大家都會(huì)在開發(fā)中遇到, 處理一個(gè)浮點(diǎn)型的數(shù)據(jù)時(shí), 有時(shí)候會(huì)有一個(gè)精度的丟失, 造成數(shù)據(jù)不準(zhǔn)確. float的精度為6-7位有效數(shù)字，double的精度為15-16位，所以在處理浮點(diǎn)型數(shù)據(jù)時(shí)盡量選擇double類型。但是在一些涉及數(shù)據(jù)計(jì)算，大小比較時(shí)責(zé)任重大，非常容易出錯(cuò)，因此使用系統(tǒng)自帶的NSDecimalNumber做處理, 能解決這問題。

1.NSDecimalNumber

NSDecimalNumber是NSNumber的不可變子類。蘋果針對(duì)浮點(diǎn)型計(jì)算時(shí)存在精度計(jì)算誤差的問題而提供的一個(gè)計(jì)算類,它是基于10進(jìn)制的定點(diǎn)計(jì)算保證了精度不會(huì)缺失。同時(shí)也可以定制精度的取正類型：向上取正、向下去正、四舍五入等。相對(duì)與浮點(diǎn)類型的計(jì)算，NSDecimalNumber提供了更加精準(zhǔn)的計(jì)算。

2.NSDecimalNumber 基本運(yùn)算

   //1、字符串 -> NSDecimalNumber
   NSDecimalNumber *num1 = [NSDecimalNumber decimalNumberWithString:@"123"];
   //2、NSNumber -> NSDecimalNumber(基礎(chǔ)類型的話，先轉(zhuǎn)成NSNumber)
   NSDecimalNumber *num2 = [NSDecimalNumber decimalNumberWithDecimal:[[NSNumber numberWithInt:123] decimalValue]];
   //相關(guān)運(yùn)算
   /* +
   - (NSDecimalNumber *)decimalNumberByAdding:(NSDecimalNumber *)decimalNumber;
   - (NSDecimalNumber *)decimalNumberByAdding:(NSDecimalNumber *)decimalNumber withBehavior:(nullable id <NSDecimalNumberBehaviors>)behavior;//帶保留小數(shù)位數(shù)
    */
   NSDecimalNumber *num3 = [num1 decimalNumberByAdding:num2];
   /*減 -
    - (NSDecimalNumber *)decimalNumberBySubtracting:(NSDecimalNumber *)decimalNumber;
    - (NSDecimalNumber *)decimalNumberBySubtracting:(NSDecimalNumber *)decimalNumber withBehavior:(nullable id <NSDecimalNumberBehaviors>)behavior;//帶保留小數(shù)位數(shù)
    */
   NSDecimalNumber *num4 = [num1 decimalNumberBySubtracting:num2];
   /*乘 *
    - (NSDecimalNumber *)decimalNumberByMultiplyingBy:(NSDecimalNumber *)decimalNumber;
    - (NSDecimalNumber *)decimalNumberByMultiplyingBy:(NSDecimalNumber *)decimalNumber withBehavior:(nullable id <NSDecimalNumberBehaviors>)behavior;//帶保留小數(shù)位數(shù)
    */
   NSDecimalNumber *num5 = [num1 decimalNumberByMultiplyingBy:num2];
   /*除 \
    - (NSDecimalNumber *)decimalNumberByDividingBy:(NSDecimalNumber *)decimalNumber;
    - (NSDecimalNumber *)decimalNumberByDividingBy:(NSDecimalNumber *)decimalNumber withBehavior:(nullable id <NSDecimalNumberBehaviors>)behavior;//帶保留小數(shù)位數(shù)
    */
   NSDecimalNumber *num6 = [num1 decimalNumberByDividingBy:num2];
   /*比較 
    - (NSComparisonResult)compare:(NSNumber *)decimalNumber;
    */
   NSComparisonResult result = [num1 compare:num2];
   NSLog(@"result == %ld",(long)result);
   //保留小數(shù)及相關(guān)參數(shù)含義
   /*保留小數(shù)
    + (instancetype)decimalNumberHandlerWithRoundingMode:(NSRoundingMode)roundingMode scale:(short)scale raiseOnExactness:(BOOL)exact raiseOnOverflow:(BOOL)overflow raiseOnUnderflow:(BOOL)underflow raiseOnDivideByZero:(BOOL)divideByZero;
    */
     // Rounding policies :
// Original
//    value 1.2  1.21  1.25  1.35  1.27
// Plain    1.2  1.2   1.3   1.4   1.3  四舍五入
// Down     1.2  1.2   1.2   1.3   1.2  向下取正
// Up       1.2  1.3   1.3   1.4   1.3  向上取正
// Bankers  1.2  1.2   1.2   1.4   1.3  (特殊的四舍五入，碰到保留位數(shù)后一位的數(shù)字為5時(shí)，根據(jù)前一位的奇偶性決定。為偶時(shí)向下取正，為奇數(shù)時(shí)向上取正。如：1.25保留1為小數(shù)。5之前是2偶數(shù)向下取正1.2；1.35保留1位小數(shù)時(shí)。5之前為3奇數(shù)，向上取正1.4）


   typedef NS_ENUM(NSUInteger, NSRoundingMode) {
       NSRoundPlain,   // Round up on a tie
       NSRoundDown,    // Always down == truncate
       NSRoundUp,      // Always up
       NSRoundBankers  // on a tie round so last digit is even
   };
  //scale：保留有效小數(shù)的個(gè)數(shù)（為0的無效小數(shù)后自動(dòng)過濾).
  //Exactness：進(jìn)度異常、Overflow:向上溢出、Underflow：向下溢出、DivideByZero：除數(shù)為0。當(dāng)參數(shù)為YES出錯(cuò)會(huì)
  拋出異常，為NO時(shí)忽略異常。返回nil.
   NSDecimalNumberHandler *roundUp = [NSDecimalNumberHandler
                                      decimalNumberHandlerWithRoundingMode:NSRoundUp
                                      scale:2
                                      raiseOnExactness:NO
                                      raiseOnOverflow:NO
                                      raiseOnUnderflow:NO
                                      raiseOnDivideByZero:YES];

所以在處理有關(guān)浮點(diǎn)型數(shù)據(jù)，后臺(tái)傳字符串的格式，防止丟失精度。