ios - OC中一个关于数组的问题。

Question

最近刚学iOS不久 跟着斯坦福上学着做计算器app

当用数组把操作数存入到栈中 发现数组中永远只有一个元素

---

@interface ViewController ()
@property (strong, nonatomic) IBOutlet UILabel *display;
@end

bool userIsInTheMiddleOfTyping = FALSE;
@implementation ViewController
- (IBAction)appendDigit:(UIButton *)sender {
    if(userIsInTheMiddleOfTyping){
    NSString *digit = sender.currentTitle;
    _display.text = [_display.text stringByAppendingString:digit];
    }else{
        _display.text = sender.currentTitle;
        userIsInTheMiddleOfTyping=TRUE;
    }
    
}

- (IBAction)enter {
    userIsInTheMiddleOfTyping = FALSE;
    NSMutableArray *operandstack = [[NSMutableArray alloc]init];
    [operandstack addObject:_display.text];
    NSLog(@"operandstack = %li", operandstack.count);
    NSLog(@"operandstack = %@", operandstack);

}

当点击一个enter按钮的时候 数组里就会加一个元素 但是情况是数组里永远只有一个元素

第一次点击后NSLog输出数组的count和内容的时候

2017-01-15 08:47:50.620 MYCalculator[32235:2359081] operandstack = 1
2017-01-15 08:47:50.622 MYCalculator[32235:2359081] operandstack = (
    3
)

之后第二次 第三次 都是显示有一个元素 永远是Label上那个最新的元素

2017-01-15 08:47:50.620 MYCalculator[32235:2359081] operandstack = 1
2017-01-15 08:47:50.622 MYCalculator[32235:2359081] operandstack = (
    3
)
2017-01-15 08:52:20.332 MYCalculator[32235:2359081] operandstack = 1
2017-01-15 08:52:20.334 MYCalculator[32235:2359081] operandstack = (
    236
) 

我就想知道之前加入数组的3 怎么不见了

小白一个 大神们轻喷

Answer 1

In Objective-C, alloc is to open up a memory space, and init is to initialize.
Every time you click the button, the array operandstack is re-initialized in the method - (IBAction)enter , and then new elements are added. - (IBAction)enter 中重新初始化了数组 operandstack ，然后加入新的元素。
把 operandstack 数组写成 property ，然后在 - (void)viewDidLoadWrite the operandstack array as property, and then initialize it once in the - (void)viewDidLoad method:

- (void)viewDidLoad  {
    self.operandstack = [NSMutableArray array];
}

Answer 2

Because the initialization code of NSMutablearray is placed in the click event, NSMutablearray is re-initialized every time it is clicked, which means that the previous elements are lost, so naturally, there is only one element. You can use NSMutablearray as a member variable or attribute to solve this problem.