i'm trying to get the values of an array randomly but i'm getting an error
here is my code so far:
NSMutableArray *validMoves = [[NSMutableArray alloc] init];
for (int i = 0; i < 100; i++){
[validMoves removeAllObjects];
for (TileClass *t in tiles ) {
if ([self blankTile:t] != 0) {
[validMoves addObject:t];
}
}
NSInteger pick = arc4random() % validMoves.count;
[self movePiece:(TileClass *)[validMoves objectAtIndex:pick] withAnimation:NO];
}
The error you're getting (an arithmetic exception) is because validMoves is empty and this leads to a division by zero when you perform the modulus operation.
You have to explicitly check for the case of an empty validMoves array.
Also you should use arc4random_uniform for avoiding modulo bias.
if (validMoves.count > 0) {
NSInteger pick = arc4random_uniform(validMoves.count);
[self movePiece:(TileClass *)[validMoves objectAtIndex:pick] withAnimation:NO];
} else {
// no valid moves, do something reasonable here...
}
As a final remark not that arc4random_uniform(0) returns 0, therefore such case should be avoided or you'll be trying to access the first element of an empty array, which of course will crash your application.
Related
I have two arrays: array1 and array2. Each object of arrays is an array too (2D arrays). In this way I multiple them. So how I have big arrays I use dispatch_apply. Every time i receive different results include a right result. Maybe somebody knows how to fix it?
dispatch_apply([array2 count], queue, ^(size_t j)
{
k = 0;
for (int l = 0; l < [[array1 objectAtIndex:0] count]; l++) {
k += [[[array1 objectAtIndex:i] objectAtIndex:l] intValue] *
[[[array2 objectAtIndex:j] objectAtIndex:l] intValue];
}
kNSNumber = [NSNumber numberWithInt:k];
[multipliedArrayInto replaceObjectAtIndex:j withObject:kNSNumber];
});
[resulArray insertObject:multipliedArrayInto atIndex:i];
}
There's two things, I can suggest, and I bet one of them (or both) is the overarching solution to your problem.
First, I would declare k local to the block, so there would be no question that you are overwriting it or not. You likely have the same problem with kNSNumber inside the block. If you are just using that NSNumber instance to slam into the multipliedArrayInto accumulator, you may as well remove kNSNumber, and use #(k) in it's place (if only to be more readable). Similarly, make sure multipliedArrayInto is declared just before the dispatch_apply, in what looks like an outer for loop (where ever i is coming from). And finally, make sure resulArray is instantiated, or otherwise readied just before that outer for loop.
Second, is queue a concurrent or serial queue? If you are using dispatch_apply like a parallel-executing for/enumeration -- which is likely, I think, so you are taking about handling "big arrays" efficiently -- then you are practically guaranteeing that k is being overwritten. If you change it to serial, it may work as designed. If you want it to be parallel, you will need to move the declaration of your k accumulator inside the block, and make sure the declaration of other variables makes sense, too.
Update to reflect question updates:
#antonytonies ideally, your followup answer on this thread should be moved into the question itself, so that people can follow this thread easier.
So, it looks like what I described is exactly your problem.
The global queues are all concurrent queues, which means that (hypothetically) all the dispatch blocks are executing at once, and the contents of k and other variables are getting blown away depending on how the order of the blocks executes.
I've taken your update (in the "answer" you added), and modified it to probably work:
// I renamed your method, because nameless parameters pain me. This is cosmetic, and doesn't
// matter for the problem at hand.
- (NSMutableArray *)multiplicationArrays:(NSMutableArray *)array vector:(NSMutableArray *)vector
{
// IMHO, you want to set resultArray to nil here. Another option is to set it to nil in the
// else case, below. Properties in Objective-C are initalized to nil,0,false,etc; you can
// rely on ARC to initialize pointer to objc objects on the stack, too. However, someone
// reading this code may or may not know that. IMHO, using the explicitly assignement makes it
// clear that you're going to be returning `nil` or an instance of `NSMutableArray`.
NSMutableArray *resultArray = nil;
if ([[array objectAtIndex:0] count] == [vector count]) {
// Nicely done w/ pre-allocating the result array here, so that there's no question
// of the indexes matches the results later on.
resultArray = [[NSMutableArray alloc] initWithCapacity:[array count]];
for (int i=0; i < [array count]; i++) {
[resultArray insertObject:[NSNull null] atIndex:i];
}
// 'queue' here is a concurrent queue. This means that you are proclaiming to the runtime
// that the blocks being executed are able to operate correctly w/o interference from each
// other. This is also thought of in terms of parallel execution: all these blocks may run
// **at once**. This *also* means, that you must not share storage between them.
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_apply([array count], queue, ^(size_t j) {
// Moved 'result' inside the block.
NSInteger result = 0;
for (int l = 0; l < [[array objectAtIndex:0] count]; l++) {
// These array reads are **NOT** thread safe. They probably don't cause must trouble in
// practice, but you may want to reconfigure this.
result += [[[array objectAtIndex:j] objectAtIndex:l] intValue] * [[vector objectAtIndex:l] intValue];
}
// The replace of the object into resultArray is **NOT** thread-safe.
