What I am trying to do is create an array that pulls even numbers from another array. I'm not sure if I have gone about it the right way. I've look for ways of returning from statements like you would functions/methods and I can't find anything, not even sure if it is possible.
Anyway, the issue I am having here is the 'return evenArray' below 'cannot find symbol.' I am not sure what this means?
public static int[] getEvenArray(int[] array)
{
int dividedBy = 2;
int evenElement;
int evenCount = 0;
for(int i = 0; i < array.length; i++)
{
int[] evenArray;
evenElement = array[i] % dividedBy;
if(evenElement == 0)
{
evenCount++;
}
else
{
array[i] = 0;
}
evenArray = new int[evenCount];
for(int x = 0; x < evenArray.length; x++)
{
if(array[i] != 0)
{
evenArray[x] = array[i];
}
}
}
return evenArray;
}
This is for a tutorial from one of my lectures, it's a little bit challenging to say the least :-)0
evenArray is defined within the scope of the for loop. (Actually a little worse than that; you're redeclaring it on each iteration so discarding the previous contents).
So once you're outside the for loop you can't refer to it.
Quickest fix is to use a std::vector<int> for this type, and declare it at the start of the function. Also change the return type of the function to the same. Don't forget to size the vector appropriately.
(Moving on, a smart lecturer will ask you about returning a std::vector which could potentially take a deep copy of that vector. Pre C++11 you'd mention return value optimisation, now you can talk about r-value references. No deep copy will be taken since the move constructor will be used).
Variable declared inside a block is not visible outside of it; move this int[] evenArray; to very start of function.
Related
The task was to write a method to return the least value of an array.
Would someone quickly look over my code?
public static int findMinimum (int [] array) {
for (int kohlrabi = 0; kohlrabi < array.length; kohlrabi++) {
for (int zwiebel= 0; zwiebel < array.length; zwiebel ++) {
if (array [zwiebel] < array [kohlrabi]) {
kohlrabi = zwiebel -1;
break;
}
int spinat = array [kohlrabi];
if (zwiebel == array.length-1) {
return spinat;
}
}
}
}
Exception in thread "main" java.lang.Error: Unresolved compilation
problem: This method must return a result of type int
at Abgabe7.ArrayExercises.findMinimum(ArrayExercises.java:38)
It's a homework for my school and I definitely understood the logic behind it but cannot find my fault.
Thanks Max
I don't think you need to have two loops. One loop would work.
Simply loop though the array keeping a variable which is the lowest you've found.
You should declare a global variable before the loop then use only one for loop in your code as follows:
int zwiebel= 0;
for (int kohlrabi = 0; kohlrabi < array.length; kohlrabi++) {
if (kohlrabi == 0){
zwiebel = array[kohlrabi];
}
if (array[kohlrabi] < zwiebel) {
zwiebel = array[kohlrabi];
}
}
The lowest value in your array is now stored in the variable zwiebel.
The real mistake is that you are not taking the possibility of an empty array into account. One thing to learn in programming is to think of all possibilities (maybe you’ve discovered that already). An array in Java (and most other languages) can have length 0, that is, have no elements in it. When array.length is 0, your outer for loop doesn’t execute, so we get down to the bottom of the method without having returned anything and without having anything to return.
Java is dumb, but nevertheless smart enough to discover this problem for you. When your method is declared to return an int, it insists that it too can see that it will return an int in all cases. In your method it cannot, which is what the following message is trying to tell you:
This method must return a result of type int
One possible fix — and I may be showing you something that you haven’t learned in school yet — is to insert the following statement in the end of your method:
throw new IllegalArgumentException("Cannot find the minimum of an empty array");
Throwing an exception is an alternative to returning a value, so this statement will make Java happy. If you actually try to find the minimum of an empty array, your program will crash and give you the message “Cannot find the minimum of an empty array”. But finding the minimum of an array that has numbers in it should work now.
That said the others are correct: Your way of finding the minimum is overly complex. I have been staring at your code and still have not understood how it works. Such code is not good. In real life you will be writing code that others will need to read and change after you, and no one can change code they don’t understand, so your code would not be useful. More important than writing code that works correctly is writing readable code.
