原值:2.7999999999

typedef CF_ENUM(CFIndex, CFNumberFormatterRoundingMode) {
    kCFNumberFormatterRoundCeiling = 0,//四舍五入,直接輸出3
    kCFNumberFormatterRoundFloor = 1,//保留小數輸出2.8
    kCFNumberFormatterRoundDown = 2,//加上了人民幣標志,原值輸出￥2.8
    kCFNumberFormatterRoundUp = 3,//本身數值乘以100后用百分號表示,輸出280%
    kCFNumberFormatterRoundHalfEven = 4,//輸出2.799999999E0
    kCFNumberFormatterRoundHalfDown = 5,//原值的中文表示,輸出二點七九九九。。。。
kCFNumberFormatterRoundHalfUp = 6//原值中文表示,輸出第三
};

NSNumberFormatter *formatter = [[NSNumberFormatter alloc] init];
formatter.numberStyle = kCFNumberFormatterRoundHalfDown;
NSString *string = [formatter stringFromNumber:[NSNumber numberWithInt:12]];
NSLog(@"str = %@", string);