當我在NSMutableArray上調用方法時，出現錯誤「發送給實例的無法識別的選擇器」

Question

我有一個可變數組，allRows。在循環中，我將allRows（其中也包含數組）的每個索引分配給另一個數組。但是，在該數組上調用shuffle方法時，程序會崩潰。該方法是一個類別，適用於其他陣列;只是不是row。

- (void)selectQuestions { 

self.quizQuestions = [[NSMutableArray alloc] init]; 

for (int j = 0; j<self.maxRowNumber; j++) { 
    //shuffle each row 
    NSMutableArray *row = [allRows objectAtIndex:j]; 
    [row shuffle]; 

}

@implementation NSMutableArray (shuffle) 

-(void) shuffle { 

NSUInteger count = [self count]; 
for (NSUInteger i = 0; i< count; ++i) { 

    int nElements = count - i; 
    int randomIndex = arc4random() % (nElements) + i ; 
    [self exchangeObjectAtIndex:i withObjectAtIndex:randomIndex]; 
} 
} 

EDIT

allRows在類的init方法被實例化。我添加了整個方法，以防有其他需要查看的內容。

- (id)init { 
self = [super init]; 
if (self) { 

    // Question, Reactants, Products, Elements 
    NSArray *R1Q1 = [[NSArray alloc] initWithObjects:@"Methanol is burned completely in air", @"2CH₃OH(l) + 3O₂(g)", @"2CO₂(g) + 4H₂O", @"C,H,O", nil]; 
    NSArray *R1Q2 = [[NSArray alloc] initWithObjects:@"Ammonia is burned in excess oxygen gas", @"4NH₃(g) + 7H₂O(l)", @"4NO₂(g) + 6H₂O(l)", @"N,H,O", nil]; 
    NSArray *R1Q3 = [[NSArray alloc] initWithObjects:@"Hydrogen sulfide gas is burned in excess oxygen gas", @"2H₂S(g) + 3O₂(g)", @"CO₂(g) + 2SO₂(g)", @"H,S,O", nil]; 

    NSArray *R2Q1 = [[NSArray alloc] initWithObjects:@"Solid potassium is added to a flask of oxygen gas", @"K(s) + O₂(g)", @"KO₂(s)", @"K,O", nil]; 
    NSArray *R2Q2 = [[NSArray alloc] initWithObjects:@"Sodium metal is dropped into a flask of pure water", @"2Na(s) + H₂O(l)", @"2Na⁺(aq) + 2OH⁻(aq) + H₂(g)", @"Na,H,O", nil]; 
    NSArray *R2Q3 = [[NSArray alloc] initWithObjects:@"A piece of lithium is heated strongly in oxygen", @"4Li(s) + O₂(g)", @"2Li₂O(s)", @"Li,O", nil]; 

    NSArray *R3Q1 = [[NSArray alloc] initWithObjects:@"Solutions of potassium chloride and silver nitrate are mixed", @"Ag⁺(aq) + Cl⁻(aq)", @"AgCl(s)", @"K,Cl,Ag,N,O", nil]; 
    NSArray *R3Q2 = [[NSArray alloc] initWithObjects:@"Solutions of iron(III) nitrate and sodium hydroxide are mixed", @"Fe³⁺(aq) + 3OH⁻(aq)", @"Fe(OH)₃(s)", @"Fe,N,O,Na,H", nil]; 
    NSArray *R3Q3 = [[NSArray alloc] initWithObjects:@"Solutions of nickel iodide and barium hydroxide are mixed", @"Ni²⁺(aq) + 2OH⁻(aq)", @"Ni(OH)₂(s)", @"Ni,I,Ba,OH", nil]; 

    row1 = [[NSArray alloc] initWithObjects:R1Q1, R1Q2, R1Q3, nil]; 
    row2 = [[NSArray alloc] initWithObjects:R2Q1, R2Q2, R2Q3, nil]; 
    row3 = [[NSArray alloc] initWithObjects:R3Q1, R3Q2, R3Q3, nil]; 
    //add rest 

    allRows = [[NSMutableArray alloc] initWithObjects:row1, row2, row3, nil]; 
    self.maxRowNumber = 3; //hypothetical 
    self.questionsPerRow = 2; // " " 
} 

Answer 1

根據錯誤您發佈它看起來像[allRows objectAtIndex:j]將返回一個NSArray不是一個NSMutableArray。快速解決就是這樣的。

NSMutableArray *row = [NSMutableArray arrayWithArray:[allRows objectAtIndex:j]]; 

但它可能更好，你把NSMutableArrays如果你想要得到一個。