编码篇－开发中关于数字的那些事儿

+ (NSDecimalNumber *)decimalNumberWithMantissa:(unsigned long long)mantissa exponent:(short)exponent isNegative:(BOOL)flag;
  mantissa：长整形；exponent：指数；flag：正负数。
  NSDecimalNumber *subtotalAmount = [NSDecimalNumber decimalNumberWithMantissa:
  1275 exponent:-2 isNegative:NO];   //12.75
  subtotalAmount = [NSDecimalNumber decimalNumberWithMantissa:
  1275 exponent:2 isNegative:YES];   //-127500

+ (NSDecimalNumber *)decimalNumberWithString:(nullable NSString *)numberValue;
将字符串转成一个十进制数。
  NSDecimalNumber *discountAmount = [NSDecimalNumber decimalNumberWithString:@"-12.74"];      //-12.74
  discountAmount = [NSDecimalNumber decimalNumberWithString:@"127.4"];      //127.4

+ (NSDecimalNumber *)decimalNumberWithString:(nullable NSString *)numberValue locale:(nullable id)locale;
这个有点复杂，locale代表一种格式，就像date的格式化一样。这里的locale可以传递两种格式

  NSDictionary *locale = [NSDictionary dictionaryWithObject:@"," forKey:NSLocaleDecimalSeparator];    //以","当做小数点格式
  NSDecimalNumber *discountAmount = [NSDecimalNumber decimalNumberWithString:@"123,40" locale:locale];    //123.4

  NSLocale *locale = [[NSLocale alloc] initWithLocaleIdentifier:@"fr_FR"];    //法国数据格式,法国的小数点是','逗号
  NSDecimalNumber *discountAmount = [NSDecimalNumber decimalNumberWithString:@"123,40" locale:locale];    //123.4

  +(NSDecimalNumber *)zero; //0
  +(NSDecimalNumber *)one; //1
  +(NSDecimalNumber *)minimumDecimalNumber;
  //-3402823669209384634633746074317682114550000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
   +(NSDecimalNumber *)maximumDecimalNumber;
  //3402823669209384634633746074317682114550000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
  +(NSDecimalNumber *)notANumber;
  //非数字，常用于对比，比如：
  [[NSDecimalNumber notANumber] isEqualToNumber:myNumber];

NSDecimalNumberBehaviors对象可以通过下述方法创建
NSDecimalNumberHandler *roundUp = [NSDecimalNumberHandler
                                   decimalNumberHandlerWithRoundingMode:NSRoundBankers
                                   scale:2
                                   raiseOnExactness:NO
                                   raiseOnOverflow:NO
                                   raiseOnUnderflow:NO
                                   raiseOnDivideByZero:YES];

 #枚举:
       NSRoundPlain,   // Round up on a tie ／／四舍五入
        NSRoundDown,    // Always down == truncate  ／／只舍不入
        NSRoundUp,      // Always up    ／／ 只入不舍
        NSRoundBankers 四舍六入, 中间值时, 取最近的,保持保留最后一位为偶数

 参照一下图片, 理解上面枚举值:

当他们试图除以0或产生一个数表示太大或太小的时候发生异常。
下面列出了各种异常的名字 表明NSDecimalNumber计算错误。

  extern NSString *NSDecimalNumberExactnessException; //如果出现一个精确的错误
  extern NSString *NSDecimalNumberOverflowException; // 溢出
  extern NSString *NSDecimalNumberUnderflowException; //下溢
  extern NSString *NSDecimalNumberDivideByZeroException; //除数为0

  NSDecimalNumber *sub = [[NSDecimalNumber alloc]initWithFloat:1.23];
  sub = [sub decimalNumberByAdding:sub 
                      withBehavior:[NSDecimalNumberHandler decimalNumberHandlerWithRoundingMode:NSRoundDown 
                            scale:1 raiseOnExactness:NO raiseOnOverflow:NO raiseOnUnderflow:NO raiseOnDivideByZero:YES]];
     
  # 这里特别提醒一下：RoundingMode  中 NSRoundDown模式下的 NSDecimalNumber数值  floatValue、doubleValue  后依然会出现不精确的问题。
  # 其他模式下倒没有这样的现象。
 .
 ..

