J. Dave J. Dave - 1 month ago 8
Objective-C Question

How to split an NSArray into two equal pieces?

I have an NSArray, and I want to split it into two equal pieces (if odd "count" then add to the latter new array) - I want to split it "down the middle" so to speak.

The following code does exactly what I want, but is there a better way?:

// NOTE: `NSArray testableArray` is an NSArray of objects from a class defined elsewhere;
NSMutableArray *leftArray = [[NSMutableArray alloc] init];
NSMutableArray *rightArray = [[NSMutableArray alloc] init];

for (int i=0; i < [testableArray count]; i=i+1) {
if (i < [testableArray count]/2) {
[leftArray addObject:[testableArray objectAtIndex:i]];
}
else {
[rightArray addObject:[testableArray objectAtIndex:i]];
}
}


Once leftArray and rightArray are made, I will not change them, so they do not need to be "mutable". I think there may be a way to accomplish the above code with the ObjectsAtIndexes method or some fast enumeration method?, but I cannot get the following code to work (or other variations):

NSArray *leftArray = [[NSArray alloc] initWithObjects:[testableArray objectsAtIndexes:[NSIndexSet indexSetWithIndexesInRange:NSMakeRange(????, ????)]]];
NSArray *rightArray = [[NSArray alloc] initWithObjects:[testableArray objectsAtIndexes:[NSIndexSet indexSetWithIndexesInRange:NSMakeRange(????, ????)]]];


Does anyone know if I am going in the right direction with this or point me in the correct direction?

Thanks!

Answer Source

You also have the option of using -subarrayWithRange: detailed in the NSArray documentation:

NSArray *firstHalfOfArray;
NSArray *secondHalfOfArray;
NSRange someRange;

someRange.location = 0;
someRange.length = [wholeArray count] / 2;

firstHalfOfArray = [wholeArray subarrayWithRange:someRange];

someRange.location = someRange.length;
someRange.length = [wholeArray count] - someRange.length;

secondHalfOfArray = [wholeArray subarrayWithRange:someRange];

This method returns new, autorelease-d arrays.