I've been working on my assignment all day and am stuck at just one part. I am pretty good at searching the internet and finding answers and have checked here as well. I still can't seem to figure out what is going on.
For the assignment we need to write a class with methods that will test a triangle. I am struggling with understanding classes, but have been able to get most of the code to work. The only issue I have now is with the is_equilibrium(); method. When running with the program provided with the assignment, the equilibrium method will return true every time.
problem code:
public boolean is_equilateral()
{
if(longest == shortest)
{
return true;
}
else
{
return false;
}
}
When I change it to:
public boolean is_equilateral()
{
if(side1 == side2 && side2 == side3&& side3 == side1)
{
return true;
}
else
{
return false;
}
}
I the code works fine and will return the true or false values.
I hope I am asking everything right, I am still trying to learn java so please bear with me! Any help is greatly appreciated!
I got the following problem. I have implemented transposition tables, which seem to work well or at least I can not see they are not working.
Additionally, I wanted to implement some move-ordering.
Basically, I search the move first that's saved in my transposition table. For some reason, that doesn't improve the search at all. Either I haven't implemented move-ordering properly or my implementation of the alpha-beta algorithm along with transposition tables is just wrong.
To measure the performance I counted the number of nodes visited during search. My implementation of move-ordering didn't have any effect at all.
Here is the Java-Code I have come up with:
public int alphabeta(final CheckerBoard board,int alpha,int beta,int depth,boolean nullMoveAvailable){
nodeCount++;
int alphaOrig=alpha;
Transposition entry =table.find(board.key);
if(entry!=null && entry.depth>=depth){
if(entry.flag==Transposition.EXACT){
return entry.value;
}else if(entry.flag==Transposition.UPPER){
beta = Math.min(beta,entry.value);
}else if(entry.flag == Transposition.LOWER){
alpha =Math.max(alpha,entry.value);
}
if(alpha>=beta){
return entry.value;
}
}
if(depth==0 || board.isTerminalState()){
return quiesceneSearch2(board,alpha,beta);
}
ArrayList<Move>sucessors =MGenerator.getMoves(board);
Move currentBest =null;
for( int i=0;i<sucessors.size();i++){
if(entry!=null && entry.depth<depth && (entry.flag == Transposition.UPPER || entry.flag == Transposition.EXACT) && sucessors.get(i).equals(entry.best)){
Collections.swap(sucessors,0,i);
break;
}
}
int bestValue =Integer.MIN_VALUE;
for(Move move : sucessors){
board.makeMove(move);
int value =-alphabeta(board, -beta, -alpha, depth - 1, true);
board.undoMove(move);
if(value>bestValue){
bestValue =value;
currentBest = move;
}
alpha =Math.max(alpha,value);
if(alpha>=beta){
break;
}
}
Transposition next =new Transposition();
next.depth=depth;
next.value =bestValue;
next.zobrisKey=board.key;
next.best = currentBest;
if(bestValue<=alphaOrig){
next.flag =Transposition.UPPER;
}else if(bestValue>=beta){
next.flag = Transposition.LOWER ;
}else{
next.flag = Transposition.EXACT;
}
table.insert(next);
return alpha;
}
and the following code to start the search:
public int findBestMove(int depth){
if (table != null) {
table.clear();
}
ArrayList<Move> sucessors =MGenerator.getMoves(board);
int max = Integer.MIN_VALUE;
for(Move m : sucessors){
board.makeMove(m);
int value =-alphabeta(board, -1000000, 1000000, depth, true);
board.undoMove(m);
if(value>max){
max=value;
bestMove=m;
}
}
board.makeMove(bestMove);
return max;
}
Would appreciate it if someone looked at my code. Maybe there isn't anything wrong with this part of my code but I didn't want to post everything since it's already quite a lot of code to look at.
I see a couple of things "off" compared to this reference site for alphabeta: https://web.archive.org/web/20071031100051/http://www.brucemo.com/compchess/programming/hashing.htm
1) You're not storing any transposition entries for depth 0; and
2) You're storing a transposition value of alpha instead of beta when alpha>=beta.
Don't know if that would make the difference, but...
