How to split an NSArray into two equal pieces?

问题:

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!

回答1:

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.

回答2:

Have you tried adding nil to the end of the -initWithObjects: method?

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

回答3:

If you want an extra object on first array (in case total items are odd), use following code modified from Alex's answer:

NSArray *arrayTotal = [NSArray arrayWithObjects:@"A", nil];
//NSArray *arrayTotal = [NSArray arrayWithObjects:@"A", @"B", nil];
//NSArray *arrayTotal = [NSArray arrayWithObjects:@"A", @"B", @"C", nil];
//NSArray *arrayTotal = [NSArray arrayWithObjects:@"A", @"B", @"C", @"D", nil];
//NSArray *arrayTotal = [NSArray arrayWithObjects:@"A", @"B", @"C", @"D", @"E", nil];

NSArray *arrLeft;
NSArray *arrRight;

NSRange range;
range.location = 0;
range.length = ([arrayTotal count] % 2) ? ([arrayTotal count] / 2) + 1 : ([arrayTotal count] / 2);

arrLeft = [arrayTotal subarrayWithRange:range];

range.location = range.length;
range.length = [arrayTotal count] - range.length;

arrRight = [arrayTotal subarrayWithRange:range];

NSLog(@"Objects: %lu", (unsigned long)[arrLeft count]);
NSLog(@"%@", [arrLeft description]);

NSLog(@"Objects: %lu", (unsigned long)[arrRight count]);
NSLog(@"%@", [arrRight description]);