I'm writing for the iPhone and I'm trying to measure the time between maximum accelerations. Imagine you've attached your iPhone to an oscillating spring. What I want to do is measure the frequency of the oscillation. The way I am going about doing this is storing a certain number acceleration values (updated 20 times per second) in an NSMutableArray and comparing the current acceleration value with previous acceleration values.
How do I add each updated acceleration value to the NSMutableArray and at the same time delete values from the back of the array as I don't need them any more?
Thanks in advance!
Are NSMutableArrays -addObject:/-insertObject:atIndex: and -removeLastObject/-removeObjectAtIndex: what you're looking for? See this one for more information.
Think of the array as a wheel. Once you have filled it up, use a rotating index for the replace location.
myIndex = ( ( myIndex + 1 ) % myCount );
[myArray replaceObjectAtIndex:myIndex withObject:newValue];
You can initialize it with a bunch of [NSNull null] objects that you will replace, so you do not need other special processing the first time through.
Once you have finished collecting data, organize the array to have the appropriate element first if it simplifies other operations.
NSRange range = NSMakeRange( 0 , myIndex );
NSArray *temp = [myArray subarrayWithRange:range];
[myArray removeObjectsInRange:range];
[myArray addObjectsFromArray:temp];
Related
I am not sure about how NSSet's anyObject work. What does it mean that "The object returned is chosen at the set’s convenience" (from the NSSet class reference) ?
Further, how can I best extract objects randomly from a NSSet? I was thinking about getting allObjects in an array and then myArray[arc4random_uniform(x)] where x is the number of objects in the array.
Quote from NSSet Class Reference:
The object returned is chosen at the set’s convenience—the selection is not guaranteed to be random.
For "randomness", convert the NSSet to an NSArray using [theSet allObjects].
Next, pick any object randomly using arc4random_uniform().
Usually, NSSet instances are created with a CFHash backing, so they almost always return the first object in that hash, as it is the fastest to look up. The reason it says
The object returned is chosen at the set’s convenience—the selection is not guaranteed to be random.
Is because you don't always know it will have a backing array. For all you know, the NSSet instance you have has a NSDictionary backing it, or some other similar data structure.
So, in conclusion, if you need a random object from a NSSet, don't use -anyObject, instead use allObjects: and then shuffle that array.
The documentation reads that anyObject returns
One of the objects in the set, or nil if the set contains no objects.
The object returned is chosen at the set’s convenience—the selection
is not guaranteed to be random.
Most likely there is some deterministic algorithm at work.
The most reliable thing to do would be, as you suggest, to create an NSArray using the NSSet method allObjects, and then choose a random element from that with arc4random() % N where N is the count of the NSArray.
I use arc4random() and two mutable arrays to get a random and unique set of objects:
NSMutableArray *selectionPool = ...;
int numberOfObjectsToSelect = x;
NSMutableArray *selectedObjects = [[NSMutableArray alloc] initWithCapacity:numberOfObjectsToSelect];
int modulus = selectionPool.count - 1;
for (int i = 0; i < numberOfObjectsToSelect; i++) {
int j = arc4random() % (modulus--);
[selectedObjects addObject:[selectionPool objectAtIndex:j]];
[selectionPool removeObjectAtIndex:j];
}
I'm not sure how efficient it would be for large collections, but it's worked for me with collections that number in the low 100s of objects.
I am trying to remove objects at array starting at index 5 to the end of the list. I have a for loop to do this, however now I discovered
- (void)removeObject:(id)anObject inRange:(NSRange)aRange
The question is what is the anObject here? I only need range as far as I know
removeObject:inRange: deletes an object within a certain range. This method would be useful if you wanted delete the string #"Hello World" only if it is one of the first 5 elements.
It sounds like what you are trying to do is delete all objects after the 5th element. If that is what you are trying to do, you should use the removeObjectsInRange: method. For example:
NSRange r;
r.location = 5;
r.length = [someArray count]-5;
[someArray removeObjectsInRange:r];
You want
- (void)removeObjectsInRange:(NSRange)aRange
Removes from the array each of the objects within a given range.
I am working on a small isometric engine for my next iPhone game. To store the map cells or tiles I need a 2 dimensionel array. Right now I am faking it with a 1-dimensionel, and it is not good enough anymore.
So from what I have found out looking around the net is that in objective-c I need to make an array of arrays.
So here is my question: How do I dynamicly create arrays at runtime based on how many map-rows I need?