And, oh, 3) You're also returning alpha instead of beta from the function when alpha>=beta.
I have four methods that check whether or not a given grid location is next to an occupied location (value of 1). The grid is assumed to wrap around, ie, if in a 50x50 grid[0][1] is the given location and grid[49][1] is occupied, the method should return true/ My checkNorth and checkEast method are working fine, but I get an ArrayIndexOutofBoundsException: -1 error for either the south or west methods every time I run the program. I checked my math and I think it should work - am I using the modulo incorrectly, or am I missing something else?
EDIT: Clarified the wrapping criterion, word use correction.
boolean checkWest(int indexA, int indexB)
{
if (indexA-1 > 0)
{
if (grid[indexA-1][indexB] == 1)
{
return true;
}
}
if (indexA-1 < 0)
{
if (grid[(indexA-1)%width][indexB] == 1)
{return true;}
else return false;
}
return false;
}
I see a couple problems. First, Java arrays are zero-indexed, which means that the first element is at index 0. So it's okay to check grid[indexA-1][indexB] when indexA-1 is equal to 0. Second, you're not properly handling when indexA equals 0. Here is my implementation. I also simplified the logic a bit.
boolean checkWest(int indexA, int indexB)
{
if (indexA > 0)
return grid[indexA - 1][indexB] == 1;
else
return grid[width + indexA - 2][indexB] == 1;
}
EDIT: I'm pretty sure I butchered the math with the second return statement. It should be right now...
So I'm making a TicTacToe-program, and I made a method for checking if there are 3 pieces in a row, to check if the game is over or not.
public boolean hasWon() {
if (tictac[0][0].equals(tictac[1][0]) && tictac[0][0].equals(tictac[2][0]) && tictac[1][0].equals(tictac[2][0])) { //Vertical
return false;
} else if (tictac[0][1].equals(tictac[1][1]) && tictac[0][1].equals(tictac[2][1]) && tictac[1][1].equals(tictac[2][1])) { //Vertical
return false;
} else if (tictac[0][2].equals(tictac[1][2]) && tictac[0][2].equals(tictac[2][2]) && tictac[1][2].equals(tictac[2][2])) { //Vertical
return false;
} else if (tictac[0][0].equals(tictac[0][1]) && tictac[0][0].equals(tictac[0][2]) && tictac[0][1].equals(tictac[0][2])) { //Horizontal
return false;
} else if (tictac[1][0].equals(tictac[1][1]) && tictac[1][0].equals(tictac[1][2]) && tictac[1][1].equals(tictac[1][2])) { //Horizontal
return false;
} else if (tictac[2][0].equals(tictac[2][1]) && tictac[2][0].equals(tictac[2][2]) && tictac[2][1].equals(tictac[2][2])) { //Horizontal
return false;
} else if (tictac[0][0].equals(tictac[1][1]) && tictac[0][0].equals(tictac[2][2]) && tictac[1][1].equals(tictac[2][2])) { //Diagonal
return false;
} else if (tictac[0][2].equals(tictac[1][1]) && tictac[0][2].equals(tictac[2][0]) && tictac[1][1].equals(tictac[2][0])) { //Diagonal
return false;
} else {
return true;
}
}
The problem here is, with these conditions I figured if, say, all of the indexes "in a row" where f.ex. X, " X X X " it would be a win for the player X. However, my program calls for a winner if any kind of piece fill 3 in a row, f.ex. " X O X " would end the game, which obviously is wrong.
Have I misunderstood .equals?
EDIT: after the wonderful feedback, is there a way that I can make the if-statements to ignore blank spaces (" ")?
You should consider creating a class, Board. Have this class provide an interface that includes getLines. This should return a List<Line>. There are eight lines in a tic tac toe board: three horizontal, three vertical, two diagonal. Create a method isWin(String player, Line line). Return true or false based on whether the count of the player's marker in the cells of the line is three.
I would guess you can write the whole thing in the same amount of space as the horrible nested if with all its inscrutable && clauses, and the functioning will be obvious. (This is also good practice for encapsulation.)
Your prog returns true if all three values on some line are the same. So, it won't return true on XOX. But would on OOO, XXX and SpaceSpaceSpace.
