My flood fill method:
public void fillNeighbours(int x, int y) {
for(int i = -1; i < 2; i++) {
for(int j = -1; j < 2; j++) {
try {
visible[x+i][y+j] = true;
if(num[x+i][y+j] == 0) {
fillNeighbours(x+i, y+j);
}
} catch (ArrayIndexOutOfBoundsException ignored) {}
}
}
}
That catch (ArrayIndexOutOfBoundsException ignored) {} is there for avoiding x and/or y position from going outside of the array. Array size is 30 by 30. I'm making minesweeper like game. So you may know why I need this method and how it should work. If u don't know what minesweeper is then here is quick video about that game: Introduction to minesweeper
The code revisits fields which are set to visible already.
Try something like
if(!visible[x+i][y+j]){
visible[x+i][y+j] = true;
if(num[x+i][y+j] == 0) {
fillNeighbours(x+i, y+j);
}
}
It looks like you're recursively calling fillNeighbours without any break out clause (base case) so the calls fill the stack.
From
Wikistack
The tree laws of recursion are
A recursive algorithm must have a base case.
A recursive algorithm
must change its state and move toward the base case.
A recursive
algorithm must call itself, recursively.
Once fillNeighbours finds a cell and calls itself, the next loop will always call another when i and j are equal to zero. So it will never exit, and crash once the stack is full.
Aside from that, it will produce a very deep tree as it is not keeping track of which cells have been recursed, and will call fillNeighbours on the same cell multiple times.
I have some code that I would like to make more efficient by recursion. Trouble is I don't know where to start. The code compares two arraylists a and b to see if they are equal. Assume the sizes of both arrays are equal.
The code is
public boolean isEqual(A B) {
boolean answer = false;
if (lessThanOrEqualTo(B) == true);
for (int i = 0; i < DList.size(); i++) {
if (DList.get(i) == B.DList.get(i)) answer = true;
else answer = false;
}
return answer;
}
I have currently written
public boolean isEqualRecursion(A B) {
if DList.size() == 0;
return false();
} else {
}
I know the stopping case is 0 as when size is 0 nothing happens. I have no idea what to write next
Any help will be appreciated
Thanks
I have some code that I would like to make more efficient by recursion.
It is unlikely that you can make it more efficient by recursion. The chances are that it will be less efficient, and also fragile. This is because standard Java compilers don't implement tail-call optimization. The fragility occurs because a recursive comparison algorithm is liable to trigger a stack overflow if the input arrays are large enough.
However, if you want to continue with this as "an exercise", then my HINT is to add an index argument to the isEqualRecursion signature ...
I think that this is a pretty good start for you. This looks through all your elements, assuming they are an array, and then checks if they are equal in size.
public boolean isEqual(ArrayList<?> a, ArrayList<?> b) {
if (a.size() != b.size())
return false;
for (int i = 0; i < a.size(); i++) {
if (!isEqual((ArrayList<?>)a.get(i), (ArrayList<?>)b.get(i))) {
return false;
}
}
return true;
}
Now a couple of things to consider:
This assumes that the content of a(and b) must be an ArrayList at line (ArrayList<?>)a.get(i) what if our ArrayList actually contains something else, like an Integer?
What if our array lists contain null as an item?
What if we pass in two null ArrayLists? (or even just one?)
I'm not sure the point of your function lessThanOrEqualTo(B) is this part of the question or did you write this down wrong?
Also what is a DList?
This is a typical recursion question. You might want to try something like this:
int x = 0;
if(Dlist.get(x) != B.Dlist.get(x)) {
return false;
} else {
x+1;
}
if( x!= dList.size()) {
recursion;
}
return true;
I implemented a SuDoku backtracking algorithm, but it keeps giving me StackOverflow Error. Any other methods or algorithms to avoid this, because I can't get my head around forming a loop for this.
public boolean guess(int istart){
int i=istart, j=0; boolean found=false;
for(i=istart; i<9; i++){//find first empty cell
for(j=0; j<9; j++){
if(get(i,j)==0) {found=true; break;}
}
if(found) break;
}
boolean[] pos=pos_copy;//pos_copy is a length-9 boolean array with all elements set to true
for(int n=0; n<9; n++){//store all possible numbers in pos[]
if(get(i,n)!=0) pos[get(i,n)-1]=false;
if(get(n,j)!=0) pos[get(n,j)-1]=false;
if(get(start[i]+n/3, start[j]+n%3)!=0) pos[get(start[i]+n/3, start[j]+n%3)-1]=false;
}
for(int n=0; n<9; n++) if(pos[n]) {
set(i,j,n+1);
if(i==8 && j==8) return true;
if(guess(i)) return true;//recurse; gives Stackoverflow at this line
}
set(i,j,0);
return false;
}
There is no (realistic) way to put this in a loop, but you can circumvent the recursion using a Dequeue approach (in the form of a stack).