The first array is easy enough:
NSMutableArray *OuterArray = [NSMutableArray arrayWithCapacity:mapSize];
now I have the first array that should contain an array for each row needed.
Problem is, it can be 10 but it can also be 200 or even more. So I dont want to manually create each array and then add it. I am thinking there must be a way to create all these arrays at runtime based on input, such as the chosen mapsize.
Hope you can help me
Thanks in advance
Peter
I think this previous question should help.
2d arrays in objective c
Nothing to do with me. I have never owned an iphone or tried to write code for one.
The whole point of NSMutableArray is that you don't care. Initialize both dimensions with an approximate size and then add to them. If your array grows beyond your initial estimate the backing storage will be increased to accomodate it. This counts for both your columns (first order array), and rows (second order array).
EDIT
Not sure what you meant in your comment. But this is one way to dynamically create a 2-dimensional mutable array in Objective-C.
NSUInteger columns = 25 ; // or some random, runtime number
NSUInteger rows = 50; // once again, some random, runtime number
NSMutableArray * outer = [NSMutableArray arrayWithCapacity: rows];
for(NSUInteger i; i < rows; i++) {
NSMutableArray * inner = [NSMutableArray arrayWithCapacity: columns];
[outer addObject: inner];
}
// Do something with outer array here
NSMutableArray can hold as many elements as you want to add to it. (Based on available heap though).
All you have to do is when you want to add an element(Array) to this mutable array you can add it using the addObject method.
So you create a MutableArray as follows:
NSMutabaleArray *outerArray = [NSMutableArray array]; // initially contains 0 elements.
[outerArray addobject:<anotherArray>];
From your rejection of the other answers I think you don't know how to add them in a loop, or am I wrong?
Try:
for (i = 0; i < mapSize; i++)
[outerArray addObject: [[NSMutableArray alloc] init]];
or, if you know or can estimate the size of the second dimension:
for (i = 0; i < mapSize; i++)
[outerArray addObject: [[NSMutableArray alloc]
initWithCapacity: your_2nd_d_size]];
Now, how you fill the arrays, i.e. where you get the contents depends on you. In one or more loops, you do:
[(NSMutableArray *)[outerArray objectAtIndex: i]
addObject: your_current_object];
My aim is to produce an array, which I can use to add section headers for a UITableView. I think the easiest way to do this, is to produce a sections array.
I want to create section headers for dates, where I'll have several or no rows for each.
So in my populate data array function, I want to populate a display array. So record 1, look for the first date in my display array, create a new array item if it doesn't exist, if it does exist add 1 to the count.
So I should end up with something like this.
arrDisplay(0).description = 1/June/2001; arrDisplay(0).value = 3;
arrDisplay(1).description = 2/June/2001; arrDisplay(1).value = 0;
arrDisplay(2).description = 3/June/2001; arrDisplay(2).value = 1;
arrDisplay(3).description = 5/June/2001; arrDisplay(3).value = 6;
My question is how do I create and use such an array with values, where I can add new elements of add to the count of existing elements and search for existing elements ?
I think, if i understand you, an NSMutableDictionary would work. (as NR4TR said) but, i think the object would be the description and the key would be the count. you could check for the key and get the count in the same gesture. if the return value of objectForKey is nil, it doesn't exist.
NSMutableDictionary *tableDictionary = [[NSMutableDictionary alloc] init];
NSString *displayKey = #"1/June/2001";
NSNumber *displayCount = [tableDictionary objectForKey:displayKey];
if (displayCount != nil) {
NSNumber *incrementedCount = [[NSNumber alloc] initWithInteger:[displayCount integerValue] + 1];
[tableDictionary removeObjectForKey:displayKey];
[tableDictionary setValue:incrementedCount
forKey:displayKey];
[incrementedCount release];
}
else {
NSNumber *initialCount = [[NSNumber alloc] initWithInteger:1];
[tableDictionary setValue:initialCount
forKey:displayKey];
[initialCount release];
}
EDIT: Hopefully this isn't pedantic, but I think a couple pointers will help.
Dictionaries, Sets, and Arrays all hold objects for retrieval. The manner of holding and retrieval desired drives the decision. I think of it based on the question 'what is the nature of the information that I have when I need an object being held?'
NSDictionary and NSMutableDictionary
Hold n objects per key. (I think...I haven't had to test a limit, but i know you can get an NSSet back as a value.)
