In NSArray, NSMutuableArray,... data types, the addobject: method, can I
set the size of that to some limit?
Are you trying to constrain the amount of elements that can be added to an NSMutableArray?
if so, then you can just check that the current count is less then your desired max prior to adding a new object:
if (someMutableArray.count < max) {
[someMutableArray addObject:anObject];
};
Related
I only know method addObject, which is add object to the next element in the array. I want to be able to add / set / update object to an arbitrary position at the NSMutableArray, ex:
arr[105] = #(true);
arr[709] = #(30);
arr[1010] = #"Hello world!";
NSLog (#"%#", arr[1010]);
I have been trying something like this, but the next time I tried to retrieve the value, it says nil. How to do this? Thanks.
EDIT: last time I tried, it gave me error: index 1010 beyond bounds for empty array.
you specify a size when you create an array, the specified size is regarded as a “hint”; the actual size of the array is still 0. This means that you cannot insert an object at an index greater than the current count of an array. For example, if an array contains two objects, its size is 2, so you can add objects at indices 0, 1, or 2. Index 3 is illegal and out of bounds.
read more at https://developer.apple.com/library/prerelease/ios/documentation/Cocoa/Reference/Foundation/Classes/NSMutableArray_Class/index.html
below method use for replace existing object at index with new object
[self.arr replaceObjectAtIndex:<#(NSUInteger)#> withObject:<#(nonnull id)#>]
below method use for insert new object at index
[self.arr insertObject:<#(nonnull id)#> atIndex:<#(NSUInteger)#>]
Ex:
if object already available at index and replace with new object
[self.arrayBuyers replaceObjectAtIndex:1010 withObject:#"Hello world!"]
add new object at index
[self.arr insertObject:1 atIndex:#"Hi"];
Is there a way to set the maximum index size of an array. For example I have an array of UIImage but I only want the array to store 6 images. How would I set a restriction on that array so it can only hold 6 images
There is no such functionality. You would have to implement it yourself:
if array.count < 6 {
array.append(element)
}
or perhaps:
while array.count >= 6 {
array.removeFirst()
}
array.append(element)
Initialize your array with size of 6 and then do any one of the following checks:
Check element count of the array before inserting new element
You can surround your insertion code with try / catch block with 'ArrayIndexOutOfBounds' Exception being handled
This question already has answers here:
Getting a random object from NSArray without duplication
(3 answers)
Closed 7 years ago.
I have an array of random properties I would like to assign to equipment within the game I'm developing.
The code that I use below is returning an NSArray. I'm interested if there's way to get item indices from that array without getting duplicate values. The obvious solution is to create a mutable array with the returned array, do random, remove item that was returned and loop until the number of items is received.
But is there a different way of getting X random items from NSArray without getting duplicates?
//get possible enchantments
NSPredicate *p = [NSPredicate predicateWithFormat:#"type = %i AND grade >= %i", kEnchantmentArmor,armor.grade];
NSArray* possibleEnchantments = [[EquipmentGenerator allEnchantmentDictionary] objectForKey:#"enchantments"];
//get only applicable enchantments
NSArray *validEnchantments = [possibleEnchantments filteredArrayUsingPredicate:p];
NSMutableArray* mutableArray = [NSMutableArray arrayWithArray:validEnchantments];
NSDictionary* enchantment = nil;
if(mutableArray.count>0)
{
//got enchantments, assign number and intensity based on grade
for (int i = 0; i<3;i++)
{
enchantment = mutableArray[arc4random()%mutableArray.count];
[mutableArray removeObject:enchantment];
//create enchantment from dictionary and assign to item.
}
}
You can shuffle the array using one of the following techniques:
What's the Best Way to Shuffle an NSMutableArray?
Non repeating random numbers
Then, take the first X elements from the array.
Many years ago, I was working on card game and I realized that shuffling the deck was an inefficient way to get random cards. What I would do in your shoes is pick a random element, and then replace it with the element at the end of the array, like so:
#interface NSMutableArray (pickAndShrink)
- (id) pullElementFromIndex:(int) index // pass in your random value here
{
id pickedItem = [self elementAtIndex:index];
[self replaceObjectAtIndex:index withObject:[self lastObject]];
[self removeLastObject];
return pickedItem;
}
#end
The array will shrink by one every time you pull an element this way.
You could use a random number generator to pick a starting index, and then pick the subsequent indices based on some kind of math function. You would still need to loop depending on how many properties you want.