Edit: There are variations on how to do this in a simple way. For a school exercise using a for loop I would write:
public static int findMinimum (int [] array) {
if (array.length == 0) {
return 42; // or whichever value is desired in this case
}
int niedrichsteSoWeit = array[0];
for (int index = 1; index < array.length; index++) {
if (array[index] < niedrichsteSoWeit) {
niedrichsteSoWeit = array[index];
}
}
return niedrichsteSoWeit;
}
For production code I probably would not write a method but use the built-in functionality, for example:
IntStream.of(myIntArray)
.min()
.ifPresentOrElse(min -> {
// do something with min
}, () -> {
// do whatever you want to do when the array is empty
});
Don’t worry if you don’t understand a bit of this snippet. It’s mostly for more experienced programmers who might happen to read this answer.
I need this for an array, but basically the idea is that a for loop will run, and whatever number you tell it to skip, it won't do. So for(int x=0; x<50; x++) if I want 1-50 except 22, how would I write that?
This would give me the ability to skip a certain number in my array.
Sorry if this is an extremely simple question, I am not too familiar with Java.
Make use of continue, something like this:
for(int x=0; x<50; x++) {
if(x == 22)
continue;
// do work
}
Suggested reading: http://docs.oracle.com/javase/tutorial/java/nutsandbolts/branch.html
public static final void doSkippedIteration(final int[] pArray, final int pSkipIndex) {
for(int i = 0; i < pSkipindex; i++) {
// Do something.
}
for(int i = pSkipIndex + 1; i < pArray.length; i++) {
// Do something.
}
}
You would have to do some basic check to see whether pIndex lies within the confines of the array. This saves you from having to perform a check for every single iteration, but does require you to duplicate your code in this specific example. You could of course avoid this by wrapping the code in a wider control block which handles the two iterations in a cleaner manner.
Good evening people,
I have a method that creates, populates, and returns an array to the function call as so:
public double[] getTotalDistances(){
double[] distance;
distance = new double[3];
for(Activity r: diary ){
if(r instanceof Run){
distance[0] += r.getDistance();
}
}
return distance;
}
and i can't seem to find a way to access this returned array to print it out in the main method, i have tried this: (where m is the object i have instantiated)
for(int i = 0; i< m.getTotalDistances().length; i++){
System.out.println(m.getTotalDistances().distance[i]);
}
this says it cannot find the variable distance.
i am aware that i can do something like:
for(double i: m.getTotalDistances()){
System.out.println(i);
}
this will print out the returned array, however, i would like to know how to do it in the "classic" way.I know that this must be extremely novice, but i couldn't find an answer. Any kind of help will be greatly appreciated.
It should be m.getTotalDistances()[i] and not m.getTotalDistances().distance[i]
Use a variable to store it before iterating.
double[] distance = m.getTotalDistances();
for(int i = 0; i < distance.length; i++){
System.out.println(distance[i]);
}
Your approach would call your getTotalDistances() method over and over inside the loop. You only need it once to work with.
You get this error
this says it cannot find the variable distance.
because the variable distance is only known in the scope of your method getTotalDistances() and thus you cannot use it outside of that (and it wouldn't make sense either).
The way it is written, distance is not defined. You will need to create a pointer the the returned value if you want to reference it.
double[] distance = getTotalDistances();
for(int i = 0; i < distance.length; i++) {
System.out.println(distance[i]);
}
Also, as it is written, any values other than the first will always be 0, and an accumulator makes more sense.
Another thing to note is that, as it is written, getTotalDistances() will run twice on each iteration of your for loop; once for the condition and again for the println(). If you were to scale this concept to a larger use case, the performance implications would be huge.
I am trying to write a method to remove a chromosome from my population. The method I have written is below. I am getting an out of bounds error when I run the code. Population is constructed with an ArrayList. The getChromosomeFitness method returns an int value score. Can someone spot my error?
void removeWorst()
{
int worst = population.get(0).getChromosomeFitness();
int temp = 0;
for(int i = 1; i < population.size(); i++)
{
if (population.get(i).getChromosomeFitness() < population.get(worst).getChromosomeFitness())
{
worst = population.get(i).getChromosomeFitness();
temp = i;
}
}
Chromosome x = population.get(temp);
population.remove(x);
}
You should probably change
if (population.get(i).getChromosomeFitness() < population.get(worst).getChromosomeFitness())
to
if (population.get(i).getChromosomeFitness() < worst)
You don't assure that in this line population has an element with the index 0:
int worst= population.get(0).getChromosomeFitness();
Try to add this to your method:
void removeWorst() {
if (population.isEmpty()) {
return;
}
...