First create a class that holds the current state of numbers entered into the Sodoku-field. Then instead of calling set(...) create a copy of that field and set the value in that copy. Then put that copy in a Dequeue and terminate the function.
Your search loop then becomes:
SodokuField field;
while (((field = dequeue.pollLast()) != null) && (field.isComplete() == false)) {
guess(field);
}
if (field != null) {
showSolution(field);
}
This approach has two benefits: first you won't get any StackOverflowException anymore, and second: you can easily put the code part above in the run() method of a Runnable and have multiple threads wait on a ConcurrentLinkedDeque.
Note: it is important to work stack-based, as otherwise you would create every possible combination of fields before finding the solution and therefore very soon run into memory issues.
i have recrusive function which works fine. The problem is it gives stackoverflow error when the number of lines are huge. I want to put it in iterative, probably using a for loop. Need some help in doing it.
private TreeSet validate(int curLine, TreeSet errorSet) {
int increment = 0;
int nextLine = 0;
if (curLine == lines.length || errorSet.size() != 0) {
return errorSet;
} else {
String line = lines[curLine];
//validation starts. After validation, line is incremented as per the requirements
increment = 1 //As per requirement. Depends on validation results of the line
if (increment > 0) {
try{
Thread.currentThread().sleep(100);
}catch(Exception ex){
System.out.println(ex);
}
nextLine = (curLine + increment);
validate(nextLine, errorSet);
}
}
return errorSet;
}
Poster's description of the method:
The method does validates textlines, these lines has instructions of how much line has to be skipped, if the line is valid. So, if the line is valid that many of lines will be skipped using the increment. if the line is not valid increment will be 0.
I'm not sure why this was ever recursive in the first place. This is perfectly suited for the use of a FOR loop. use something like so:
private TreeSet validate(int curLine, TreeSet errorSet) {
int increment = 0;
if (errorSet.size() != 0)
return errorSet;
for (int curLine = 0; curLine < lines.Length; curLine += increment)
{
// put your processing logic in here
// set the proper increment here.
}
}
If the increment is always going to be 1, then you can just use curr++ instead of curLine += increment
for(String line : lines) {
// validate line here
if(!errorSet.isEmpty()) {
break;
}
}
The solution for your problem could be simple for loop or while, with logical expression for stop condition. Typically we use for loop when we have to pass through all elements of Iterable or array. In case when we are not aware how many loops we are going to do we use a while loop. Advantage of for loop over while, is that we for free have localized variables so we ca not use them out side of the loop, therefore we reduce possibility to have some bug.
You problem is that you have to break the program on two conditions:
When errorSet is not empty.
When the array of lines have no longer items.
As contradiction, we can say that your program should continue:
Until errorSet is empty,
and until line number is smaller than array size where they are stored.
This provide us to simply expression
errorSet.isEmpty()
lineNumber < lines.length()
We can combine them using logical operator && and use as a stop rule in for loop.
for(int lineNumber= 0; errorSet.isEmpty() && lineNumber< lines.length(); lineNumber++) {
//code to operate
}
Note:
Typically for logical expression is used operator &&, that assure that every part of the logical expression is evaluated. An alternative for this is &, that in case of false do not operate longer and return false. We could be tempted to use this operator for this expression but i would be bad idea. Because when we would traversed all lines without error code will generate IndexOutOfBoundException, if we switch the places then we would not have any optimization as first expression would be evaluated same number of times.
