I need help dealing with an array in my java program. in my first class, "test", I set 4 variables and then send them to my other class (test2).
arr[i] = new test2(id, fname, lname, case);
at that point, variables are set and then I want to return those variables. So in the test2 class, I have a method that strictly returns one of those variables
public int getId(){
return id;
}
I understand this is a little stupid, but professor gets what professor wants I guess. What I want to do now is in my main method in "test" I want to retrieve that variable and sort the array based on that int. Unfortunately, I have to create my own sort function, but I think this would work for what I want to do.
for(j = 0; j < arr.length; j++){
int indexMin =j;
for(i = j; i < arr.length;i++){
if(arr[i] < arr[indexMin]){
indexMin = i;
}
}
int tmp = arr[j];
arr[j] = arr[indexMin];
arr[indexMin] = tmp;
}
I appreciate any help anyone could provide.
Thank you
So a few comments:
-Your loop looks like this:
for(i = j; i < arr.length; i++)
You should be declaring
for(int i = j; i< arr.length; i++);
Either you hadn't declared i yet, which would give you a compilation error, or you declared i earlier which is not ideal...you avoid bugs better by declaring variables as locally as possible.
-In this line you directly compare objects:
if(arr[i] < arr[indexMin]){
but if I understand your intent correctly, you want to be comparing the IDs, so this should look like
if(arr[i].getId() < arr[indexMin].getId()){
-It looks like arr is an array of test2 objects, but you assign one to an int for your swapping code:
int tmp = arr[j];
This should be
test2 tmp = arr[j];
As far as your algorithm, why don't you get your code up and running and then try testing with a few results. What about 4 objects with IDs 1, 2, 3 and 4? How about 4, 3, 2, and 1? You'll learn more by playing around with it manually than if I tell you an algorithm here. Don't be afraid to add some statements that help you see exactly what's going on when. For example, maybe you might change the last 4 lines to look like:
System.out.println("About to swap id " + arr[j].getId() + " from index " + j + " with minimum " + arr[indexMin].getId() + " at index " + indexMin);
test2 tmp = arr[j];
arr[j] = arr[indexMin];
arr[indexMin] = tmp;
This will help you sooner get to the bottom of what's happening when in your program, and don't be afraid to add more similar stuff.
Make Test2 implements Comparable<Test2> and use java.util.Arrays.sort(Object[]) instead.
The compareTo(Test2) would look something like this:
#Override int public compareTo(Test2 other) {
return
(this.id < other.id) ? -1 :
(this.id > other.id) ? +1 :
0;
}
Even better is if you can switch to List<Test2> (Effective Java 2nd Edition: Item 25: Prefer lists to arrays).
See also
Java Integer: what is faster comparison or subtraction?
DO NOT be tempted to be "clever" and use this.id - other.id. This comparison-by-subtraction trick is broken.
Java Naming convention
Class names should be nouns, in mixed case with the first letter of each internal word capitalized.
If you have to implement your own sort, then in place of arr[i] < arr[indexMin], you want arr[i].getId() < arr[indexMin].getId(). This assumes that it was declared Test2[] arr;
If for some reason it was declared as Object[] arr instead, then you'd just need to cast them to Test2 before you can invoke getId().
((Test2) arr[i]).getId() < ((Test2) arr[indexMin]).getId()
You'd also want to declare Test2 tmp instead of int tmp for the swap.
As for printing, you can use a for-each loop:
for (Test2 t : arr) {
System.out.println(t);
}
This assumes that Test2 has #Overridepublic String toString() that is sensible for printing, but otherwise you can access the member methods individually.
See also
Java language guide/for-each loop
Related
I have a problem, my variable tabOffrandes change without I change this so I don't understand. This is my code:
public static int[] tri_selection(int[] tab) {
int a;
for (int i = 0; i < tab.length; i++) {
for (int j = i + 1; j < tab.length; j++) {
if (tab[i] > tab[j]){
a = tab[j];
tab[j] = tab[i];
tab[i] = a;
}
}
}
System.out.println();
return tab;
}
public static Joueur[] offrande(int[] offrandes) {
int[] tabOffrandes = offrandes;
printTab(tabOffrandes);
int[] ordreOffrande = tri_selection(offrandes);
printTab(tabOffrandes);
The terminal respond:
8 - 3 - 5 - 1
1 - 3 - 5 - 8
The function printTab just print all of the numbers in int[], I think it's not very important to show this.
