ios 科學計數法，保留n個有效值

/* 科學計數法，保留n個有效值 */-(NSString *) toExponent:(double)d rms:(unsigned)n{    if(n==0)    {        return nil;    }    CFLocaleRef currentLocale = CFLocaleCopyCurrent();    CFNumberFormatterRef customCurrencyFormatter = CFNumberFormatterCreate    (NULL, currentLocale, kCFNumberFormatterCurrencyStyle);    NSString *s_n = @"#";    if(n > 1)    {        for(int j = 0; j < n; j++)        {            NSString *temp = s_n;            if(j == 0)            {                s_n = [temp stringByAppendingString:@"."];            }            else            {                s_n = [temp stringByAppendingString:@"0"];            }                    }    }    CFNumberFormatterSetFormat(customCurrencyFormatter, (CFStringRef)s_n);        double i=1;    int exponent = 0;    while (1) {        i = i*10;        exponent++;        if(d < i)        {            break;        }    }    double n1 = d * 10 / i;        CFNumberRef number1 = CFNumberCreate(NULL, kCFNumberDoubleType, &n1);    CFStringRef string1 = CFNumberFormatterCreateStringWithNumber    (NULL, customCurrencyFormatter, number1);        NSString * result = [NSString stringWithFormat:@"%s E%d",CFStringGetCStringPtr(string1, CFStringGetSystemEncoding()),exponent];        CFRelease(currentLocale);    CFRelease(customCurrencyFormatter);    CFRelease(number1);    CFRelease(string1);        return result;    }