I have a loop that looks something like this:
for (int i = 0; i < max; i++) {
String myString = ...;
float myNum = Float.parseFloat(myString);
myFloats[i] = myNum;
}
This is the main content of a method whose sole purpose is to return the array of floats. I want this method to return null if there is an error, so I put the loop inside a try...catch block, like this:
try {
for (int i = 0; i < max; i++) {
String myString = ...;
float myNum = Float.parseFloat(myString);
myFloats[i] = myNum;
}
} catch (NumberFormatException ex) {
return null;
}
But then I also thought of putting the try...catch block inside the loop, like this:
for (int i = 0; i < max; i++) {
String myString = ...;
try {
float myNum = Float.parseFloat(myString);
} catch (NumberFormatException ex) {
return null;
}
myFloats[i] = myNum;
}
Is there any reason, performance or otherwise, to prefer one over the other?
Edit: The consensus seems to be that it is cleaner to put the loop inside the try/catch, possibly inside its own method. However, there is still debate on which is faster. Can someone test this and come back with a unified answer?
PERFORMANCE:
There is absolutely no performance difference in where the try/catch structures are placed. Internally, they are implemented as a code-range table in a structure that is created when the method is called. While the method is executing, the try/catch structures are completely out of the picture unless a throw occurs, then the location of the error is compared against the table.
Here's a reference: http://www.javaworld.com/javaworld/jw-01-1997/jw-01-hood.html
The table is described about half-way down.
Performance: as Jeffrey said in his reply, in Java it doesn't make much difference.
Generally, for readability of the code, your choice of where to catch the exception depends upon whether you want the loop to keep processing or not.
In your example you returned upon catching an exception. In that case, I'd put the try/catch around the loop. If you simply want to catch a bad value but carry on processing, put it inside.
The third way: You could always write your own static ParseFloat method and have the exception handling dealt with in that method rather than your loop. Making the exception handling isolated to the loop itself!
class Parsing
{
public static Float MyParseFloat(string inputValue)
{
try
{
return Float.parseFloat(inputValue);
}
catch ( NumberFormatException e )
{
return null;
}
}
// .... your code
for(int i = 0; i < max; i++)
{
String myString = ...;
Float myNum = Parsing.MyParseFloat(myString);
if ( myNum == null ) return;
myFloats[i] = (float) myNum;
}
}
All right, after Jeffrey L Whitledge said that there was no performance difference (as of 1997), I went and tested it. I ran this small benchmark:
public class Main {
private static final int NUM_TESTS = 100;
private static int ITERATIONS = 1000000;
// time counters
private static long inTime = 0L;
private static long aroundTime = 0L;
public static void main(String[] args) {
for (int i = 0; i < NUM_TESTS; i++) {
test();
ITERATIONS += 1; // so the tests don't always return the same number
}
System.out.println("Inside loop: " + (inTime/1000000.0) + " ms.");
System.out.println("Around loop: " + (aroundTime/1000000.0) + " ms.");
}
public static void test() {
aroundTime += testAround();
inTime += testIn();
}
public static long testIn() {
long start = System.nanoTime();
Integer i = tryInLoop();
long ret = System.nanoTime() - start;
System.out.println(i); // don't optimize it away
return ret;
}
public static long testAround() {
long start = System.nanoTime();
Integer i = tryAroundLoop();
long ret = System.nanoTime() - start;
System.out.println(i); // don't optimize it away
return ret;
}
public static Integer tryInLoop() {
int count = 0;
for (int i = 0; i < ITERATIONS; i++) {
try {
count = Integer.parseInt(Integer.toString(count)) + 1;
} catch (NumberFormatException ex) {
return null;
}
}
return count;
}
public static Integer tryAroundLoop() {
int count = 0;
try {
for (int i = 0; i < ITERATIONS; i++) {
count = Integer.parseInt(Integer.toString(count)) + 1;
}
return count;
} catch (NumberFormatException ex) {
return null;
}
}
}
I checked the resulting bytecode using javap to make sure that nothing got inlined.
The results showed that, assuming insignificant JIT optimizations, Jeffrey is correct; there is absolutely no performance difference on Java 6, Sun client VM (I did not have access to other versions). The total time difference is on the order of a few milliseconds over the entire test.
Therefore, the only consideration is what looks cleanest. I find that the second way is ugly, so I will stick to either the first way or Ray Hayes's way.
While performance might be the same and what "looks" better is very subjective, there is still a pretty big difference in functionality. Take the following example:
Integer j = 0;
try {
while (true) {
++j;
if (j == 20) { throw new Exception(); }
if (j%4 == 0) { System.out.println(j); }
if (j == 40) { break; }
}
} catch (Exception e) {
System.out.println("in catch block");
}
The while loop is inside the try catch block, the variable 'j' is incremented until it hits 40, printed out when j mod 4 is zero and an exception is thrown when j hits 20.
