Blocks are fine but what about writing C arrays?
Given this simplified situation:
CGPoint points[10];
[myArray forEachElementWithBlock:^(int idx) {
points[idx] = CGPointMake(10, 20); // error here
// Cannot refer to declaration with an array type inside block
}];
after searching a while found this possible solution, to put it in a struct:
__block struct {
CGPoint points[100];
} pointStruct;
[myArray forEachElementWithBlock:^(int idx) {
pointStruct.points[idx] = CGPointMake(10, 20);
}];
this would work but there is a little limitation I have to create the c array dynamically:
int count = [str countOccurencesOfString:#";"];
__block struct {
CGPoint points[count]; // error here
// Fields must have a constant size: 'variable length array in structure' extension will never be supported
} pointStruct;
How can I access my CGPoint array within a block?
OR
Is it even possible at all or do I have to rewrite the block method to get the full functionality?
Another simple answer which works for me is the following:
CGPoint points[10], *pointsPtr;
pointsPtr = points;
[myArray forEachElementWithBlock:^(int idx) {
pointsPtr[idx] = CGPointMake(10, 20);
}];
Maybe you can allocate the array on the heap?
// Allocates a plain C array on the heap. The array will have
// [myArray count] items, each sized to fit a CGPoint.
CGPoint *points = calloc([myArray count], sizeof(CGPoint));
// Make sure the allocation succeded, you might want to insert
// some more graceful error handling here.
NSParameterAssert(points != NULL);
// Loop over myArray, doing whatever you want
[myArray forEachElementWithBlock:^(int idx) {
points[idx] = …;
}];
// Free the memory taken by the C array. Of course you might
// want to do something else with the array, otherwise this
// excercise does not make much sense :)
free(points), points = NULL;
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 have an NSMutableArray of NSNumbers, I want to enumerate through all of them with Objective-C styled enumeration. Here's what I've done so far.
for ( NSNumber* number in array )
{
//some code
}
I want to be able to recognize the first object fast, I am able to do this of course,
if ( [array indexOfObject:number] == 0 )
{
//if it's the first object
}
Is there any way to do this faster? There's of course the old-fashioned C style way, and remove the object from array first, and then put it back after enumeration. Just want to know if there's a better technic.
You can try using a method that provides the index of the object currently being enumerated:
[array enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
if (idx == 0) {
// this is the first object
}
}];
Or if you simply want to access the first object of an array:
id obj = [array objectAtIndex:0];
or with the new Objective-C style/syntax:
id obj = array[0];
This solution is faster than accessing and comparing the first array element:
BOOL firstIteration = YES;
for (NSNumber *number in array) {
if (firstIteration) {
// Handle first iteration
firstIteration = NO;
}
// Do something
}
In fast enumeration you cant alter the array. So if you want to remove you have to use old style for(;;) loop.
To find the first object simply use [array objectAtIndex:0]
I have seen this over and over, why exactly is it faster to use fast enumeration in loops rather than an NSEnumerator using nextObject:.
NSEnumerator is the old way to enumerate over collections. It involves creating an object to represent the enumeration, then calling a method on it for every single iteration. While this was perfectly serviceable for many years, it's not terribly efficient, as it involves at least one message send for every iteration of the loop. NSFastEnumeration is the more modern approach, which leverages native language support to provide a much more efficient enumeration. The way it works under the hood is it creates a struct that represents the current enumeration state and repeatedly calls -countByEnumeratingWithState:objects:count: on the collection. This method returns a C array of objects in the objects out-param as well as a counter in the count out-param. This allows the caller to then iterate over the C array. In essence, this means one message call per chunk of objects, which, depending on the collection, could be as efficient as a single message call to get all objects.
If you have a bit of code that looks like
for (id obj in myArray) {
[obj doSomething];
}
This gets translated by the compiler into something roughly equivalent to
NSFastEnumerationState __enumState = {0};
id __objects[MAX_STACKBUFF_SIZE];
NSUInteger __count;
while ((__count = [myArray countByEnumeratingWithState:&__enumState objects:__objects count:MAX_STACKBUFF_SIZE]) > 0) {
for (NSUInteger i = 0; i < __count; i++) {
id obj = __objects[i];
[obj doSomething];
}
}
The actual variables used are hidden, and the maximum size of the object buffer is also implementation-dependent, but the basic idea is there. It translates iteration over an obj-c collection into iteration over a C array.
GCC 8.9.4 Fast enumeration
protocol
GNUstep libs/base/trunk/Source/NSEnumerator.m countByEnumeratingWithState:objects:count:
It is not same as Apple's implementation but it is helpful to understand.
- (NSUInteger) countByEnumeratingWithState: (NSFastEnumerationState*)state
objects: (id*)stackbuf
count: (NSUInteger)len
{
IMP nextObject = [self methodForSelector: #selector(nextObject)];
int i;
state->itemsPtr = stackbuf;
state->mutationsPtr = (unsigned long*)self;
for (i = 0; i < len; i++)
{
id next = nextObject(self, #selector(nextObject));
if (nil == next)
{
return i;
}
*(stackbuf+i) = next;
}
return len;
}
NSArray *array = something;
array = { {1,2}, {2,3}, {3,4} }
that means array is an array of array. so how can you access all the arrays and their values.
we can use for loop like this
for (int i = 0; i < array.count; i++)
{
NSArray x = [array objectAtIndex:i];
}
or a fast enum works like this
for(NSArray array2 in array)
{
// do what ever you want with this new array2.
}
this is a sample example.
PS. I forgot how the array looks in console.
I have created a few sprites using a spriteclass and I have loaded them into an array. In my app, I loop over the array checking for particular conditions (position, etc.). I want to create an explosion method that I can pass one of these objects to and then using the pointer pull the position of the object on the screen and show an explosion there. I don't know how to pass the pointer/object that is in my array to the method to be used.
Here is essentially what I had in mind:
for (int i=1; i<4; i++) {
EnemySprite *currentenemy = [enemies objectAtIndex:i-1];
//Blow this guy up
[self explosion:currentenemy]
}
-(void)explosion someobject {
explosion.position = someobject.position
someobject.setHidden=YES;
}
You would write it like this for one param
// definition
-(void) explosion:(EnemySprite*) someObject
{
}
// usage
[self explosion: object];
For two params things are a bit more involved. Consider;
// definition
-(void) explosion:(EnemySprite*) someObject radius:(float)explosionRadius
{
...
if (pos < explosionRadius)
...
}
// usage
[self explosion: object radius:10.0f];
Everything before the : is used for the external name, everything after is the name internal to the function.
This is why you will often see Objective-C functions written with function names that end with the name of the first type:
-(void) explodeSprite:(EnemySprite*) sprite radius:(float)radius;
Both the sprite and radius params then appear to be "named" when the function is written;
[self explodeSprite:sprite radius:10.0f];
Why not make your object the receiver of the explosion?
for (id currentenemy in enemies)
{
[currentenemy explode];
}