Eg:
-(NSMutableArray*)getRandomPropertiesFromArray:(NSArray*)myArray
{
int lengthOfMyArray = myArray.count;
int startingIndex = arc4random()%lengthOfMyArray;
NSMutableArray *finalArray = [[NSMutableArray alloc]init]autorelease];
for(int i=0; i<numberOfPropertiesRequired; i++)
{
int index = [self computeIndex:i usingStartingIndex:startingIndex origninalArray:myArray];
[finalArray addObject:[myArray objectAtIndex:index]];
}
return finalArray;
}
-(int)computeIndex:(int)index usingStartingIndex:(int)startingIndex
{
//You write your custom function here. This is just an example.
//You will have to write some code to make use you don't pick an Index greater than the length of your array.
int computedIndex = startingIndex + index*2;
return startingIndex;
}
EDIT: Even your computeIndex function could use randomness in picking the subsequent indices. Since you have a startingIndex, and another index, you could use that to offset your function so that you never pick a duplicate.
EDIT: If your array is very large, and the subset you need to pick is small, then rather than shuffle the entire array (maybe more expensive), you could use this method to pick the number of items you need. But if your array is small, or if the number of items you need to pick are almost the size of the array, then the godel9's solution is better.
You can use a mutable array and then remove them as the are selected, use something like random()%array.count to get a random index. If you don't want to modify the array then copy it with [array mutableCopy].
I have the need to obtain the maximum value of a property of a collection of custom objects of the same class. The objects are stored in a NSArray, and the property happens to be another NSArray of numbers.
Let me explain in detail:
NSArray *samples; // of CMData, 4000 elements
CMData is a class that models a sample, for a specific moment in time, of a set of different channels that can have different values.
#interface CMData : NSObject
#property (nonatomic) NSUInteger timeStamp;
#property (nonatomic, strong) NSArray *analogChannelData; // of NSNumber, 128 elements
#end
(I have stripped other properties of the class not relevant to the question)
So for example, sample[1970] could be:
sample.timeStamp = 970800
sample.analogChannelData = <NSArray>
[
[0] = #(153.27)
[1] = #(345.35)
[2] = #(701.02)
...
[127] = #(-234.45)
]
Where each element [i] in the analogChannelData represents the value of that specific channel i for the timeStamp 970800
Now I want to obtain the maximum value for all the 4000 samples for channel 31. I use the following code:
NSUInteger channelIndex = 31;
NSMutableArray *values = [[NSMutableArray alloc] init]; // of NSNumber
// iterate the array of samples and for each one obtain the value for a
// specific channel and store the value in a new array
for (CMData *sample in samples) {
[values addObject:sample.analogChannelData[channelIndex]];
}
// the maximum
NSNumber *maxValue = [values valueForKeyPath:#"#max.self"];
I want to replace this programming structure by a filter through an NSPredcicate or use valueForKeyPath: to obtain the maximum of the data I need.
Anyone knows how to do this without a for loop? Just using NSPredicates and/or valueForKeyPath?
Thank you very much in advance for your help.
Update 1
Finally I benckmarked the for-loop version against the keyPath version (see accepted answer) and it runs much faster so it is better to go with a for loop.
Recalling some lessons from my algorithms classes, I implemented an even faster version that doesn't need an array to store the values. I just iterate over the selected channel and just choose the maximum in each iteration. This is by far the fastest version.
So:
version 1: for loop (see code above)
version 2: version with custom property (see selected answer from Marcus, update 2)
version 3: new code
Code for version 3:
NSUInteger channelIndex = 31;
NSNumber *maxValue = #(-INFINITY);
for (CMTData *sample in samples) {
NSNumber *value = sample.analogChannelData[channelIndex];
if (value) { // I allow the possibility of NSNull values in the NSArray
if ([value compare:maxValue] == NSOrderedDescending)
maxValue = value;
}
}
// the maximum is in maxValue at the end of the loop
Performance:
After 20.000 iterations in iOS simulator:
Version 1: 12.2722 sec.
Version 2: 21.0149 sec.
Version 3: 5.6501 sec.
The decision is clear. I'll use the third version.
Update 2
After some more research, it is clear to me now that KVC does not work for infividual elements in the inner array. See the following links: KVC with NSArrays of NSArrays and Collection Accessor Patterns for To-Many Properties
Anyway because I wanted to compute the maximum of the elements it is better to iterate the array than use some tricks to make KVC work.
You can solve this with using Key Value Coding and the collection operators.