Before any details, here the other example:
Integer i = 0;
while (true) {
try {
++i;
if (i == 20) { throw new Exception(); }
if (i%4 == 0) { System.out.println(i); }
if (i == 40) { break; }
} catch (Exception e) { System.out.println("in catch block"); }
}
Same logic as above, only difference is that the try/catch block is now inside the while loop.
Here comes the output (while in try/catch):
4
8
12
16
in catch block
And the other output (try/catch in while):
4
8
12
16
in catch block
24
28
32
36
40
There you have quite a significant difference:
while in try/catch breaks out of the loop
try/catch in while keeps the loop active
I agree with all the performance and readability posts. However, there are cases where it really does matter. A couple other people mentioned this, but it might be easier to see with examples.
Consider this slightly modified example:
public static void main(String[] args) {
String[] myNumberStrings = new String[] {"1.2345", "asdf", "2.3456"};
ArrayList asNumbers = parseAll(myNumberStrings);
}
public static ArrayList parseAll(String[] numberStrings){
ArrayList myFloats = new ArrayList();
for(int i = 0; i < numberStrings.length; i++){
myFloats.add(new Float(numberStrings[i]));
}
return myFloats;
}
If you want the parseAll() method to return null if there are any errors (like the original example), you'd put the try/catch on the outside like this:
public static ArrayList parseAll1(String[] numberStrings){
ArrayList myFloats = new ArrayList();
try{
for(int i = 0; i < numberStrings.length; i++){
myFloats.add(new Float(numberStrings[i]));
}
} catch (NumberFormatException nfe){
//fail on any error
return null;
}
return myFloats;
}
In reality, you should probably return an error here instead of null, and generally I don't like having multiple returns, but you get the idea.
On the other hand, if you want it to just ignore the problems, and parse whatever Strings it can, you'd put the try/catch on the inside of the loop like this:
public static ArrayList parseAll2(String[] numberStrings){
ArrayList myFloats = new ArrayList();
for(int i = 0; i < numberStrings.length; i++){
try{
myFloats.add(new Float(numberStrings[i]));
} catch (NumberFormatException nfe){
//don't add just this one
}
}
return myFloats;
}
As already mentioned, the performance is the same. However, user experience isn't necessarily identical. In the first case, you'll fail fast (i.e. after the first error), however if you put the try/catch block inside the loop, you can capture all the errors that would be created for a given call to the method. When parsing an array of values from strings where you expect some formatting errors, there are definitely cases where you'd like to be able to present all the errors to the user so that they don't need to try and fix them one by one.
If its an all-or-nothing fail, then the first format makes sense. If you want to be able to process/return all the non-failing elements, you need to use the second form. Those would be my basic criteria for choosing between the methods. Personally, if it is all-or-nothing, I wouldn't use the second form.
As long as you are aware of what you need to accomplish in the loop you could put the try catch outside the loop. But it is important to understand that the loop will then end as soon as the exception occurs and that may not always be what you want. This is actually a very common error in Java based software. People need to process a number of items, such as emptying a queue, and falsely rely on an outer try/catch statement handling all possible exceptions. They could also be handling only a specific exception inside the loop and not expect any other exception to occur.
Then if an exception occurs that is not handled inside the loop then the loop will be "preemted", it ends possibly prematurely and the outer catch statement handles the exception.
If the loop had as its role in life to empty a queue then that loop very likely could end before that queue was really emptied. Very common fault.
My perspective would be try/catch blocks are necessary to insure proper exception handling, but creating such blocks has performance implications. Since, Loops contain intensive repetitive computations, it is not recommended to put try/catch blocks inside loops. Additionally, it seems where this condition occurs, it is often "Exception" or "RuntimeException" which is caught. RuntimeException being caught in code should be avoided. Again, if if you work in a big company it's essential to log that exception properly, or stop runtime exception to happen. Whole point of this description is PLEASE AVOID USING TRY-CATCH BLOCKS IN LOOPS
In your examples there is no functional difference. I find your first example more readable.
You should prefer the outer version over the inner version. This is just a specific version of the rule, move anything outside the loop that you can move outside the loop. Depending on the IL compiler and JIT compiler your two versions may or may not end up with different performance characteristics.
