removeObject工作，但addObject不，都具有相同的參數

Question

在下面的代碼中，removeObject工作正常，但addObject不起作用。哪裏不對？

NSMutableArray *sourceList = self.questions; 
NSMutableArray *randomizedList; 

for (NSInteger i = sourceList.count; i > 0; i--) { 
    //other things 

    [randomizedList addObject:sourceList[index]]; 
    [sourceList removeObject:sourceList[index]]; 
} 

randomizedList相同之前，執行[randomizedList addObject:sourceList[index]];後：NSArray > NAObject >isa Class 0x0

Answer 1

的這裏的問題是，randomizedList未分配。

NSMutableArray *sourceList = self.questions; 
NSMutableArray *randomizedList = [[NSMutableArray alloc] init]; 

for (NSInteger i = sourceList.count; i > 0; i--) { 
    //other things 

    [randomizedList addObject:sourceList[index]]; 
    [sourceList removeObject:sourceList[index]]; 
}