为什么NSMutableArray有重复的对象？

Question

我有一些代码可以创建一个字符串并将该字符串添加到可变数组中。下面是代码：

ExportBookData *abe = [[ExportBookData alloc] initWithCategory:@"ABE"]; 
    abe.builtFileList = [[NSMutableDictionary alloc] initWithCapacity: 2]; 
    abe.exportData = [NSMutableArray array]; 

     for(int i = 0; i < booksArray.count; i++) { 
     [tabString setString: @""]; // clear it... 
     [tabString appendString:[NSString stringWithFormat:@"%@\t%@\t%@\t%@\t%@\t%@\t%@\t%@\t%@\t%@\t" 
           "%@\t%@\t%@\t%@\t%@\t%@\t%@\t%@\t%@\t%@\t%@\t%@\t%@\t%@\t%@\t%@\n", 

           [[booksArray objectAtIndex:i] sku], 
           [[booksArray objectAtIndex:i] title], 
           [[booksArray objectAtIndex:i] author], 
           [[booksArray objectAtIndex:i] illustrator], 
           [[booksArray objectAtIndex:i] price], 
           [(Books *)[booksArray objectAtIndex:i] quantity], 
           @"Book", // book type 
           [[booksArray objectAtIndex:i] bookDescription], 
           [[booksArray objectAtIndex:i] binding], 
           [[booksArray objectAtIndex:i] bookCondition], 
           [[booksArray objectAtIndex:i] pubName], 
           [[booksArray objectAtIndex:i]pubLocation ], 
           [[booksArray objectAtIndex:i] pubYear], 
           [[booksArray objectAtIndex:i] isbn], 
           [[booksArray objectAtIndex:i] primaryCatalog], 
           @"",@"", // seller catalogs secondary and third catalog entries (not used) 
           @"", // ABE category <----- TODO 
           [[booksArray objectAtIndex:i] keywords], 
           [[booksArray objectAtIndex:i] jacket], 
           [[booksArray objectAtIndex:i] edition], 
           [[booksArray objectAtIndex:i] printing], 
           [[booksArray objectAtIndex:i] signedBy], 
           [[booksArray objectAtIndex:i]volume], 
           [[booksArray objectAtIndex:i] bookSize], 
           @"" // image url 
           ]]; 

     NSLog(@"\n\ntabString: %@",tabString); 

     [abe.exportData addObject: tabString]; // add to array 

     NSLog(@"\n\nexportData: %@", abe.exportData); // <-------------- overwritten with last entry, so all entries are the same TODO 
    } 

booksArray是通过读取存储CoreData填补了的NSMutableArray; exportData也是一个NSMutableArray。 books数组中的数据是有效的，计数为2; tabString中的数据也是有效的，包含来自booksArray的正确行。该生产线

[abe.exportData addObject: tabString]; 

拥有的最后一个记录的booksArray重复。这是从日志方法的输出：

---->booksArray.count: 2 

tabString: 120 Women Who Run With The Wolves: Myths And Stories Of The Wild Woman Archetype Clarissa Pinkola Estes  2.1 1 Book  Hardcover  Ballantine Books  1992 0345377443      Like New  (null)    

exportData: (
"120\tWomen Who Run With The Wolves: Myths And Stories Of The Wild Woman Archetype\tClarissa Pinkola Estes\t\t2.1\t1\tBook\t\tHardcover\t\tBallantine Books\t\t1992\t0345377443\t\t\t\t\t\tLike New\t\t(null)\t\t\t\t\n" 
) 

tabString: 121 Colossus: The Secrets Of Bletchley Park's Code-breaking Computers B. Jack Copeland  35 1 Book  Paperback (Reprint)  Oxford University Press, USA  2010 9780199578146      No Dust Jacket  (null)    

exportData: (
"121\tColossus: The Secrets Of Bletchley Park's Code-breaking Computers\tB. Jack Copeland\t\t35\t1\tBook\t\tPaperback (Reprint)\t\tOxford University Press, USA\t\t2010\t9780199578146\t\t\t\t\t\tNo Dust Jacket\t\t(null)\t\t\t\t\n", 
"121\tColossus: The Secrets Of Bletchley Park's Code-breaking Computers\tB. Jack Copeland\t\t35\t1\tBook\t\tPaperback (Reprint)\t\tOxford University Press, USA\t\t2010\t9780199578146\t\t\t\t\t\tNo Dust Jacket\t\t(null)\t\t\t\t\n" 
) 

我都看了又看（谷歌，SO）几个小时，现在，我是在做正确的一切，从我在其他问题的回答看，和我不看看有什么可能造成这种情况。非常感谢帮助。 SD

Answer 1

您的tabString似乎是您尝试反复使用的NSMutableString。这是行不通的。您最终将同一个可变字符串添加到数组中多次。只需将stringWithFormat分配到正常的本地NSString即可。

为什么一遍又一遍地拨打[booksArray objectAtIndex:i]？获取对象一次。更好的是，在循环中使用快速枚举。

for (YourBookClass *book in booksArray) { 
    NSString *tabString = [NSString stringWithFormat:@"%@\t%@\t%@\t%@\t%@\t%@\t%@\t%@\t%@\t%@\t" 
          "%@\t%@\t%@\t%@\t%@\t%@\t%@\t%@\t%@\t%@\t%@\t%@\t%@\t%@\t%@\t%@\n", 
     book.sku, 
     book.title, 
     // and all of the rest of the properties 
     ]]; 

    [abe.exportData addObject:tabString]; 
} 

还请记住，如果任何值包含制表符或换行符，您的代码将生成无效的CSV文件。这些值需要加上引号。