NSNumber *result = [sample valueForKeyPath:#"#max.analogDataChannel"];
Update 1
As Arcanfel mentioned, you can join the arrays together:
NSNumber *result = [samples valueForKeyPath:#"#max.#unionOfArrays.#analogChannelData"];
I would suggest reading the documentation that we both linked to. There are some very powerful features in there.
Update 2
Further to HRD's answer, he has your solution, you need to combine his changes with KVC.
Add a propert to your CMData object for currentChannel. Then you can call
[samples setValue:#(channelIndex) forKey:#"currentChannel"];
Which will set it in every instance in the array. Then call:
[samples valueForKeyPath:#"#max.analogDataForCurrentChannel"];
Then you are done.
I have not tested out the code yet, but I think this is exactly what you are looking for:
[samples valueForKeyPath:#"#max.(#unionOfArrays.analogChannelData)"];
I guess you can also use #distinctUnionOfArray to remove duplicate values.
Here is the link to Apple Documentation that covers collection operators.
Hope this is helpful!
Cheers!
A suggestion for further exploration only
Offhand it is not clear you can do this as-is with a single KVC operator. What you might consider is adding two properties to your class: currentChannel, which sets/gets the current channel; and analogChannelDataForCurrentChannel, which is equivalent to analogChannelData[currentChannel]. Then you can:
samples.currentChannel = channelIndex;
... [samples valueForKeyPath:"#max.analogChannelDataForCurrentChannel"];
with any appropriate locking between the two calls if thread-safety is required (so one thread does not set currentChannel, then a second, and then the first do the KVC operator with the second's channel...).
HTH
Currently I'm trying to find a compact way to average a matrix. The obvious solution is to sum the matrix, then divide by the number of elements. I have, however, come across a method on the apple developer website that claims this can be done in a simpler way using valueForKeyPath. This is linked here:
http://developer.apple.com/library/ios/#documentation/cocoa/conceptual/KeyValueCoding/Articles/CollectionOperators.html
Here is the example I'm currently working on to try and get it to work:
-(void)arrayAverager
{
NSMutableArray *myArray = [NSMutableArray arrayWithCapacity:25];
[myArray addObject:[NSNumber numberWithInteger:myValue]];
NSNumber *averageValue = [myArray valueForKeyPath:#"#avg.self"];
NSLog(#"avg = %#", averageValue);
}
The problem is: instead of averaging the array it merely prints out the elements in the array 1 by 1.
UPDATE
-(void) pixelAverager
{
//Put x-coordinate value of all wanted pixels into an array
NSMutableArray *xCoordinateArray = [NSMutableArray arrayWithCapacity:25];
[xCoordinateArray addObject:[NSNumber numberWithInteger:xCoordinate]];
NSLog(#"avg = %#", [xCoordinateArray valueForKeyPath:#"#avg.intValue"]);
}
You need to use #avg.floatValue (or #avg.doubleValue, or what have you). The #avg operator will average the property of the objects in the array specified by the name after the dot. The documentation is confusing on this point, but that is what:
to get the values specified by the property specified by the key path
to the right of the operator
Is saying. Since you have a collection of NSNumber objects, you use one of the *value accessors, e.g. floatValue to get the value of each object. As an example:
#include <Foundation/Foundation.h>
int main(void) {
NSMutableArray *ma = [NSMutableArray array];
[ma addObject:[NSNumber numberWithFloat:1.0]];
[ma addObject:[NSNumber numberWithFloat:2.0]];
[ma addObject:[NSNumber numberWithFloat:3.0]];
NSLog(#"avg = %#", [ma valueForKeyPath:#"#avg.floatValue"]);
return 0;
}
Compiling and running this code returns:
$ clang avg.m -framework Foundation -o avg
steve:~/code/tmp
$ ./avg
2013-01-18 12:33:15.500 avg[32190:707] avg = 2
steve:~/code/tmp
The nice thing about this approach is that this work for any collection, homogenous or otherwise, as long as all objects respond to the specified method, #avg will work.
EDIT
As pointed in the comments, the OP's problem is that he is averaging a collection with one element, and thus it appears to simply print the contents of the collection. For a collection of NSNumber objects, #avg.self works just fine.
No, you can't do like this. The object Transaction is a Modal Class. This class is having three properties, namely
payee
amount
date
Each row in this image represents one Transaction modal object.
transactions is an array which is holding all these rows (Transaction Modal Objects).
In these transactions array, they are trying to calculate the Transaction Modal amount field average using the operator #avg. So, its like
NSNumber *transactionAverage = [transactions valueForKeyPath:#"#avg.amount"];
your array doesn't have the key self. So that's the problem