best way to populate NSArray in this algorithm - objective-c

Question

I intend to make a program that does the following:
Create an NSArray populated with numbers from 1 to 100,000.
Loop over some code that deletes certain elements of the NSArray when certain conditions are met.
Store the resultant NSArray.
However the above steps will also be looped over many times and so I need a fast way of making this NSArray that has 100,000 number elements.
So what is the fastest way of doing it?
Is there an alternative to iteratively populating an Array using a for loop? Such as an NSArray method that could do this quickly for me?
Or perhaps I could make the NSArray with the 100,000 numbers by any means the first time. And then create every new NSArray (for step 1) by using method arraywithArray? (is it quicker way of doing it?)
Or perhaps you have something completely different in mind that will achieve what I want.
edit: replace NSArray with NSMutableArray in above post

Answer 1

It is difficult to tell in advance which method will be the fastest. I like the block based functions, e.g.
NSMutableArray *array = ...; // your mutable array
NSIndexSet *toBeRemoved = [array indexesOfObjectsPassingTest:^BOOL(NSNumber *num, NSUInteger idx, BOOL *stop) {
// Block is called for each number "num" in the array.
// return YES if the element should be removed and NO otherwise;
}];
[array removeObjectsAtIndexes:toBeRemoved];
You should probably start with a correctly working algorithm and then use Instruments for profiling.

Answer 2

You may want to look at NSMutableIndexSet. It is designed to efficiently store ranges of numbers.
You can initialize it like this:
NSMutableIndexSet *set = [[NSMutableIndexSet alloc]
initWithIndexesInRange:NSMakeRange(1, 100000)];
Then you can remove, for example, 123 from it like this:
[set removeIndex:123];
Or you can remove 400 through 409 like this:
[set removeIndexesInRange:NSMakeRange(400, 10)];
You can iterate through all of the remaining indexes in the set like this:
[set enumerateIndexesUsingBlock:^(NSUInteger i, BOOL *stop) {
NSLog(#"set still includes %lu", (unsigned long)i);
}];
or, more efficiently, like this:
[set enumerateRangesUsingBlock:^(NSRange range, BOOL *stop) {
NSLog(#"set still includes %lu indexes starting at %lu",
(unsigned long)range.length, (unsigned long)range.location);
}];

Answer 3

I'm quite certain it will be fastest to create the array using a c array, then creating an NSArray from that (benchmark coming soon). Depending on how you want to delete the numbers, it may be fastest to do that in the initial loop:
const int max_num = 100000;
...
id *nums = malloc(max_num * sizeof(*nums));
int c = 0;
for(int i = 1; i <= max_num; i++) {
if(!should_skip(i)) nums[c++] = #(i);
}
NSArray *nsa = [NSArray arrayWithObjects:nums count:c];
First benchmark was somewhat surprising. For 100M objects:
NSArray alloc init: 8.6s
NSArray alloc initWithCapacity: 8.6s
id *nums: 6.4s
So an array is faster, but not by as much as I expected.

Answer 4

You can use fast enumeration to search through the array.
for(NSNumber item in myArrayOfNumbers)
{
If(some condition)
{
NSLog(#"Found an Item: %#",item);
}
}

Answer 5

You might want to reconsider what you are doing here. Ask yourself why you want such an array. If your goal is to manipulate an arbitrarily large collection of integers, you'll likely prefer to use NSIndexSet (and its mutable counterpart).
If you really want to manipulate a NSArray in the most efficient way, you will want to implement a dedicated subclass that is especially optimized for this kind of job.