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NSDecimalNumber是NSNumber的子类，比NSNumber的功能更为强大，可以指定一个数的幂，四舍五入等操作。
NSString类型转化成CGFloat 精度不高的情况。例如 @"8" -> 7.99999 @"1.0" -> 1.00001
由于NSDecimalNumber精度较高，所以会比基本数据类型费时，所以需要权衡考虑，苹果官方建议在货币以及要求精度很高的场景下使用。
一、一些科学计数法的简单方法。
//一个关于科学计数法的方法：参数 1.mantissa 实数 2.exponent 指数 3.flag 正负
- (instancetype)initWithMantissa:(unsigned long long)mantissa exponent:(short)exponent isNegative:(BOOL)flag;
+ (NSDecimalNumber *)decimalNumberWithMantissa:(unsigned long long)mantissa exponent:(short)exponent isNegative:(BOOL)flag;
例：mantissa ＝ 6 ，exponent ＝ 2 ，flag ＝ false 结果：6*10^2 = 600。
//一个科学计数法的初始化方法：numberValue 一定格式的字符串（详情见官方文档）
- (instancetype)initWithString:(nullable NSString *)numberValue;
+ (NSDecimalNumber *)decimalNumberWithString:(nullable NSString *)numberValue;
例：NSDecimalNumber *a = [[NSDecimalNumber alloc] initWithString:@"-4e3"]; 输出：-4000。
二、NSDecimalNumber 对象值进行操作
//一些基本赋值
+ (NSDecimalNumber *)zero; //Returns an NSDecimalNumber object equivalent to the number 1.0.
+ (NSDecimalNumber *)one; //Returns an NSDecimalNumber object equivalent to the number 0.0.
+ (NSDecimalNumber *)minimumDecimalNumber; //-340282366920938463463374607431768211455
+ (NSDecimalNumber *)maximumDecimalNumber;
+ (NSDecimalNumber *)notANumber; //Returns an NSDecimalNumber object that specifies no number.非数字
例：NSDecimalNumber * a = [NSDecimalNumber decimalNumberWithString:@"2.5301"];
NSDecimalNumber * b = [NSDecimalNumber decimalNumberWithString:@"7.5001"];
NSDecimalNumberHandler *roundingBehavior = [NSDecimalNumberHandler decimalNumberHandlerWithRoundingMode:NSRoundBankers
scale:2
raiseOnExactness:NO
raiseOnOverflow:NO
raiseOnUnderflow:NO
raiseOnDivideByZero:NO];
//加法运算 参数 behavior 对的到的结构做一些限制（后面有说明）
- (NSDecimalNumber *)decimalNumberByAdding:(NSDecimalNumber *)decimalNumber;
- (NSDecimalNumber *)decimalNumberByAdding:(NSDecimalNumber *)decimalNumber withBehavior:(nullable id <NSDecimalNumberBehaviors>)behavior;
eg. NSDecimalNumber *c = [a decimalNumberByAdding:b]; 结果：10.032
NSDecimalNumber *c = [a decimalNumberByAdding:b withBehavior:roundingBehavior]; 结果：10.03
//减法运算
- (NSDecimalNumber *)decimalNumberBySubtracting:(NSDecimalNumber *)decimalNumber;
- (NSDecimalNumber *)decimalNumberBySubtracting:(NSDecimalNumber *)decimalNumber withBehavior:(nullable id <NSDecimalNumberBehaviors>)behavior;
eg.NSDecimalNumber *c = [a decimalNumberBySubtracting:b]; 结果：-4.97
NSDecimalNumber *c = [a decimalNumberBySubtracting:b withBehavior:roundingBehavior]; 结果：-4.97
//乘法运算
- (NSDecimalNumber *)decimalNumberByMultiplyingBy:(NSDecimalNumber *)decimalNumber;
- (NSDecimalNumber *)decimalNumberByMultiplyingBy:(NSDecimalNumber *)decimalNumber withBehavior:(nullable id <NSDecimalNumberBehaviors>)behavior;
