Double数字精度处理

今天在用Double的数字的时候，发现精度有问题，不管怎么处理，后面都会有一长串的0.0000000002,0.0000000003,0.0000000001,0.99999999997等问题

解决问题：使用NSDecimalNumber用于精度准确的计算

1. 在处理金额计算时，往往会涉及到小数，由于Double类型不准确，无法做到产品的要求。为了保证金额计算的准确性，建议使用NSDecimalNumber。

2.创建对象（常用的方法）

// mantissa：长整型数值；exponent：指数（几次方）；flag：正负数。
+ (NSDecimalNumber *)decimalNumberWithMantissa:(unsigned long long)mantissa exponent:(short)exponent isNegative:(BOOL)flag;

NSDecimalNumber *decimalNumber = [NSDecimalNumber decimalNumberWithMantissa:531 exponent:-2 isNegative:NO];   //5.31
decimalNumber = [NSDecimalNumber decimalNumberWithMantissa:531 exponent:2 isNegative:YES]; //-53100

// locale代表一种格式
+ (NSDecimalNumber *)decimalNumberWithString:(nullable NSString *)numberValue locale:(nullable id)locale;
NSDictionary *locale = [NSDictionary dictionaryWithObject:@"," forKey:NSLocaleDecimalSeparator];    //以","当做小数点格式
NSDecimalNumber *discountAmount = [NSDecimalNumber decimalNumberWithString:@"123,45" locale:locale];    //123.45

+ (NSDecimalNumber *)decimalNumberWithString:(nullable NSString *)numberValue;

3.逻辑运算

加：
- (NSDecimalNumber *)decimalNumberByAdding:(NSDecimalNumber *)decimalNumber;
- (NSDecimalNumber *)decimalNumberByAdding:(NSDecimalNumber *)decimalNumber withBehavior:(nullable id )behavior;

减：
- (NSDecimalNumber *)decimalNumberBySubtracting:(NSDecimalNumber *)decimalNumber;
- (NSDecimalNumber *)decimalNumberBySubtracting:(NSDecimalNumber *)decimalNumber withBehavior:(nullable id )behavior;

乘：
- (NSDecimalNumber *)decimalNumberByMultiplyingBy:(NSDecimalNumber *)decimalNumber;
- (NSDecimalNumber *)decimalNumberByMultiplyingBy:(NSDecimalNumber *)decimalNumber withBehavior:(nullable id )behavior;

除：
- (NSDecimalNumber *)decimalNumberByDividingBy:(NSDecimalNumber *)decimalNumber;
- (NSDecimalNumber *)decimalNumberByDividingBy:(NSDecimalNumber *)decimalNumber withBehavior:(nullable id )behavior;

a的n次方：
- (NSDecimalNumber *)decimalNumberByRaisingToPower:(NSUInteger)power;
- (NSDecimalNumber *)decimalNumberByRaisingToPower:(NSUInteger)power withBehavior:(nullable id )behavior;

指数运算：
- (NSDecimalNumber *)decimalNumberByMultiplyingByPowerOf10:(short)power;
- (NSDecimalNumber *)decimalNumberByMultiplyingByPowerOf10:(short)power withBehavior:(nullable id )behavior;

四舍五入运算：
- (NSDecimalNumber *)decimalNumberByRoundingAccordingToBehavior:(nullable id )behavior;

比较运算：
-(NSComparisonResult)compare:(NSNumber *)decimalNumber;

4.数值格式设置

// roundingMode:舍入模式； scale：保留几位小数；exact：发生精确错误时是否抛出异常，一般为NO；overflow：发生溢出错误时是否抛出异常，一般为NO； underflow：发生不足错误时是否抛出异常，一般为NO； divideByZero：被0除时是否抛出异常，一般为YES
+ (instancetype)decimalNumberHandlerWithRoundingMode:(NSRoundingMode)roundingMode scale:(short)scale raiseOnExactness:(BOOL)exact raiseOnOverflow:(BOOL)overflow raiseOnUnderflow:(BOOL)underflow raiseOnDivideByZero:(BOOL)divideByZero;

NSDecimalNumberHandler *roundUp = [NSDecimalNumberHandler
                                       decimalNumberHandlerWithRoundingMode:NSRoundDown
                                       scale:2
                                       raiseOnExactness:NO
                                       raiseOnOverflow:NO
                                       raiseOnUnderflow:NO
                                       raiseOnDivideByZero:YES];

Swift中使用

/// 对数字进行保留2位有效数字处理
let item: Double = 5321209
let speedFormat = NSDecimalNumber.init(value: item / 1024 * 8)
let roundup = NSDecimalNumberHandler.init(roundingMode: .down, scale: 2, raiseOnExactness: false, raiseOnOverflow: false, raiseOnUnderflow: false, raiseOnDivideByZero: true)
let res = speedFormat.rounding(accordingToBehavior: roundup)