The problem in your code is that you refer to the same array in the memory, even if you provide different names for them.
This is only an assumption, but what I think you are trying to do by writing:
int[] tabOffrandes = offrandes;
is to create a copy of your original array, right?
To achieve this, you must write the code as:
int[] tabOffrandes = Arrays.copyOf(offrandes, offrandes.length);
This way, your original offrandes array will not be affected when you call the tri_selection function.
Not related to your question, but it might help you in the future. I'm not sure what's your implementation for printTab function. I assume that you manually iterate through the array and print the items? Just wanted to let you know that there is a shorter solution, by using Arrays.toString(...), in case you didn't know:
System.out.println(Arrays.toString(tabOffrandes));
I need to create a Method that has 2 parameters in Java, upperborder and lowerborder. This method must create an array from the number 2 to the number 10.
Then I must implement another method, that calculates the factorial for a given number.
Then I must implement a third method that calculates the factorial for every element in the created array and test all these methods in a TestClass.
I know how to do this, but apparently I'm making some kind of a mistake in my code and it gives me the StackOverflow exception. I read the code a couple of times, but I can't seem to quite understand where I'm wrong.
package fakultaetinarray;
public final class FakultaetinArray{
private int i;
private int[] a;
private int j;
public FakultaetinArray(){
System.out.println("Given array : ");
createArray(1, 9);
System.out.println("Factorial for every element : ");
FakinArray();
}
public int fakRe(int n){
if(n == 1){
return n;
}
return n * fakRe(n - 1);
}
public void createArray(int untergrenze, int obergrenze){
this.a = new int[obergrenze];
for(this.j = 1; j <= a.length; j++){
a[i] = j + 1;
System.out.println(a[i]);
}
}
public void FakinArray(){
a[0] = 2;
for(i = 1; i < a.length; i++){
int fak = fakRe(a[i]);
a[i] = fak;
System.out.println(fak);
}
}
}
The reason you're getting StackOverflowErrors is due to your recursive method not having a case when n == 0.
The reason that your values are coming in as 0 is due to how you're constructing your loop.
for(this.j = 1; j <= a.length; j++){
a[i] = j + 1;
System.out.println(a[i]);
}
It's unclear why you're using j here at all, and i is initialized to its default value of 0, so in all reality, you're only ever filling one element of your array with a positive value and all of the others are at zero.
You need to reconsider how your loops are constructed. I would strongly encourage you not to make them fields, but declare them as part of the loop construct instead.
if(n == 1){ is not a strong enough condition to block the recursion: n can go below 1. In your particular case, you have a situation where n is 0.
Consider unwinding the recursion to a simple loop in any case. As a rule of thumb, recursion is not good for O(N) stuff.
I'm in dire need of help with this project. I'm trying to implement a Bag class for a programming assignment, and I'm getting hung up on the addAll(), Union(), and equals(), methods.
Edit: According to the assignment, addAll() is supposed to add all of the the objects from the second array into the first. I'm no longer getting an error when I run it, but for some reason it will not add all of the elements from the second array, it will only add the first 2. Thanks guys, this one is working perfectly now!
Edit: For Union(), I'm supposed to create a third bag that will contain all the contents of the first 2 bags. I was getting an ArrayIndexOutOfBoundsException when running this method. I've updated the code following biddulph.r and it's also working great. Thanks again!
Edit: "First attempt" And for equals(), it's supposed to check the size of the bags to make sure they are equal in size, then check to see if they contain the same numbers. So as it's written now, my equals() method will compare sizes and return the boolean value for that, but I'm unsure of how to make it compare the actual values.