// This probably hasn't caused you much trouble, since you can guarantee that
// you aren't writing at the same index. However, I would strongly suggest to
// change this to be thread-safe.
[resultArray replaceObjectAtIndex:j withObject:#(result)];
});
}
else {
NSLog(#"matrix count isn't correspond");
}
return resultArray;
}
Finally: consider just using Apple's Accelerate framework for this sort of problem solving. It's available on OSX and iOS, so you should have all of your bases covered.
it's the same thing if I multiple 2D-array and vector
-(NSMutableArray*)multiplicationArraysWithVector:(NSMutableArray *)array :(NSMutableArray *)vector
{
NSMutableArray* resultArray;
if ([[array objectAtIndex:0] count] == [vector count])
{
resultArray = [[NSMutableArray alloc] initWithCapacity:[array count]];
for (int i=0; i < [array count]; i++) {
[resultArray insertObject:[NSNull null] atIndex:i];
}
__block NSInteger result;
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_apply([array count], queue, ^(size_t j)
{
result = 0;
for (int l = 0; l < [[array objectAtIndex:0] count]; l++) {
result += [[[array objectAtIndex:j] objectAtIndex:l] intValue] * [[vector objectAtIndex:l]intValue];
}
[resultArray replaceObjectAtIndex:j withObject:#(result)];
});
}
else
{
NSLog(#"matrix count isn't correspond");
}
return resultArray;
}
In this case I can get a right or wrong data result.
I'm getting the error "undeclared identifier" on the commented line:
- (BOOL) isInIntArray:(NSInteger[])array theElem:(int)elem{
int i = 0;
NSInteger sizeOfArray = (sizeof array) / (sizeof array[0]);
while(i < sizeOfArray){
if(array[i] == elem){
return TRUE;
}
i++;
}
return FALSE;
}
- (int)getNextUnusedID{
int i = rand()%34;
while ([isInIntArray:idsUsed theElem:i]) { //here: Use of undeclared identifier 'isInIntArray'
i = rand()%34;
}
return i;
}
I really don't understand why, they are in the same .m file.
Why would that be?
Also, this code:
NSInteger sizeOfArray = (sizeof array) / (sizeof array[0]);
is giving me the warning:
Sizeof on array function will return Sizeof 'NSInteger *' (aka: 'int *') instead of 'NSInteger[]'"
How should I properly determine the size of an array?
It looks like you've missed out self from this line
while ([isInIntArray:idsUsed theElem:i])
This should be:
while ([self isInIntArray:idsUsed theElem:i])
As #CaptainRedmuff pointed out, you are missing the target object in method invocation, that is self.
//[object methodParam:x param:y];
[self isInIntArray:idsUsed theElem:i];
To your second Q. In C language you cannot determine the size of an array. That's why they are not used, since we have objects for this. I recommend you to use these:
NSMutableArray *array = [[NSMutableArray alloc] init]; // to create array
array[0] = #42; // to set value at index, `#` creates objects, in this case NSNumber
[array insertObject:#42 atindex:0]; // equivalent to the above
NSInteger integer = array[0].integerValue; // get the value, call integerMethod to get plain int
integer = [[array objectAtIndex:0] integerValue]; // equivalent to the above
[array containsObject:#42]; // test if given object is in the array
[array indexOfObject:#42]; // get index of object from array, NSNotFound if not found
array.count; // to get the number of objects
Important: These arrays have variable size and they are not limited! But you can access elements only at indexes 0..(n-1) (where n in number of objects) and you can set values only for indexes 0..n.
In other words, you can not do array[3] = #42; for empty array, you need to fill first 3 positions first (indexes 0, 1 and 2).
write this in .h file (declare the function)
- (BOOL) isInIntArray:(NSInteger[])array theElem:(int)elem;
and call the method using following way
while ([self isInIntArray:idsUsed theElem:i]) { //here: Use of undeclared identifier 'isInIntArray'
i = rand()%34;
}
i'm trying to get the values of an array randomly but i'm getting an error
here is my code so far:
NSMutableArray *validMoves = [[NSMutableArray alloc] init];
for (int i = 0; i < 100; i++){
[validMoves removeAllObjects];
for (TileClass *t in tiles ) {
if ([self blankTile:t] != 0) {
[validMoves addObject:t];
}
}
NSInteger pick = arc4random() % validMoves.count;
[self movePiece:(TileClass *)[validMoves objectAtIndex:pick] withAnimation:NO];
}
The error you're getting (an arithmetic exception) is because validMoves is empty and this leads to a division by zero when you perform the modulus operation.
You have to explicitly check for the case of an empty validMoves array.
Also you should use arc4random_uniform for avoiding modulo bias.
if (validMoves.count > 0) {
NSInteger pick = arc4random_uniform(validMoves.count);
[self movePiece:(TileClass *)[validMoves objectAtIndex:pick] withAnimation:NO];
} else {
// no valid moves, do something reasonable here...