  #  NSDecimal 与 NSDecimalNumber 之间的转化
  NSDecimalNumber *price = [NSDecimalNumber decimalNumberWithString:@"15.99"];
  NSDecimal asStruct = [price decimalValue];
  NSDecimalNumber *asNewObject = [NSDecimalNumber decimalNumberWithDecimal:asStruct];

  NSDecimal的使用中需要注意
  C接口使用类似的功能NSDecimalAdd()， NSDecimalSubtract()不是返回结果，这些函数用计算的值填充第一个参数。
  这使得可以重用现有NSDecimal的几个操作，并避免分配不必要的结构只是为了保存中间值。

  NSDecimal price1 = [[NSDecimalNumber decimalNumberWithString:@"15.99"] decimalValue];
  NSDecimal price2 = [[NSDecimalNumber decimalNumberWithString:@"29.99"] decimalValue];
  NSDecimal coupon = [[NSDecimalNumber decimalNumberWithString:@"5.00"] decimalValue];
  NSDecimal discount = [[NSDecimalNumber decimalNumberWithString:@".90"] decimalValue];
  NSDecimal numProducts = [[NSDecimalNumber decimalNumberWithString:@"2.0"] decimalValue];
  NSLocale *locale = [NSLocale currentLocale];
  NSDecimal result;

  NSDecimalAdd(&result, &price1, &price2, NSRoundUp);
  NSLog(@"Subtotal: %@", NSDecimalString(&result, locale));
  NSDecimalSubtract(&result, &result, &coupon, NSRoundUp);
  NSLog(@"After coupon: %@", NSDecimalString(&result, locale));
  NSDecimalMultiply(&result, &result, &discount, NSRoundUp);
  NSLog(@"After discount: %@", NSDecimalString(&result, locale));
  NSDecimalDivide(&result, &result, &numProducts, NSRoundUp);
  NSLog(@"Average price per product: %@", NSDecimalString(&result, locale));
  NSDecimalPower(&result, &result, 2, NSRoundUp);
  NSLog(@"Average price squared: %@", NSDecimalString(&result, locale));

.
  # 浮点型小数四舍五入     afterPoint:  小数点后几位
+(NSString *)notRounding:(float)price afterPoint:(int)position{
   NSDecimalNumberHandler* roundingBehavior = [NSDecimalNumberHandler decimalNumberHandlerWithRoundingMode:NSRoundPlain scale:position raiseOnExactness:NO raiseOnOverflow:NO raiseOnUnderflow:NO raiseOnDivideByZero:NO];
    NSDecimalNumber *ouncesDecimal;
    NSDecimalNumber *roundedOunces;

  ouncesDecimal = [[NSDecimalNumber alloc] initWithFloat:price];
  roundedOunces = [ouncesDecimal decimalNumberByRoundingAccordingToBehavior:roundingBehavior];
  return [NSString stringWithFormat:@"%@",roundedOunces];
}

  # 浮点数处理并去掉多余的0
- (NSString *)stringDisposeWithFloat:(double)floatValue
{
    NSString *str = [NSString stringWithFormat:@"%f",floatValue];
    NSInteger len = str.length;
    for (NSInteger i = 0; i < len; i++)
    {
      if (![str  hasSuffix:@"0"])
           break;
      else
          str = [str substringToIndex:[str length]-1];
    }
    if ([str hasSuffix:@"."])//避免像2.0000这样的被解析成2.    以。。。结尾
    {
        return [str substringToIndex:[str length]-1];//s.substring(0, len - i - 1);
    }
    else
    {
        return str;
    }   
}

  # 数字3位加一个逗号
+(NSString *)countNumAndChangeformat:(NSString *)num  
{  
    int count = 0;  
    long long int a = num.longLongValue;  
    while (a != 0)  
    {  
        count++;  
        a /= 10;  
    }  
    NSMutableString *string = [NSMutableString stringWithString:num];  
    NSMutableString *newstring = [NSMutableString string];  
    while (count > 3) {  
        count -= 3;  
        NSRange rang = NSMakeRange(string.length - 3, 3);  
        NSString *str = [string substringWithRange:rang];  
       [newstring insertString:str atIndex:0];  
        [newstring insertString:@"," atIndex:0];  
        [string deleteCharactersInRange:rang];  
    }  
    [newstring insertString:string atIndex:0];  
    return newstring;  
 }