import java.util.Arrays;
import javax.swing.*;
public class bag {
int maxSize = 10; //Size of the arrays
int count = 0; //Number of items stored in the array
int[] a;
int[] b;
bag c;
bag d;
public bag() {
//for(int i = 0; i < maxSize; i++){
//a[i] = (int)(1+Math.random()*100);
//}
a = new int[maxSize];
}
public String bagString() {
return Arrays.toString(a);
}
public void add(int b) {
try {
a[count] = b;
count++;
} catch (ArrayIndexOutOfBoundsException n) {
JOptionPane.showMessageDialog(null, "Array is full, element will not be added");
}
}
public void removeRandom() {
int i = (int)(1 + Math.random() * (count - 1));
a[i] = a[count - 1];
a[count - 1] = 0;
count--;
}
public void remove(int b) {
for (int i = 0; i < maxSize; i++) {
if (contains(b)) {
a[i] = a[count - 1];
}
}
}
public boolean isEmpty() {
if (count == 0) return true;
else return false;
}
public boolean contains(int b) {
int tf = 0;
for (int i = 0; i < maxSize; i++) {
if (a[i] == b) tf = 1;
}
if (tf == 1) return true;
else return false;
}
public int size() {
return count;
}
public void addAll(bag c, bag d) {
if (a.length >= c.size() + d.size()) {
for (int i = 0; c.size() <= d.size(); i++) {
c.add(d.a[i]);
}
}
}
public void union(bag c, bag d) {
bag bigger = new bag();
for (int i = 0; i < c.size(); i++) {
bigger.add(c.a[i]);
}
for (int i = 0; count < d.size() - 1; i++) {
bigger.add(d.a[i]);
}
System.out.println(bigger.bagString());
}
public boolean equals(bag c, bag d){
if(c.size() != d.size()){
return false;
}else{
for(int i = 0; i < c.union(c, d).size(); i++){
if(c.union(c, d).contains(c.a[i]) && c.union(c, d).contains(d.a[i])){
return true;
}
}
return false;
}
}
}
I really appreciate any help you guys can give me, thanks.
EDIT: Thanks to everyone for your help, you guys are life savers.
Your problem for addAll() is here
if (a.length >= c.size() + d.size()) {
for (int i = 0; c.size() <= d.size(); i++) {
c.add(d.a[i]);
}
}
You shouldn't be adding elements until your c bag becomes bigger than d, you should be adding all of d's elements to c.
for (int i = 0; i < d.size(); i++) {
c.add(d.a[i]);
}
So the part of the assignment you are having issue with is:
public void addAll(bag c, bag d){
if (a.length >= c.size() + d.size()) {
for (int i = 0; c.size() <= d.size(); i++) {
c.add(d.a[i]);
}
}
}
which you say is supposed to add all of the the objects from the second array into the first.
If you break that down and apply it to your addAll() method, it sounds like you are supposed to be adding all of the items in bag "d" into bag "c".
Your for loop is saying start i at 0, and add 1 to it until the size of c is less than or equal to d.
What it should be saying is start i at 0, and add 1 to it until you have gone through every item in d.
That would look like this:
for (int i = 0; i < d.size(); i++){
c.add(d.a[i]);
}
i is increased every time you go through the for loop, and i will stop increasing when you have got to the size of d (the second condition). At this point you will exit the for loop. You don't have to worry about the size of c.
In fact you can probably get rid of the if (a.length >= c.size() + d.size()) line as well.
I hope my explanation helps you understand why the changes have been made to the method.
I think you have a lot of problems with the design of the class that you should address first. If you are representing the bag as a static or dynamic array then you only need one array, not 2. You also don't need two bags inside each bag as attributes, that doesn't make any sense; all you should have left is the size of the bag or count and the array to hold all the elements (which are integers in your case). Also, avoid naming parameters for functions and attributes for the class the same way. Not doing so might confuse the compiler and will require code like self.attributeName to use attributes; otherwise, the compiler assumes you're talking about the parameter.
If you make these changes, the rest should be straight-forward from here. Since it's an assignment, you should make these changes and try again because you won't learn if we provide the answers for you; you'll see it will be much easier once you structure it correctly.
P.S. it's a convention to start a class name with a capital letter. Bag and not bag
addAll
There's a couple of problems with this function as written. First is that it's confusing to the caller. The code using this method would be something like this:
Bag bag1 = ...
Bag bag2 = ...
