Objective-C中的NSString中的Shuffling Letters

Question

我已经编写了这个函数来调整a的内容NSString,它似乎工作,但它时不时地崩溃.这可能是一种迂回的方式,但我把字符放入一个数组,随机交换数组中的元素,然后将数组转回一个字符串.

我不确定我在做什么不安全会导致它崩溃.我认为可能是我正在设置finalLettersString = result,但我也尝试了,但finalLettersString = [NSString stringWithString:result]也崩溃了.我感到困惑的原因是因为它不会每次都崩溃.我只是按下随机按钮,有时它会崩溃.我应该看的任何地方？

/* This function shuffles the letters in the string finalLettersString */

-(IBAction)shuffleLetters:(id)sender{
    int length = [finalLettersString length];
    NSMutableArray * letters = [NSMutableArray arrayWithCapacity:length]; 
    NSLog(@"final letters: %@", finalLettersString);
    for(int i = 0; i < length; i++){
        char ch = [finalLettersString characterAtIndex:i];
        NSLog(@"%c", ch);
        NSString * cur = [NSString stringWithFormat:@"%c", ch];
        [letters insertObject:cur atIndex:i];
    }

    NSLog(@"LETTERS:: %@", letters);

    for(int i = length - 1; i >= 0; i--){
        int j = arc4random() % (i + 1);
        //NSLog(@"%d %d", i, j);
        //swap at positions i and j
        NSString * str_i = [letters objectAtIndex:i];
        [letters replaceObjectAtIndex:i withObject:[letters objectAtIndex:j]];
        [letters replaceObjectAtIndex:j withObject:str_i];      
    }
    NSLog(@"NEW SHUFFLED LETTERS %@", letters);

    NSString * result = @"";
    for(int i = 0; i < length; i++){
        result = [result stringByAppendingString:[letters objectAtIndex:i]];
    }

    NSLog(@"Final string: %@", result);
    finalLettersString = result;
    finalLetters.text = finalLettersString;
}

Answer 1

@ dreamlax代码的变体,不使用char数组.肯定不那么有效.但它没有Unicode问题.

NSMutableString *randomizedText = [NSMutableString stringWithString:currentText];

NSString *buffer;
for (NSInteger i = randomizedText.length - 1, j; i >= 0; i--)
{
    j = arc4random() % (i + 1);

    buffer = [randomizedText substringWithRange:NSMakeRange(i, 1)];
    [randomizedText replaceCharactersInRange:NSMakeRange(i, 1) withString:[randomizedText substringWithRange:NSMakeRange(j, 1)]];
    [randomizedText replaceCharactersInRange:NSMakeRange(j, 1) withString:buffer];
}

Answer 2

最好将字符串的内容复制到类型的临时缓冲区中unichar并随机缓冲缓冲区的内容,而不是创建大量的小字符串.

NSUInteger length = [finalLettersString length];

if (!length) return; // nothing to shuffle    

unichar *buffer = calloc(length, sizeof (unichar));

[finalLettersString getCharacters:buffer range:NSMakeRange(0, length)];

for(int i = length - 1; i >= 0; i--){
    int j = arc4random() % (i + 1);
    //NSLog(@"%d %d", i, j);
    //swap at positions i and j
    unichar c = buffer[i];
    buffer[i] = buffer[j];
    buffer[j] = c;
}

NSString *result = [NSString stringWithCharacters:buffer length:length];
free(buffer);

// caution, autoreleased. Allocate explicitly above or retain below to
// keep the string.
finalLettersString = result;

您需要注意的几件事情:

1. Unicode字符串可以包含复合字符和代理项对.将这些转移到最有可能导致无效的字符串.虽然代理对很少见,但是发现字符é由两个字符组成(基本小写字母e和组合的尖锐重音)并不罕见.

2. 对于大字符串,它可能会导致内存问题,因为最终使用的空间是原始字符串的3倍(原始字符串为1倍,我们使用的缓冲区为2倍,新字符串为3倍,然后返回一旦我们释放缓冲区就2×