KEY is more important than INDEX. I don't think of dictionaries as ordered. they know something and you need to ask the correct question.
NSArray and NSMutableArray
hold n objects in order.
INDEX is most important bit of information. (you can ask for the index of an object but, even here, the index is the important part)
you will typically drive table views with an array because the ordered nature of the array fits.
NSSet, NSMutableSet, and NSCountedSet
A collection of objects without order.
You can change any of these into the other with something like [nsset setFromArray:myArray];
and all of these things can hold the other as objects. I think an array as your top level is the correct thinking, but beyond that, it becomes an issue of implementation
Try array of dictionaries. Each dictionary contains two objects - section title and array of section rows.
If you want to have a description AND a rowcount then you can either create a class with those two properties and generate an NSArray of objects with that class or instead of all that you can just use an NSDictionary to store key/value lookups.
I think NSCountedSet is closest to what you want. It doesn't have an intrinsic order, but you can get an array out of it by providing a sort order.
I have 2 NSArray (Mutable, actually) that I am trying to convert to a C-style double array for a c routine i am passing them to.
Here is my Objective-C routine:
NSMutableDictionary *childDictionary = [myParentDictionary objectForKey:resort_code];
latitudeArray = [childDictionary objectForKey:#"lat"];
longitudeArray = [childDictionary objectForKey:#"lon"];
int nvert = [latitudeArray count];
double laArray[nvert];
double loArray[nvert];
for(int i=0; i<nvert; i++) {
double dLat = [[latitudeArray objectAtIndex:i]doubleValue];
double dLon = [[longitudeArray objectAtIndex:i]doubleValue];
laArray[i] = dLat;
loArray[i] = dLon;
}
This takes upwards of 3-8 seconds on the 3G iPhone (instantaneous on the simulator -- yet another reason to test on the device )
is there faster way? I have to end up with laArray[i] and loArray[i] as c-style arrays of doubles.
(to expand on the question for the benefit of a commenter):
Each array consists of #"38.448745" (lat) and #"-122.9847684" (lon) style content. I do this cos to be pushed onto an NSArray, the lat and lon need to be objects. I simply used:
[latitudeArray addObject:[NSString stringWithFormat: #"%.10f",dlat]];
[longitudeArray addObject:[NSString stringWithFormat: #"%.10f",dlon]];
I suppose I could change that to:
[latitudeArray addObject:[NSNumber numberWithDouble: #"%.10f",dlat]];
[longitudeArray addObject:[NSNumber numberWithDouble: #"%.10f",dlon]];
...which may reduce the conversion time of
double dLat = [[latitudeArray objectAtIndex:i]doubleValue];
but wouldn't I still need that exact line to convert from NSString to double? It just may work faster?
thx
dlat is a double, right?
So instead of:
[latitudeArray addObject:[NSString stringWithFormat: #"%.10f",dlat]];
Do:
[latitudeArray addObject:[NSNumber numberWithDouble:dlat]];
They both respond to doubleValue but the NSNumber should not have to do any string parsing since it's stored as a numeric value already. And you never have to go to a string at all.
I suspect you have an array of strings like #"213.12385" that need to be parsed and converted when you call doubleValue on them. If that is where the issue is, the C arrays have nothing to with this.
Only thing I would add here is to throw Shark on this and see where it's spending it's time. If it's spending time in doubleValue find a different way to parse the strings with preprocessing in background or something. If it's in objectAtIndex: perhaps fast enumeration would help. If it's somewhere else entirely then you know it's not this snippet that's slow.
For the general case of converting an NSArray to a C array, you can use getObjects:. In this case, though, want you actually want is not to convert the NSArray, but to derive an array of doubles from an NSArray of some unspecified object type.
One obvious way to speed things up would be fast enumeration rather than sending a message to get the object for each iteration of the loop. I suspect the real solution, though, is outside your algorithm. The slowness probably comes from transforming whatever objects the array contains into doubles, in which case you'll need to find a way around that — maybe store doubles all along, maybe do the conversion in the background, etc. If you're creating the arrays yourself and there isn't some compelling reason for the objects to be strings, you should use NSNumbers instead. Those should be quite a bit faster.
The best solution is probably to make sure those values never end up in an NSArray as NSString values. I would attack this at the source.
So you edited your question and added that you are actually building those arrays. So why not use native arrays of doubles or floats from the start? I usually recommend against this but in your case it sounds like there is a huge performance gain.
Possibly using fast iteration, but I doubt that will really speed up your loop.