Bag bag3 = ...
bag1.addAll(bag2, bag3)
...or perhaps bag2.addAll(bag2, bag3). The function is intended to add elements from one bag in to another bag, so why does the caller have to specify three different bags? There's only two involved. You should either make the function static, so it can be called like Bag.addAll(bag1, bag2) or (better) make it totally clear who's getting elements added by making it take a single argument bag1.addAll(bag2).
Second problem is that the function isn't implemented correctly, but I think that's because you're getting confused because you've got three bags involved instead of two. To sketch out how it should be fixed:
Bag target = ...
Bag source = ...
if (target.a.length >= target.size() + source.size()) {
for (int i = 0; i < source.a.length; i++) {
target.add(source.a[i]);
}
}
Good variable naming is your friend.
union
You haven't specified what problem you're having with your implementation, so I'm not going to simply rewrite it for you. Edit your question with the problem, and I'll help.
However, this is an excellent example of a method that should be static (a Factory method, in fact). It should be able ot be called like: Bag biggerBag = Bag.union(bag1, bag2).
EDIT after his comment regarding the .union problem
The problem with .union is that you're looping through each bag using some else's size. It boils down to, if you want add each element from source in to target, you should be only counting elements from source, as so:
bag bigger = new bag();
for (int i = 0; i <= c.size(); i++) {
bigger.add(c.a[i]);
}
note that your method does not protect against the bigger bag not being big enough. You should have a check to make sure that it is BEFORE the loops, or even better just create a big enough bag.
equals
Again, you need to show that you've tried to write it, and then ask a question specifying what you need help with. Update your question and I'll help.
Your method:
public void addAll(bag c, bag d) {
if (a.length >= c.size() + d.size()) {
for (int i = 0; c.size() <= d.size(); i++) {
c.add(d.a[i]);
}
}
}
betrays your lack of understanding of Object Oriented programming.
Remember that the method addAll() is already acting on a bag, and so you should not need to specify 2 bags in the arguments.
Calling example:
mybag.addAll(yourBag);
would demonstrate a possible usage - it would add all contents of yourBag into myBag.
I'll give you this method for free (assuming that the array 'a' contains the contents of the bag - something I'm not sure about because your variable names aren't clear):
public void addAll(Bag otherBag) {
for (int i : otherBag.a) {
add(i);
}
}
The above method will copy all contents of otherBag into this bag.
Another thing I noticed - you also have a b[] instance variable - what's that for?
You also have 2 other bag instance variables. Not sure why.
Ok, my program in this specific section takes a line of data from a studentAnswer string array, the value of which would be something like TTFFTFTFTF. I am supposed to take this, and compare it against a key array, which might look like TFFFTFTFTF. A student takes a quiz, and my program calculates the points correct.
My intention is to use a separate points array to find the numeric grade for the student. The index of studentAnswer refers to a specific student. So studentAnswer[i] is TTFFTFTFTF. I use substrings to compare each individual T/F against the correct answer in key[], which would have a single T/F in each index. Then, if they are correct in their answer, I add a 1 to the correlating index in points[] and will later find the sum of points[] to find the numeric grade out of ten.
My problem here is that String origAns, used to define the student's original answer string, is getting a Java Error cannot find Symbol. I have tried placing the instantiation of origAns within each different for loop, but I can't get the program to work. Int i is meant to follow each specific student- I have four parallel arrays that will all log the student's ID number, numeric grade, letter grade, and original answers. So that is the intention of i, to go through each student. Then j should be used to go through each of these original student answer strings and compare it to the correct answer...
Logically, it makes sense to me where I would put it, but java doesn't agree. Please help me to understand this error!
for (int i = 0; i < studentAnswer.length; i++){
String origAns = studentAnswer[i];
for (int j = 0; j < key.length; j++){
if (origAns.substring[j] == key[j]){
//substring of index checked against same index of key
points[j] = 1;
}
if (origAns.substring[j] != key[j]){
points[j] = 0;
}
}
}
It sounds like you're trying to call the substring method - but you're trying to access it as if it were a field. So first change would be:
if (origAns.substring(j) == key[j])
Except that will be comparing string references instead of contents, so you might want:
if (origAns.substring(j).equals(key[j]))
Actually, I suspect you want charAt to get a single character - substring will return you a string with everything after the specified index:
if (origAns.charAt(j) == key[j])
... where key would be a char[] here.
