初级_NSDecimalNumber 十进制数计算

由于使用Double计算经常会在小数点后精度问题和上取整、下取整、四舍五入方面出现问题，所以决定使用十进制数做金额、利率的计算。 NSDecimalNumber是NSNumber的子类。

一、初始化

//实例方法

-(instancetype)initWithMantissa:(unsignedlonglong)mantissa exponent:(short)exponent isNegative:(BOOL)flag;
-(instancetype)initWithDecimal:(NSDecimal)dcm;
-(instancetype)initWithString:(nullableNSString *)numberValue;
-(instancetype)initWithString:(nullableNSString *)numberValue locale:(nullableid)locale;

//类方法

+(NSDecimalNumber *)decimalNumberWithMantissa:(unsignedlonglong)mantissa exponent:(short)exponent isNegative:(BOOL)flag;
+(NSDecimalNumber *)decimalNumberWithDecimal:(NSDecimal)dcm;+ (NSDecimalNumber *)decimalNumberWithString:(nullableNSString *)numberValue;
+(NSDecimalNumber *)decimalNumberWithString:(nullableNSString *)numberValue locale:(nullableid)locale;

**e.g. **

NSDecimalNumber *subtotalAmount = [[NSDecimalNumber alloc]initWithString: @"12.34"]; //12.34
subtotalAmount = [NSDecimalNumber decimalNumberWithMantissa:1234 exponent:-2 isNegative:NO];   //12.34 (
mantissa：长整形；exponent：指数；flag：正负数)
subtotalAmount = [NSDecimalNumber decimalNumberWithMantissa:1234 exponent:2 isNegative:YES];   //-123400
discountAmount = [NSDecimalNumber decimalNumberWithString:@"123.4"];      //123.4

//NSDecimal类型
C语言NSDecimal类型和十进制数转换

NSDecimalNumber *subtotalAmount = [NSDecimalNumber decimalNumberWithMantissa:1234 exponent:-2 isNegative:NO];   //12.34
NSDecimal decimalValue = [subtotalAmount decimalValue];
subtotalAmount = [NSDecimalNumber decimalNumberWithDecimal:decimalValue];   //12.34

☐//字符串转换十进制数格式 : locale
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

二、加减乘除

加法运算

- (NSDecimalNumber *)decimalNumberByAdding:(NSDecimalNumber *)decimalNumber;
- (NSDecimalNumber *)decimalNumberByAdding:(NSDecimalNumber *)decimalNumber withBehavior:(nullableid <NSDecimalNumberBehaviors>)behavior;

减法运算

- (NSDecimalNumber *)decimalNumberBySubtracting:(NSDecimalNumber *)decimalNumber;
- (NSDecimalNumber *)decimalNumberBySubtracting:(NSDecimalNumber *)decimalNumber withBehavior:(nullableid <NSDecimalNumberBehaviors>)behavior;

乘法运算

- (NSDecimalNumber *)decimalNumberByMultiplyingBy:(NSDecimalNumber *)decimalNumber;
- (NSDecimalNumber *)decimalNumberByMultiplyingBy:(NSDecimalNumber *)decimalNumber withBehavior:(nullableid <NSDecimalNumberBehaviors>)behavior;

除法运算

- (NSDecimalNumber *)decimalNumberByDividingBy:(NSDecimalNumber *)decimalNumber;
- (NSDecimalNumber *)decimalNumberByDividingBy:(NSDecimalNumber *)decimalNumber withBehavior:(nullableid <NSDecimalNumberBehaviors>)behavior;

a的n次方

- (NSDecimalNumber *)decimalNumberByRaisingToPower:(NSUInteger)power;
- (NSDecimalNumber *)decimalNumberByRaisingToPower:(NSUInteger)power withBehavior:(nullableid <NSDecimalNumberBehaviors>)behavior;

指数运算

- (NSDecimalNumber *)decimalNumberByMultiplyingByPowerOf10:(short)power;
- (NSDecimalNumber *)decimalNumberByMultiplyingByPowerOf10:(short)power withBehavior:(nullableid <NSDecimalNumberBehaviors>)behavior;

四舍五入运算

- (NSDecimalNumber *)decimalNumberByRoundingAccordingToBehavior:(nullableid <NSDecimalNumberBehaviors>)behavior;

比较运算

- (NSComparisonResult)compare:(NSNumber *)decimalNumber;

e.g.

NSDecimalNumber *discount1 = [NSDecimalNumber decimalNumberWithString:@"1.2"];  
NSDecimalNumber *discount2 = [NSDecimalNumber decimalNumberWithString:@"1.3"];  
NSComparisonResult result = [discount1 compare:discount2];  
if (result == NSOrderedAscending) {
      NSLog(@"discount1 < discount2");
  } else if (result == NSOrderedSame) {
      NSLog(@"discount1 == discount2");
  } else if (result == NSOrderedDescending) {
      NSLog(@"discount1 > discount2");
  }

☐三、格式化处理
NSDecimalNumberHandler

-  (instancetype)initWithRoundingMode(NSRoundingMode)roundingMode 
scale:(short)scale 
raiseOnExactness:(BOOL)exact 
raiseOnOverflow:(BOOL)overflow 
raiseOnUnderflow:(BOOL)underflow 
raiseOnDivideByZero:(BOOL)divideByZero;

+ (NSDecimalNumberHandler *)defaultDecimalNumberHandler;
+ (instancetype)decimalNumberHandlerWithRoundingMode:(NSRoundingMode)roundingMode scale:(short)scale raiseOnExactness:(BOOL)exact raiseOnOverflow:(BOOL)overflow raiseOnUnderflow:(BOOL)underflow raiseOnDivideByZero:(BOOL)divideByZero;

e.g.

NSDecimalNumberHandler *roundUp = [NSDecimalNumberHandler   decimalNumberHandlerWithRoundingMode:NSRoundBankers
                                       scale:2
                                       raiseOnExactness:NO
                                       raiseOnOverflow:NO
                                       raiseOnUnderflow:NO
                                       raiseOnDivideByZero:YES];
NSDecimalNumber *subtotal = [NSDecimalNumberdecimalNumberWithString:@"11.4035"];
NSDecimalNumber *discount = [NSDecimalNumberdecimalNumberWithString:@"3.22"];
NSDecimalNumber *total = [subtotal decimalNumberByAdding:discount withBehavior:roundUp];//14.62  两个数相加然后舍去小数点后两位以后的部分

枚举

NSRoundPlain,   // Round up on a tie // 貌似是四舍五入
NSRoundDown,    // Always down == truncate  // 只舍不入
NSRoundUp,      // Always up   //  只入不舍
NSRoundBankers  // on a tie round so last digit is even  // 貌似是： if(四舍五入位 == 5)（四舍五入位（5）+  保留位 ）%2 == 0 ？ 入 ： 舍；  if(四舍五入位 ！= 5) 遵从四舍五入

End、未补充
☐NSDecimalNumberBehaviors
☐ NSDecimalNumberHandler
☐ NSRoundingMode

End、未验证
所有NSDecimalNumber是不可变的，这意味着已经被创建后不能改变它们的值。

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