eg.NSDecimalNumber *c = [a decimalNumberByMultiplyingBy:b]; 结果：18.97600301
NSDecimalNumber *c = [a decimalNumberByMultiplyingBy:b withBehavior:roundingBehavior]; 结果：18.98
//除法运算
- (NSDecimalNumber *)decimalNumberByDividingBy:(NSDecimalNumber *)decimalNumber;
- (NSDecimalNumber *)decimalNumberByDividingBy:(NSDecimalNumber *)decimalNumber withBehavior:(nullable id <NSDecimalNumberBehaviors>)behavior;
eg.NSDecimalNumber *c = [a decimalNumberByDividingBy:b]; 结果：0.3373421687710830522259703203957280
NSDecimalNumber *c = [a decimalNumberByDividingBy:b withBehavior:roundingBehavior]; 结果：0.34
//a的n次方 参数
- (NSDecimalNumber *)decimalNumberByRaisingToPower:(NSUInteger)power;
- (NSDecimalNumber *)decimalNumberByRaisingToPower:(NSUInteger)power withBehavior:(nullable id <NSDecimalNumberBehaviors>)behavior;
eg.NSDecimalNumber *c = [a decimalNumberByRaisingToPower:3]; 结果：16.196197345901
NSDecimalNumber *c = [a decimalNumberByRaisingToPower:3 withBehavior:roundingBehavior]; 结果：16.2
//指数运算
- (NSDecimalNumber *)decimalNumberByMultiplyingByPowerOf10:(short)power;
- (NSDecimalNumber *)decimalNumberByMultiplyingByPowerOf10:(short)power withBehavior:(nullable id <NSDecimalNumberBehaviors>)behavior;
//四舍五入
- (NSDecimalNumber *)decimalNumberByRoundingAccordingToBehavior:(nullable id <NSDecimalNumberBehaviors>)behavior;
eg.NSDecimalNumber *c = [a decimalNumberByRoundingAccordingToBehavior:roundingBehavior]; 结果：2.53
//两数比较
- (NSComparisonResult)compare:(NSNumber *)decimalNumber;
eg.NSLog(@"%ld",(long)[a compare:b]); 结果 -1.
//设置默认的behaviors对象
+ (void)setDefaultBehavior:(id <NSDecimalNumberBehaviors>)behavior; + (id <NSDecimalNumberBehaviors>)defaultBehavior; // One behavior per thread - The default behavior is // rounding mode: NSRoundPlain // scale: No defined scale (full precision) // ignore exactnessException // raise on overflow, underflow and divide by zero.
//NSDecimalNumberBehaviors对象的创建 参数 1.RoundingMode 一个取舍枚举值 2.scale 处理范围 3.raiseOnExactness 提高精准度 4.Overflow 溢出 5.Underflow 下溢 6.DivideByZero 可否除以0.
NSDecimalNumberHandler *roundUp = [NSDecimalNumberHandler
decimalNumberHandlerWithRoundingMode:NSRoundBankers
scale:2
raiseOnExactness:NO
raiseOnOverflow:NO
raiseOnUnderflow:NO
raiseOnDivideByZero:NO];
//四舍五入相关枚举
typedef NS_ENUM(NSUInteger, NSRoundingMode) {
NSRoundPlain, // Round up on a tie 取整
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 四舍五入
};
注：NSDecimalNumberHandler 后面全部设为 NO 的话，默认只进行后两位的的四舍五入操作，再多就是直接舍去。 需要更精确的四舍五入需要scale后面参数某个改为YES。
目前 NSDecimalNumber 主要用于 货币计算 和 四舍五入 两个处。如果有更多地方能用到希望补充，谢谢。
【2016-5-25】NSDecimalNumber 知识点的简单整理
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原文地址：http://www.cnblogs.com/airende/p/5526305.html