}
As a final remark not that arc4random_uniform(0) returns 0, therefore such case should be avoided or you'll be trying to access the first element of an empty array, which of course will crash your application.
i have a strange issue , if i remove my item at forin enumeration , it would crash , so like this:
for (Obstacle *obstacleToTrack in _obstaclesToAnimate) {
//this if else not so important for happening crash
if(obstacleToTrack.distance > 0){
obstacleToTrack.distance -= _playerSpeed * _elapsed;
}else{
if (obstacleToTrack.watchOut) {
obstacleToTrack.watchOut = NO;
}
obstacleToTrack.x -= (_playerSpeed + obstacleToTrack.speed) * _elapsed;
}
if (obstacleToTrack.x < -obstacleToTrack.width || _gameState == GS_OVER) {
[self removeChild:obstacleToTrack];
//this line makes crash happen , if remove this line code work fine
[_obstaclesToAnimate removeObject:obstacleToTrack];
}
}
if i change my code to
NSMutableArray *forRemoving = [[NSMutableArray alloc]init];
for (Obstacle *obstacleToTrack in _obstaclesToAnimate) {
//this if else not so important for happening crash
if(obstacleToTrack.distance > 0){
obstacleToTrack.distance -= _playerSpeed * _elapsed;
}else{
if (obstacleToTrack.watchOut) {
obstacleToTrack.watchOut = NO;
}
obstacleToTrack.x -= (_playerSpeed + obstacleToTrack.speed) * _elapsed;
}
if (obstacleToTrack.x < -obstacleToTrack.width || _gameState == GS_OVER) {
// code change here
[self removeChild:obstacleToTrack];
[forRemoving addObject:obstacleToTrack];
}
}
for(Obstacle *obstacleToTrack in forRemoving){
[_obstaclesToAnimate removeObject:obstacleToTrack];
[forRemoving removeObject:obstacleToTrack];
}
[forRemoving release];
this would work perfect , could someone tell me why?
The answer is that if you remove an object the other objects in that array move postion in the array since an item is removed.
For example we have an array with 4 items, if we remove the first item (item 0) the item that used to be at index 1 is now at index 0 and the item at 2 is now at 1.
Thus the enumeration breaks.
You could solve this by looping thru the array from the count down to 0:
for (int i = [array count]-1; i >= 0; i--) {
id object = [array objectAtIndex:i];
if (some check) {
[array removeObjectAtIndex:i];
}
}
Like rckoenes said, you break the enumaration by removing stuff in the array while iterating through it.
What you can do is to have a second array where you insert the objects that you want to remove. Then, after your enumeration is finished you can remove all the objects that are found in the second array, from your first array.
You must not modify a collection while iterating through its items.
If you iterate index based (i.e. the classical for loop), you can remove things, but be careful about adjusting your index.
Check this link
https://developer.apple.com/library/mac/documentation/cocoa/conceptual/Collections/Articles/Enumerators.html
Enumeration is “safe”—the enumerator has a mutation guard so that if
you attempt to modify the collection during enumeration, an exception
is raised.
I am developing an application for the iPhone. The question I have is how to display a new label with a different text every .5 seconds. For example, it would display Blue, Red, Green, Orange and Purple; one right after one another. Right now I am doing this:
results = aDictionary;
NSArray *myKeys = [results allKeys];
NSArray *sortedKeys = [myKey sortedArrayUsingSelector:#selector(caseInsensitiveCompare:)];
int keyCount = [sortedKeys count];
while (flag == NO) {
NSTimeInterval timeMS = [startDate timeIntervalSinceNow] * -10000.0;
if (timeMS >= i) {
ii++;
i += 1000;
NSLog(#"endDate = %f", timeMS);
int randomNumber = rand() % keyCount + 1;
lblResult.text = [results valueForKey:[sortedKeys objectAtIndex:(randomNumber - 1)]];
result = [results valueForKey:[sortedKeys objectAtIndex:(randomNumber - 1)]];
lblResult.text = result;
}
if (ii > 25) {
flag = YES;
}
}
lblResult.text = [results valueForKey:[sortedKeys objectAtIndex:(sortedKeys.count - 1)]];
this function is called at the viewDidAppear Function and currently isn't displaying the new labels. It only displays the one at the end. Am I doing anything wrong? What would be the best method to approach this?
The problem is that you're not giving the run loop a chance to run (and therefore, drawing to happen). You'll want to use an NSTimer that fires periodically and sets the next text (you could remember in an instance variable where you currently are).
Or use something like this (assuming that items is an NSArray holding your strings):
- (void)updateText:(NSNumber *)num
{
NSUInteger index = [num unsignedInteger];
[label setText:[items objectAtIndex:index]];
index++;
// to loop, add
// if (index == [items count]) { index = 0; }
if (index < [items count]) {
[self performSelector:#selector(updateText:) withObject:[NSNumber numberWithUnsignedInteger:index] afterDelay:0.5];
}
}
At the beginning (e.g. in viewDidAppear:), you could then call
[self updateText:[NSNumber numberWithUnsignedInteger:0]];
to trigger the initial update.
You'd of course need to ensure that the performs are not continuing when your view disappears, you could do this by canceling the performSelector, or if you're using a timer, by simply invalidating it, or using a boolean, or ...
And if you want to get really fancy, use GCD :)