有时会遇到价格类似精度的问题，尤其是float相减时遇到的差距，处理方案：转为NSDecimalNumber类型
//最终的真实价格
NSString * finalResult = [NSString stringWithFormat:@"%f",[self getOriginalAmount:self.finalResultArr]];
NSDecimalNumber *finalNumber = [NSDecimalNumber decimalNumberWithString:finalResult];
//售价
NSString * saleStr = [NSString stringWithFormat:@"%f",self.saleAmount];
NSDecimalNumber *saleNumber = [NSDecimalNumber decimalNumberWithString:saleStr];
//计算差价
NSDecimalNumber*cha = [finalNumber decimalNumberBySubtracting:saleNumber];
filat chafloat = [NSString stringWithFormat:@"特价折扣优惠 %.2f元",cha.floatValue];