On another note you should probably look at float.TryParse or Convert.ToFloat.
If you put the try/catch inside the loop, you'll keep looping after an exception. If you put it outside the loop you'll stop as soon as an exception is thrown.
I's like to add my own 0.02c about two competing considerations when looking at the general problem of where to position exception handling:
The "wider" the responsibility of the try-catch block (i.e. outside the loop in your case) means that when changing the code at some later point, you may mistakenly add a line which is handled by your existing catch block; possibly unintentionally. In your case, this is less likely because you are explicitly catching a NumberFormatException
The "narrower" the responsibility of the try-catch block, the more difficult refactoring becomes. Particularly when (as in your case) you are executing a "non-local" instruction from within the catch block (the return null statement).
If you want to catch Exception for each iteration, or check at what iteration Exception is thrown and catch every Exceptions in an iteration, place try...catch inside the loop. This will not break the loop if Exception occurs and you can catch every Exception in each iteration throughout the loop.
If you want to break the loop and examine the Exception whenever thrown, use try...catch out of the loop. This will break the loop and execute statements after catch (if any).
It all depends on your need. I prefer using try...catch inside the loop while deploying as, if Exception occurs, the results aren't ambiguous and loop will not break and execute completely.
setting up a special stack frame for the try/catch adds additional overhead, but the JVM may be able to detect the fact that you're returning and optimize this away.
depending on the number of iterations, performance difference will likely be negligible.
However i agree with the others that having it outside the loop make the loop body look cleaner.
If there's a chance that you'll ever want to continue on with the processing rather than exit if there an invalid number, then you would want the code to be inside the loop.
If it's inside, then you'll gain the overhead of the try/catch structure N times, as opposed to just the once on the outside.
Every time a Try/Catch structure is called it adds overhead to the execution of the method. Just the little bit of memory & processor ticks needed to deal with the structure. If you're running a loop 100 times, and for hypothetical sake, let's say the cost is 1 tick per try/catch call, then having the Try/Catch inside the loop costs you 100 ticks, as opposed to only 1 tick if it's outside of the loop.
The whole point of exceptions is to encourage the first style: letting the error handling be consolidated and handled once, not immediately at every possible error site.
put it inside. You can keep processing (if you want) or you can throw a helpful exception that tells the client the value of myString and the index of the array containing the bad value. I think NumberFormatException will already tell you the bad value but the principle is to place all the helpful data in the exceptions that you throw. Think about what would be interesting to you in the debugger at this point in the program.
Consider:
try {
// parse
} catch (NumberFormatException nfe){
throw new RuntimeException("Could not parse as a Float: [" + myString +
"] found at index: " + i, nfe);
}
In the time of need you will really appreciate an exception like this with as much information in it as possible.
That depends on the failure handling. If you just want to skip the error elements, try inside:
for(int i = 0; i < max; i++) {
String myString = ...;
try {
float myNum = Float.parseFloat(myString);
myFloats[i] = myNum;
} catch (NumberFormatException ex) {
--i;
}
}
In any other case i would prefer the try outside. The code is more readable, it is more clean. Maybe it would be better to throw an IllegalArgumentException in the error case instead if returning null.
I'll put my $0.02 in. Sometimes you wind up needing to add a "finally" later on in your code (because who ever writes their code perfectly the first time?). In those cases, suddenly it makes more sense to have the try/catch outside the loop. For example:
try {
for(int i = 0; i < max; i++) {
String myString = ...;
float myNum = Float.parseFloat(myString);
dbConnection.update("MY_FLOATS","INDEX",i,"VALUE",myNum);
}
} catch (NumberFormatException ex) {
return null;
} finally {
dbConnection.release(); // Always release DB connection, even if transaction fails.
}
Because if you get an error, or not, you only want to release your database connection (or pick your favorite type of other resource...) once.
Another aspect not mentioned in the above is the fact that every try-catch has some impact on the stack, which can have implications for recursive methods.
If method "outer()" calls method "inner()" (which may call itself recursively), try to locate the try-catch in method "outer()" if possible. A simple "stack crash" example we use in a performance class fails at about 6,400 frames when the try-catch is in the inner method, and at about 11,600 when it is in the outer method.
In the real world, this can be an issue if you're using the Composite pattern and have large, complex nested structures.