objective-c - 关于NSString字符串反转的问题

Question

今天看到这个“Objective-C分割NSString”，有人用正则表达式进行解答，感觉很特别。所以就把以前遇到的字符串反转的问你拿出来讨论讨论，可不可以用正则表达式来解决呢？

下面是个最简单的实现的如有什么效率问题，边界问题也希望多给意见。

/**** NSString+Reverse.h ****/
#import <Foundation/Foundation.h>
@interface NSString (Reverse)
- (NSString *)stringByReversed;
@end

/**** NSString+Reverse.m ****/
#import "NSString+Reverse.h"
@implementation NSString (Reverse)
- (NSString *)stringByReversed
{
  NSMutableString *s = [NSMutableString string];
  for (NSUInteger i=self.length; i>0; i--) {
    [s appendString:[self substringWithRange:NSMakeRange(i-1, 1)]];
  }
  return s;
}
@end

分割线

3月24日补充

根据@ParagonLight同学的回答我用如下代码做了个测试：

  NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
  [dateFormatter setDateFormat:@"ss.SSSS"];

  NSLog(@"S %@", [dateFormatter stringFromDate:[NSDate date]]);
  NSString *reversed = [string stringByReversed];
  NSLog(@"E %@", [dateFormatter stringFromDate:[NSDate date]]);

测试用的字符串长度为：8970

记录了10次结果，我制作了一个图表（算法1为我问题中的；算法2为@ParagonLight同学的）

只看代码的话算法2可以提高一半的效率，之所以花的时间比算法1高出许多，是因为每次循环都会生成一个新string对象，所耗费时间确实超乎我的预想了。

分割线

3月25日补充

由于算法1和算法3相差很小，为了让结果更明显，修改了测试用例，将测试字符串长度放大100倍，现在测试字符串长度为：897000

@ParagonLight同学的算法3的性能提升还是很明显的

Answer 1

I used the stringByReplacingCharactersInRange: method here. But this method actually creates a new string, so it doesn't feel like the most efficient approach. In fact, you can convert the string into a char array, then set two pointers to point to the head and tail of the array, and then exchange the pointed values ​​in sequence until i>j.
I haven't been in contact with iOS for a long time, and I don't know much about its features. Please point out if there is anything wrong.

- (NSString *)stringByReversed
{
    NSUInteger i = 0;
    NSUInteger j = self.length - 1;
    NSString *temp;
    NSString *newString = self;
    NSLog(@"%@", self);
    while (i < j) {
        temp = [newString substringWithRange:NSMakeRange(i, 1)];
        newString = [newString stringByReplacingCharactersInRange:NSMakeRange(i, 1) withString:[self substringWithRange:NSMakeRange(j, 1)]];
        newString = [newString stringByReplacingCharactersInRange:NSMakeRange(j, 1) withString:temp];
        NSLog(@"%@",newString);
        i ++;
        j --;
    }
    NSLog(@"%@", newString);
    return newString;
}

Separating line

Try this one

-(NSString *)stringByReversed{
    NSUInteger i = 0;
    NSUInteger j = self.length - 1;
    unichar characters[self.length];
    while (i < j) {
        characters[j] = [self characterAtIndex:i];
        characters[i] = [self characterAtIndex:j];
        i ++;
        j --;
    }
    if(i == j)
        characters[i] = [self characterAtIndex:i];
    return [NSString stringWithCharacters:characters length:self.length];
}

Update: Algorithm 4

Visual inspection indicates that the unichar array is out of bounds. Just malloc directly. . . But I have to say, this is no longer OC. . .

- (NSString *)stringByReversed{
  uint64_t  i = 0;
  uint64_t j = self.length - 1;
//  unichar characters[self.length];
  unichar *characters = malloc(sizeof([self characterAtIndex:0]) * self.length);
  while (i < j) {
    characters[j] = [self characterAtIndex:i];
    characters[i] = [self characterAtIndex:j];
    i ++;
    j --;
  }
  if(i == j)
    characters[i] = [self characterAtIndex:i];
  return [NSString stringWithCharacters:characters length:self.length];
}