there are probably huge syntax errors or logical errors in that but I'd like to sort through those on my own. Is there some way i could reference objects with the strings?
Is there some way i could reference objects with the strings?
Yes, via a Map<String, SomeType> such as a HashMap<String, SomeType>.
Think of this as being similar to an array or ArrayList, but instead of using number indices, you'd be using String indices.
Now looking at your code however, you might be better off using a simple ArrayList or array, since you appear to be trying to use numeric indices.
e.g.,
// assume a class called GravMass which has Mass, position, and momentum
List<GravMass> gravMassList = new ArrayList<GravMass>();
// fill your list
for(int i = 0; i < gravMassList.size() - 1; i++) {
GravMass gravMass1 = gravMassList.get(i);
int mass1 = gravMass1.getMass();
for(int j = i + 1; j < gravMassList.size(); j++){
GravMass gravMass2 = gravMassList.get(j);
int mass2 = gravMass2.getMass();
//getting mass and position from both
//calculations here}
}
}
Related
I just recently started learning java and today I learned how I can do the so called selection sort. I have been trying for the last 3 hours to do a bucket sort, but there are some parts which I don't know how to code. Important : I am learning java completely by myself with a book. I am not a student and I am doing this as a hobby. I already googled everything I could think of and I didn't find a solution. I don't have a teacher or anybody who I can ask, so yea, any help would be appreciated!
Code:
private int[] bucketSort() {
int[]bucket=new int[maxSize+1];
int[]sortedElements = new int[elementaros.length];
for(int i=0; i<elementaros.length;i++) {
bucket[elementaros[i]]++; //it says that I can't convert from Car to int. How can I add the elements of array elemenators to bucket?
}
int outPos = 0;
for (int j = 0; j < bucket.length; j++){
for (int k = 0; k < bucket[i]; k++){
sortedElements[outPos++] = i;
}
}
return bucket;
}
The idea of the code :
I have an array elements of type Car(Car is another class of my program). It looks like this - Car[] elementaros. int maxSize shows the maximum number of administrable Car objects. What I want to do is the following - I want to sort the elements in the elementaros array alphabetically. I would really really appreciate it if somebody has the time to show me how this would function with an example code or would just give me some tips. As I said - I have nobody who I can ask.
A selection sort is a combination of searching and sorting.
The principle is quite simple but I always prefer a diagram than huge explanations.
Start a pointer at the beginning of your unsorted array. Then, for each value of the array, search for the minimum value (or search for the alphabeticaly ordered car) in your array and switch the position of the founded Car with the pointer (which is a Car too)
Then you can advance the pointer to the next element of the array.
Here is a basic implementation to do this
public static Car[] doSelectionSort(Car[] elementaros) {
for (int i = 0; i < elementaros.length - 1; i++) {
int index = i;
for (int j = i + 1; j < elementaros.length; j++) {
if (elementaros[j].getName().compareTo(elementaros[index].getName()) < 0) {
index = j;
}
}
Car nextOrderedCar = elementaros[index];
elementaros[index] = elementaros[i];
elementaros[i] = nextOrderedCar;
}
return elementaros;
}
Just for example sake, I imagine your object of type Car has a name that we could use for the comparison.
UPDATE 1:
I have read your initial question too quickly and it leds me to answer a total different sorting algorithm. My bad.
I found an implementation here that does the trick:
williamfiset bucket sort
explanation of bucket sort plus different implementation
Hope this helps.
I have an array list in another class called Ratings.
I then have a while loop that i want to use to display the data along with the time in a graph. Most of it is working except for the main data part coming from the array list.
My code is below:
XYSeries series = new XYSeries("XYSeries");
int i = 0;
int time = 0;
double e = 0;
ArrayList<Double> emotionvalues = Rating.Array;
while (i < emotionvalues.size()) {
series.add(time, emotions[e]);
e++;
time++;
i++;
}
dataset.addSeries(series);
return dataset;
}
At the moment I am getting an error saying "The type of the expression must be an array type but it resolved to ArrayList"
Two errors maybe? The List.get instead of [] and it is better style to use int i as index instead of double e.
series.add(time, emotions[e]);
series.add(time, emotionValues.get(i));
Also the loop can be written as:
for (int i = 0; i < emotionValues.size(); ++i) {
series.add(time, emotionValues.get(i));
without the later i++.
I'm trying to write a simple game where an enemy chases the player on a grid. I'm using the simple algorithm for pathfinding from the Wikipedia page on pathfinding. This involves creating two lists with each list item containing 3 integers. Here's test code I'm trying out to build and display such a list.
When I run the following code, it prints out the same numbers for each array in the ArrayList. Why does it do this?
public class ListTest {
public static void main(String[] args) {
ArrayList<Integer[]> list = new ArrayList<Integer[]>();
Integer[] point = new Integer[3];
for (int i = 0; i < 10; i++) {
for (int j = 0; j < 3; j++) {
point[j] = (int)(Math.random() * 10);
}
//Doesn't this line add filled Integer[] point to the
//end of ArrayList list?
list.add(point);
//Added this line to confirm that Integer[] point is actually
//being filled with 3 random ints.
System.out.println(point[0] + "," + point[1] + "," + point[2]);
}
System.out.println();
//My current understanding is that this section should step through
//ArrayList list and retrieve each Integer[] point added above. It runs, but only
//the values of the last Integer[] point from above are displayed 10 times.
Iterator it = list.iterator();
while (it.hasNext()) {
point = (Integer[])it.next();
for (int i = 0; i < 3; i++) {
System.out.print(point[i] + ",");
}
System.out.println();
}
}
}
First of all, several of the other answers are misleading and/or incorrect. Note that an array is an object. So you can use them as elements in a list, no matter whether the arrays themselves contain primitive types or object references.
