爲什麼這個NSNumber總和會丟回一個奇怪的結果？

Question

我正在製作一個處理計算金錢的應用程序，並且已經閱讀了它總是最好使用NSNumber的。所以我已經將我的浮點值更改爲NSNumber，但是現在我得到了下面的奇怪結果，我不知道它爲什麼會發生。

這是代碼：

- (NSDecimalNumber*) subtotal { 
    NSDecimalNumber *subtotal = [NSDecimalNumber decimalNumberWithString:@"0"]; 
    NSLog(@"subtotal initially set at %@",subtotal); 
    for (NSManagedObject *object in [fetchedResultsController fetchedObjects]) { 
     NSLog(@"Looping"); 
     NSDecimalNumber *objectRowTotalNumber = [object valueForKey:@"total"]; 
     NSLog(@"object row total number = %@",objectRowTotalNumber); 
     subtotal = [subtotal decimalNumberByAdding:objectRowTotalNumber]; 

     NSLog(@"Subtotal: %@", subtotal); 
    } 

    return 0; 
} 

這是日誌輸出：

**2011-10-19 09:27:19.289 Market[4240:b603] subtotal initially set at 0** 

**2011-10-19 09:27:19.290 Market[4240:b603] Looping** 

**2011-10-19 09:27:19.291 Market[4240:b603] object row total number = 5** 

**2011-10-19 09:27:19.292 Market[4240:b603] Subtotal: -0.00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000244200806570181766106054656** 

會有人知道是什麼原因造成的奇結果時，小計應簡單設置爲5？

最新測試：本NSDecimalNumberMethod只適用於我在會話中添加新的對象，而不是在被加載

-(void) viewDidAppear:(BOOL)animated { 
NSLog(@"I'm in ViewDidAppear"); 
// Reload subtotal via float method 
[self subtotal]; 
//Reload subtotal via decimal method 
[self subtotalDec]; 
} 

對象與浮法

-(float) subtotal { 
NSLog(@"I'm in subtotal using floats"); 

float subtotal = 0; 
NSLog(@"subtotal initially set at %@",subtotal); 
for (NSManagedObject *object in [fetchedResultsController fetchedObjects]) { 
NSLog(@"Looping"); 
NSNumber *objectRowTotalNumber = [object valueForKey:@"total"]; 
NSLog(@"object row total number = %@",objectRowTotalNumber); 
float objectRowTotal = [objectRowTotalNumber floatValue]; 
subtotal = subtotal + objectRowTotal; 
} 
NSLog(@"Subtotal: %f", subtotal); 
return 0; 
} 

與計算計算NSDecimal號碼方法

- (NSDecimalNumber*) subtotalDec { 
NSLog(@"I'm in subtotal using NSDecimalNumber"); 
NSDecimalNumber *subtotal = [NSDecimalNumber decimalNumberWithString:@"0"]; 
NSLog(@"subtotal initially set at %@",subtotal); 
for (NSManagedObject *object in [fetchedResultsController fetchedObjects]) { 
NSLog(@"Looping"); 
NSDecimalNumber *objectRowTotalNumber = [object valueForKey:@"total"]; 
NSLog(@"object row total number = %@",objectRowTotalNumber); 
subtotal = [subtotal decimalNumberByAdding:objectRowTotalNumber]; 
} 
NSLog(@"Subtotal: %@",subtotal); 
return 0; 
} 

日誌視圖會出現

2011-10-19 11:19:01.506 Market[961:b603] I'm in ViewDidAppear 
2011-10-19 11:19:01.506 Market[961:b603] I'm in subtotal using floats 
2011-10-19 11:19:01.507 Market[961:b603] subtotal initially set at (null) 
2011-10-19 11:19:01.508 Market[961:b603] Looping 
2011-10-19 11:19:01.508 Market[961:b603] object row total number = 4 
2011-10-19 11:19:01.509 Market[961:b603] Looping 
2011-10-19 11:19:01.509 Market[961:b603] object row total number = 1 
2011-10-19 11:19:01.509 Market[961:b603] Looping 
2011-10-19 11:19:01.510 Market[961:b603] object row total number = 10 
2011-10-19 11:19:01.510 Market[961:b603] Subtotal: 15.000000 
2011-10-19 11:19:01.510 Market[961:b603] I'm in subtotal using NSDecimalNumber 
2011-10-19 11:19:01.511 Market[961:b603] subtotal initially set at 0 
2011-10-19 11:19:01.511 Market[961:b603] Looping 
2011-10-19 11:19:01.511 Market[961:b603] object row total number = 4 
2011-10-19 11:19:01.512 Market[961:b603] Subtotal: -0.00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000244200806569055866199212032 
2011-10-19 11:19:01.512 Market[961:b603] Looping 
2011-10-19 11:19:01.512 Market[961:b603] object row total number = 1 
2011-10-19 11:19:01.513 Market[961:b603] Subtotal: -0.00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000488401613129104533143683072 
2011-10-19 11:19:01.513 Market[961:b603] Looping 
2011-10-19 11:19:01.513 Market[961:b603] object row total number = 10 
2011-10-19 11:19:01.514 Market[961:b603] Subtotal: -0.00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000732602419703789898877108224 