There are several potential problems in your code:
int worst= population.get(0).getChromosomeFitness();
you need to make sure that population.isEmpty() is false
population.get(worst).getChromosomeFitness()
same thing, you need to make sure that (worst >= 0 && worst < population.size()).
The issue seems that you are getting the actual fitness rather than the object itself. The issue is with this line: int worst= population.get(0).getChromosomeFitness();. This is returning an integer value which is not related to the List's dimensions, as you said, it is the fitness of the chromozome, which could be well over the size of the list.
This should solve the problem:
void removeWorst()
{
int temp=0;
for(int i=1; i <population.size();i++)
{
if (population.get(i).getChromosomeFitness() < population.get(temp).getChromosomeFitness())
{
temp=i;
}
}
Chromosome x= population.get(temp);
population.remove(x);
}
That being said, a probably neater way of doing this would be to use a custom comparator to sort the list and then simply remove the last element.
Make sure population has something in it before trying to remove something from it?
I'm trying to make a tetris game for android to help learn game programming for android. My goLeft/Rights break right when the button is pressed, the code for going left is in a class separate of the fields int array, and the list parts array. The fields array is accessed by a referenced variable (TetrisWorld tetrisworld;). While part list array is public so accessed through a variable(part) code for which is in the goLeft() code. It breaks at: if(tetrisworld.fields[x][part.y] != 0) Code for left:
public void goLeft() {
int x = 0;
for(int i = 0; i < 4; i++) {
TetrisParts part = parts.get(i);
x = part.x - 1;
if(tetrisworld.fields[x][part.y] != 0) {
noleft = true;
break;
}
}
if(noleft == false) {
for(int i = 0; i < 4; i++) {
TetrisParts part = parts.get(i);
part.x--;
}
}
}
The code for the fields int array:
int fields[][] = new int[WORLD_WIDTH][WORLD_HEIGHT];
WORLD_WIDTH and WORLD_HEIGHT are both static final ints, width being 9 and height being 19
I've tried putting if(tetrisworld.fields[0][0] == 0) and it still crashes so I don't think it has to do with the variables. Also It doesn't go out of bound even if I haven't added the code to check for that yet because I have the teroid spawning around x = 5 and since I can't go left/right once there's not a chance of that happening
I've tried moving the goLeft/Right methods to the gamescreen class which has a "world = TetrisWorld();" and it still bugs out at the same spot
UPDATE:
Ok just adding:
tetrisworld != null
to the first if statement fixed it, my question now is, why did it fix it? Why can't I move without this check? It clearly isn't null cause as far as I know; it's fully responsive now.
But an easier way to have solved this which is SOOOO easy is changing fields to static... then access it lika so: TetrisWorld.fields so my updated code is:
public void goLeft()
{
noleft = false;
for (int i = 0; i < 4; i++)
{
part = parts.get(i);
if (part.x - 1 < 0 || TetrisWorld.fields[part.x - 1][part.y] != 0)
{
noleft = true;
break;
}
}
if (noleft == false)
{
for (int i = 0; i < 4; i++)
{
part = parts.get(i);
part.x--;
}
}
}
Looks like you are hitting IndexOutOfBoundsException.
When you are doing x = part.x - 1;, your x variable can become lesser tan zero, thus your code will act like if(tetrisworld.fields[-1][part.y] != 0
It looks like you're getting a java.lang.NullPointerException when trying to access the array in tetrisworld. In the line you mention there are several ways that this could occur:
if(tetrisworld.fields[x][part.y] != 0) {
tetrisworld could be null.
The fields member of tetrisworld could be null.
The second array that you're looking up by using tetrisworld.fields[x].
The value of part could be null.
Having a quick look through your source code it looks to me like you never initialise tetrisworld, either at declaration using:
TetrisWorld tetrisworld = new TetrisWorld();
Or at some other point which is certain to have happened before your goLeft() method is called.
Ok I believe I found the answer, referencing: http://en.wikipedia.org/wiki/Null_Object_pattern
Apparently java will throw an NPE if you don't check for it first if you have a null reference? Is there any way to initialize it without doing a TetrisWorld tetrisworld = new TetrisWorld(); because it's already created in a different class so i get a thousand errors, an actual stack overflow! lul... Still not 100% positive. Please comment to verify and possibly suggest a better way to go about this.