The first two are both OK, for if you check after a move, only the last mover could have three in a line and win. But three spaces are a problem. In every if you should check the first cell for not having space.
It is longer, but if you use the advice of #ajb, you'll even shorten the prog.
As for structuring, I would advice more simple way than that of #syrion. Make an array lines[5,3,2]. It has 5 lines, 3 cells for every line and 2 coords for every cell. And you can do the check in two inserted loops. It is not a Java way to do things, but you can take it as a first step. Or you could move immediately to a more readable structure - a list of 5 x lines= array of Cell = pair of coordinates. I don't know which one is more simple for you.
I'm trying to make a method that will tell me weather or not it is true or false that a number is prime. here's the code:
class prime
{
public static boolean prime (int a, int b)
{
if (a == 0)
{
return false;
}
else if (a%(b-1) == 0)
{
return false;
}
else if (b>1)
{
prime (a, b-1) ;
}
else
{
return true;
}
}
public static void main (String[] arg)
{
System.out.println (prime (45, 45)) ;
}
}
when i try to compile this i get this error message:
prime.java:23: missing return statement
}
^
1 error
I could be misinterpreting what the error message is saying but it seems to me that there isn't a missing return statement since i have a return statement for every possible set of conditions. if a is 0 then it returns false, if it isn't then it checks to see if a is dividable by b if it is then it returns if not then if b is greater than 1 it starts over again. if b isn't greater than 1 it also returns.
Also it seems a bit messy to have to
make this method take two ints that
are the same int.
What is wrong with my syntax/ why am i getting the error message? Is there a way to make it so that the method that i use in main only has to take one int (perhaps another method splits that int into two clones that are then passed to public static boolean primeproper?
or is there a more effective way of
going about this that i'm missing
entirely?
In your prime function, there are four possible code paths, one of which doesn't return anything. That is what the error message is complaining about. You need to replace:
prime (a, b-1) ;
with:
return prime (a, b-1) ;
in the else if (b>1) case.
Having said that, this is actually not a good way to calculate if a number is prime. The problem is that every recursive call allocates a stack frame and you'll get into serious stack overflow problems if you're trying to work out whether 99,999,999 is a prime number?
Recursion is a very nice tool for a certain subset of problems but you need to be aware of the stack depth. As to more efficient solutions, the are many tests you can carry out to determine a number is not prime, then only check the others with a brute force test.
One thing you should be aware of is to check divisibility against smaller numbers first since this will reduce your search scope quicker. And don't use divide where multiply will do, multiplication is typically faster (though not always).
And some possibly sneaky tricks along the lines of:
every number other than 2 that ends in 2, 4, 6, 8 or 0 is non-prime.
every number other than 5 that ends in 5 is non-prime.
Those two rules alone will reduce your search space by 60%. Assuming you get your test number as a string, it's a simple matter to test the last digit of that string even before converting to an integral type.
There are some more complex rules for divisibility checks. If you take a multiple of 9 and sum all the digits to get a new number, then do it again to that number, then keep going until you have a single digit, you'll find that it's always 9.
That will give you another 10% reduction in search space albeit with a more time-expensive check. Keep in mind that these checks are only advantageous for really large numbers. The advantages are not so great for, say, 32-bit integers since a pre-calculated bitmap would be much more efficient there (see below).
For a simplistic start, I would use the following iterative solution:
public static boolean prime (int num) {
int t = 2;
while (t * t <= num) {
if ((num % t) == 0) {
return false;
}
t++;
}
return true;
}
If you want real speed in your code, don't calculate it each time at all. Calculate it once to create a bit array (one of the sieve methods will do it) of all primes across the range you're interested in, then simply check your values against that bit array.
If you don't even want the cost of calculating the array every time your program starts, do it once and save the bit array to a disk file, loading it as your program starts.
I actually have a list of the first 100 million primes in a file and it's easier and faster for me to use grep to find if a number is prime, than to run some code to calculate it :-)
As to why your algorithm (fixed with a return statement) insists that 7 is not prime, it will insist that every number is non-prime (haven't checked with negative numbers, I'm pretty sure they would cause some serious problems - your first check should probably be if (a < 1) ...).
