How can I reverse a NSArray in Objective
I need to reverse my NSArray .
As an example:
[1,2,3,4,5] must become: [5,4,3,2,1]
What is the best way to achieve this?
For obtaining a reversed copy of an array, look at danielpunkass' solution using reverseObjectEnumerator .
For reversing a mutable array, you can add the following category to your code:
@implementation NSMutableArray (Reverse)
- (void)reverse {
if ([self count] <= 1)
return;
NSUInteger i = 0;
NSUInteger j = [self count] - 1;
while (i < j) {
[self exchangeObjectAtIndex:i
withObjectAtIndex:j];
i++;
j--;
}
}
@end
There is a much easier solution, if you take advantage of the built-in reverseObjectEnumerator method on NSArray , and the allObjects method of NSEnumerator :
NSArray* reversedArray = [[startArray reverseObjectEnumerator] allObjects];
allObjects is documented as returning an array with the objects that have not yet been traversed with nextObject , in order:
This array contains all the remaining objects of the enumerator in enumerated order .
Some benchmarks
1. reverseObjectEnumerator allObjects
This is the fastest method:
NSArray *anArray = @[@"aa", @"ab", @"ac", @"ad", @"ae", @"af", @"ag",
@"ah", @"ai", @"aj", @"ak", @"al", @"am", @"an", @"ao", @"ap", @"aq", @"ar", @"as", @"at",
@"au", @"av", @"aw", @"ax", @"ay", @"az", @"ba", @"bb", @"bc", @"bd", @"bf", @"bg", @"bh",
@"bi", @"bj", @"bk", @"bl", @"bm", @"bn", @"bo", @"bp", @"bq", @"br", @"bs", @"bt", @"bu",
@"bv", @"bw", @"bx", @"by", @"bz", @"ca", @"cb", @"cc", @"cd", @"ce", @"cf", @"cg", @"ch",
@"ci", @"cj", @"ck", @"cl", @"cm", @"cn", @"co", @"cp", @"cq", @"cr", @"cs", @"ct", @"cu",
@"cv", @"cw", @"cx", @"cy", @"cz"];
NSDate *methodStart = [NSDate date];
NSArray *reversed = [[anArray reverseObjectEnumerator] allObjects];
NSDate *methodFinish = [NSDate date];
NSTimeInterval executionTime = [methodFinish timeIntervalSinceDate:methodStart];
NSLog(@"executionTime = %f", executionTime);
Result: executionTime = 0.000026
2. Iterating over an reverseObjectEnumerator
This is between 1.5x and 2.5x slower:
NSDate *methodStart = [NSDate date];
NSMutableArray *array = [NSMutableArray arrayWithCapacity:[anArray count]];
NSEnumerator *enumerator = [anArray reverseObjectEnumerator];
for (id element in enumerator) {
[array addObject:element];
}
NSDate *methodFinish = [NSDate date];
NSTimeInterval executionTime = [methodFinish timeIntervalSinceDate:methodStart];
NSLog(@"executionTime = %f", executionTime);
Result: executionTime = 0.000071
3. sortedArrayUsingComparator
This is between 30x and 40x slower (no surprises here):
NSDate *methodStart = [NSDate date];
NSArray *reversed = [anArray sortedArrayUsingComparator: ^(id obj1, id obj2) {
return [anArray indexOfObject:obj1] < [anArray indexOfObject:obj2] ? NSOrderedDescending : NSOrderedAscending;
}];
NSDate *methodFinish = [NSDate date];
NSTimeInterval executionTime = [methodFinish timeIntervalSinceDate:methodStart];
NSLog(@"executionTime = %f", executionTime);
Result: executionTime = 0.001100
So [[anArray reverseObjectEnumerator] allObjects] is the clear winner when it comes to speed and ease.
上一篇: NSString中子字符串的出现次数？