日誌addbuttontapped：注意NSDecimalNumberMethod如何工作只能在最後的附加價值，而不是在程序啓動

2011-10-20 07:17:03.218 Market[2199:b603] I'm in subtotal using floats 
2011-10-20 07:17:03.218 Market[2199:b603] subtotal initially set at (null) 
2011-10-20 07:17:03.219 Market[2199:b603] Looping 
2011-10-20 07:17:03.219 Market[2199:b603] object row total number = 4 
2011-10-20 07:17:03.219 Market[2199:b603] Looping 
2011-10-20 07:17:03.219 Market[2199:b603] object row total number = 1 
2011-10-20 07:17:03.220 Market[2199:b603] Looping 
2011-10-20 07:17:03.220 Market[2199:b603] object row total number = 10 
2011-10-20 07:17:03.220 Market[2199:b603] Looping 
2011-10-20 07:17:03.221 Market[2199:b603] object row total number = 2 
2011-10-20 07:17:03.221 Market[2199:b603] Subtotal: 17.000000 
2011-10-20 07:17:03.221 Market[2199:b603] I'm in subtotal using NSDecimalNumber 
2011-10-20 07:17:03.222 Market[2199:b603] subtotal initially set at 0 
2011-10-20 07:17:03.222 Market[2199:b603] Looping 
2011-10-20 07:17:03.222 Market[2199:b603] object row total number = 4 
2011-10-20 07:17:03.223 Market[2199:b603] Subtotal: -0.00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000244200806569055866199212032 
2011-10-20 07:17:03.223 Market[2199:b603] Looping 
2011-10-20 07:17:03.224 Market[2199:b603] object row total number = 1 
2011-10-20 07:17:03.224 Market[2199:b603] Subtotal: -0.00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000488401613129104533143683072 
2011-10-20 07:17:03.224 Market[2199:b603] Looping 
2011-10-20 07:17:03.225 Market[2199:b603] object row total number = 10 
2011-10-20 07:17:03.225 Market[2199:b603] Subtotal: -0.00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000732602419703789898877108224 
2011-10-20 07:17:03.225 Market[2199:b603] Looping 
2011-10-20 07:17:03.226 Market[2199:b603] object row total number = 2 
2011-10-20 07:17:03.226 Market[2199:b603] Subtotal: 2 

Answer 1

爲了讓它起作用，我不得不在代碼中添加兩行代碼。這是我必須做的。

- (NSDecimalNumber*) subtotalDec { 
NSLog(@"I'm in subtotal using NSDecimalNumber"); 
NSDecimalNumber *subtotal = [NSDecimalNumber decimalNumberWithString:@"0"]; 
NSLog(@"subtotal initially set at %@",subtotal); 
for (NSManagedObject *object in [fetchedResultsController fetchedObjects]) { 
NSLog(@"Looping"); 
NSDecimalNumber *objectRowTotalNumber = [object valueForKey:@"total"]; 
NSLog(@"object row total number = %@",objectRowTotalNumber); 
//This is the start of added code 
NSString *objectRowString = [NSString stringWithFormat:@"%@",objectRowTotalNumber]; 
NSDecimalNumber *objectRowTotal = [NSDecimalNumber decimalNumberWithString:objectRowString]; 
//This is the end of added code 
subtotal = [subtotal decimalNumberByAdding:objectRowTotal]; 

NSLog(@"Subtotal: %@",subtotal); 

} 

Answer 2

我不前中輸入的那些不認爲[object valueForKey:@"total"]是給你一個NSDecimalNumber，而是NSNumber。

更新：

顯然這是正確的方向。因此，請取消刪除帖子，以便我們可以有一個正確回答的問題。

我刪除了這個，因爲在寫完之後，我試着用一個NSNumber嘗試decimalNumberByAdding:，它崩潰了。但是這是在OSX 10.7 SDK項目中，所以顯然這在iOS中得到了不同的處理。