You can also avoid doing the "opposite" comparison by using an else clause instead.
You should also indent your code more carefully, for readability. For example:
for (int i = 0; i < studentAnswer.length; i++) {
String origAns = studentAnswer[i];
for (int j = 0; j < key.length; j++) {
if (origAns.charAt(j) == key[j]) {
points[j] = 1;
} else {
points[j] = 0;
}
}
}
And now, you can change that to use a conditional expression instead of an if/else:
for (int i = 0; i < studentAnswer.length; i++) {
String origAns = studentAnswer[i];
for (int j = 0; j < key.length; j++) {
points[j] = origAns.charAt(j) == key[j] ? 1 : 0;
}
}
When you call a method in Java, you use parentheses () instead of brackets [].
Since substring is a method, you should call it like so
if (origAns.substring(j) == key[j])
A few other notes, you should use the equals method for comparisons (especially those comparisons involving Strings.)
if (origAns.substring(j).equals(key[j]))
Also, you should use charAt to extract a single character at some position in a string. substring(j) will return a string of characters starting at position j.
if (origAns.charAt(j).equals(key[j]))
Your explanation is very long and I have not read it from the beginning to end. But I can see at least one problem in your code:
if (origAns.substring[j] == key[j])
You are comparing strings using == instead of using method equals():
if (origAns.substring[j].equals(key[j]))
Substring is a function, not a member, of String objects. Check out the example at the top of this page:
http://docs.oracle.com/javase/6/docs/api/java/lang/String.html
Notice the use of parenthesis instead of brackets.
If you are using a String use charAt function
String studentAnswer = "TTFFTFTFTF";
for (int i = 0; i < studentAnswer.length(); i++)
{
char origAns = studentAnswer.charAt(i);
}
Else if you are using an char array then
char studentAnswer[] = "TTFFTFTFTF".toCharArray();
for (int i = 0; i < studentAnswer.length; i++){
char origAns = studentAnswer[i];
}
Say you have a List of Strings or whatever, and you want to produce another List which will contain every possible combination of two strings from the original list (concated together), is there any more efficient way to do this other than using a nested for loop to combine the String with all the others?
Some sample code:
for(String s: bytes) {
for(String a: bytes) {
if(!(bytes.indexOf(a) == bytes.indexOf(s))) {
if(s.concat(a).length() == targetLength) {
String combination = s.concat(a);
validSolutions.add(combination);
}
}
}
}
The time for execution gets pretty bad pretty quickly as the size of the original list of Strings grows.
Any more efficient way to do this?
You can avoid checking i != j condition by setting j = i + 1. Also, things like bytes.length() get evaluated at each iteration of both loops - save it into a value and reuse. Calling a.length() inside the loop asks for a length of the same string multiple times - you can save some runtime on that as well. Here are the updates:
int len = bytes.length();
int aLength;
String a, b;
for(int i=0; i<len; i++) {
a = bytes[i];
aLength = a.length();
for(int j=i; j<len; j++) {
b = bytes[j];
if (b.length() + aLength == targetLength) {
validSolutions.add(b.concat(a));
validSolutions.add(a.concat(b));
}
}
}
Edit: j = i because you want to consider a combination of a string with itself; Also, you'd need to add a.concat(b) as well since this combination is never considered in the loop, but is a valid string
You can't get Better than O(N^2), because there are that many combinations. But you could speed up your algorithm a bit (from O(N^3)) by removing the indexOf calls:
for(int i=0; i<bytes.length(); i++) {
for(int j=0; j<bytes.length(); j++) {
string s = bytes[i];
string a = bytes[j];
if (i != j && s.length() + a.length() == targetLength) {
validSolutions.add(s.concat(a));
}
}
}
In addition to what Jimmy and lynxoid say, the fact that the total length is constrained gives you a further optimization. Sort your strings in order of length, then for each s you know that you require only the as such that a.length() == targetLength - s.length().
So as soon as you hit a string longer than that you can break out of the inner loop (since all the rest will be longer), and you can start at the "right" place for example with a lower-bound binary search into the array.
Complexity is still O(n^2), since in the worst case all the strings are the same length, equal to half of totalLength. Typically though it should go somewhat better than considering all pairs of strings.