Let's examine what happens when you call prime(3,3).
First time through, it hits the third condition so calls prime(3,2).
Then it hits the second condition since 3 % (2-1) == 0 is true (N % 1 is always 0).
So it returns false. This could probably be fixed by changing the third condition to else if (b>2) although I haven't tested that thoroughly since I don't think a recursive solution is a good idea anyway.
The following complete code snippet will do what you need although I appreciate your curiosity in wanting to know what you did wrong. That's the mark of someone who's actually going to end up a good code cutter.
public class prime
{
public static boolean isPrime (int num) {
int t = 2;
while (t * t <= num) {
if ((num % t) == 0) {
return false;
}
t++;
}
return true;
}
public static void main (String[] arg)
{
System.out.println (isPrime (7)) ;
}
}
You seem to be under the impression that because the recursion will eventually find a base-case which will hit a return statement, then that return will bubble up through all of the recursive calls. That's not true. Each recursive call must pass out the result like this:
return prime(a, b - 1);
If b is larger than 1, your function won't return anything.
May it be return prime (a, b-1) ; ?
To improve efficiency, think more about your conditions. Do you really need test every factor from 2 to N? Is there a different stopping point that will help tests of prime numbers complete more quickly?
To make a better API, consider making the recursive method private, with a public entry point that helps bootstrap the process. For example:
public static boolean prime(int n) {
return recurse(n, n);
}
private static boolean recurse(int a, int b) {
...
}
Making a method private means that it can't be called from another class. It's effectively invisible to users of the class. The intent here is to hide the "ugly" extra parameter by providing a public helper method.
Think about the factors of some composite numbers. 10 factors to 5×2. 12 factors to 6×2. 14 factors to 7×2. Now think about 25. 25 factors to 5×5. What about 9? Do you see a pattern? By the way, if this isn't homework, please let me know. Being this didactic is hard on me.
In answer to why 7 isn't working, pretend you're the computer and work through your logic. Here's what you wrote.
class prime
{
public static boolean prime (int a, int b)
{
if (a == 0)
{
return false;
}
else if (a%(b-1) == 0)
{
return false;
}
else if (b>1)
{
// Have to add the return statement
// here as others have pointed out!
return prime(a, b-1);
}
else
{
return true;
}
}
public static void main (String[] arg)
{
System.out.println (prime (45, 45)) ;
}
}
So let's start with 7.
if(7 == 0) // not true, don't enter this block
else if(7 % 6 == 0) // not true
else if(7 > 1) // true, call prime(7, 6)
if(7 == 0) // not true, don't enter this block
else if(7 % 5 == 0) // not true
else if(6 > 1) // true, call prime(7, 5)
if(7 == 0) // not true, don't enter this block
else if(7 % 4 == 0) // not true
else if(5 > 1) // true, call prime(7, 4)
... keep going down to calling prime(7, 2)
if(7 == 0) // not true, don't enter this block
else if(7 % 1 == 0) true, return false
When you get down to calling prime(n, 2), it will always return false because you have a logic error.
Your recursive method must return a value so it can unroll.
public static boolean prime (int a, int b)
{
if (a == 0)
{
return false;
}
else if (a%(b-1) == 0)
{
return false;
}
else if (b>1)
{
return prime (a, b-1) ;
}
else
{
return true;
}
}
I might write it a different way, but that is the reason that you are not able to compile the code.
I think the original question was answered already - you need to insert return in the body of else if (b>1) - I just wanted to point out that your code still will crash when given 1 as the value for b, throwing an ArithmeticException since a%(b-1) will be evaluated to a%0, causing a division by zero.
You can avoid this by making the first if-statement if (a == 0 || b == 1) {}
This won't improve the way the program finds primes, it just makes sure there is one less way to crash it.
Similar to #paxdiblo's answer, but slightly more efficient.
public static boolean isPrime(int num) {
if (num <= 1 || (num & 1) == 0) return false;
for (int t = 3; t * t <= num; t += 2)
if (num % t == 0)
return false;
return true;
}
Once it is determined that the number is not even, all the even numbers can be skipped. This will halve the numbers which need to be checked.