Next, declaring a variable as List<int[]> list is preferred over declaring it as ArrayList<int[]>. This allows you to easily change the List to a LinkedList or some other implementation without breaking the rest of your code because it is guaranteed to use only methods available in the List interface. For more information, you should research "programming to the interface."
Now to answer your real question, which was only added as a comment. Let's look at a few lines of your code:
Integer[] point = new Integer[3];
This line creates an array of Integers, obviously.
for (int i = 0; i < 10; i++) {
for (int j = 0; j < 3; j++) {
point[j] = (int)(Math.random() * 10);
}
//Doesn't this line add filled Integer[] point to the
//end of ArrayList list?
list.add(point);
//...
}
Here you assign values to the elements of the array and then add a reference to the array to your List. Each time the loop iterates, you assign new values to the same array and add another reference to the same array to the List. This means that the List has 10 references to the same array which has been repeatedly written over.
Iterator it = list.iterator();
while (it.hasNext()) {
point = (Integer[])it.next();
for (int i = 0; i < 3; i++) {
System.out.print(point[i] + ",");
}
System.out.println();
}
}
Now this loop prints out the same array 10 times. The values in the array are the last ones set at the end of the previous loop.
To fix the problem, you simply need to be sure to create 10 different arrays.
One last issue: If you declare it as Iterator<Integer[]> it (or Iterator<int[]> it), you do not need to cast the return value of it.next(). In fact this is preferred because it is type-safe.
Finally, I want to ask what the ints in each array represent? You might want to revisit your program design and create a class that holds these three ints, either as an array or as three member variables.
I would highly recommend to enclose the integer array of 3 numbers into a meaningful class, that would hold, display and control an array of 3 integers.
Then in your main, you can have an growing ArrayList of objects of that class.
You have an extra ) here:
element = (int[])it.next()); //with the extra parenthesis the code will not compile
should be:
element = (int[])it.next();
Besides the problem in the other answer, you cal it.next() two times, that cause the iterator move forward two times, obviously that's not what you want. The code like this:
element = (int[])it.next());
String el = (String)element;
But actually, I don't see you used el. Although it's legal, it seems meaningless.
I have two matrices for feature and weight elements.I am implementing an learning algorithm. I want to update elements of arraylist(vector for representing one sample of feature). Following is the code. But my elements of matrices(vector elements are not) updated. I have put the sample solution too. same value before and after updating is not expected. Could you please let me know where is the flaw in code?
for(int i =0 ; i< N ; i++){ //N is a large real number
ArrayList<Double> featureVector = new ArrayList<Double>();
featureVector = FeatureMatrix.get(i);
System.out.println("Before::"+ featureVector);
if(testList.contains(i)){
for(int j=0 ; j< testList.size(); j++){
if(i == testList.get(j)){
int indexInTestList= j;
List<Double> subListNextCandidate ;
subListNextCandidate = weightVectorNextCandidate.subList((10*indexIntTestList),((10)*(indexInTestList+1))); //clips a portion of member from long list of members
List<Double> approxWeight = new ArrayList<Double>();
approxWeight = getApproxWeight(FeatureVector, indexInTestList, FeatureMatrix,WeightMatrix, bias); //approxWeight is a vector of same dimension as of featureVector
for(int l=0 ; l< 10;l++){
double Update = featureVector.get(l)+ Rate*((subListCandidate.get(l)-approxWeight.get(l))-(lambda*featureVector.get(l)*(1/M)));//M is large real number
featureVector.set(l,Update);
}
}
}
}
else{
for(int l=0 ; l< 10;l++){
double Update = featureVector.get(l) -Rate*(lambda*featureVector.get(l)*(1/M));
featureVector.set(l, Update);
}
}
System.out.println("After:::"+ FeatureMatrix.get(i) );
}
Sample output is::
Before::[0.04539928251182193, -0.16233604402485394, 0.905018369795912, -1.2817141994528614, 0.7065420460225843, -0.8946090188977665, -1.74892020689701, -2.1539901172158187, 1.8229765478806985, -1.8109945435256574]
After:::[0.04539928251182193, -0.16233604402485394, 0.905018369795912, -1.2817141994528614, 0.7065420460225843, -0.8946090188977665, -1.74892020689701, -2.1539901172158187, 1.8229765478806985, -1.8109945435256574]
I can think of only a couple of reasonable reasons why this would happen:
Rate == 0
testList.contains(i) is always false
I would strongly suggest using breakpoints to debug this. At the very least, put a System.out.println where featureVector.set() is called to make sure it is ever called. I'm guessing it's never called because the conditions never become true.
Do use breakpoints, it'll be a life saver...
What is the return type of testList.get(j)? You're comparing an integer to what I suspect is a double. That's not very likely to go well ...
I'm not too familiar with Java GUI programming, and I wanted to do something where I have a loop that spits out a list of stuff and have the JTextField render it in the order it comes out.
I just do not know how the second parameter of the JTextField insert() function works. Right now when I do something like:
for(int i = 0; i < list.size(); i++){
textArea.insert(list.get(i), 0);
}
It does what I want, except it lists everything in backwards order that I put it in. I want it to display everything the other way around.
Thank you for any advice.
All you need to define a temporary string, result and for every item in the list add the string representation to that variable. When you have looped through everything, all you need to do is textArea.setText(result).
String result = "";
for(int i = 0; i < list.size(); i++)
{
result += list.get(i).toString();
}
textArea.setText(result);