创建NSDictionary

Question

在以下代码中,第一个日志语句按预期显示小数,但第二个日志语句为NULL.我究竟做错了什么？

NSDictionary *entry = [[NSDictionary alloc] initWithObjectsAndKeys:
  @"x", [NSNumber numberWithDouble:acceleration.x],
  @"y", [NSNumber numberWithDouble:acceleration.y],
  @"z", [NSNumber numberWithDouble:acceleration.z],
  @"date", [NSDate date],
  nil];
NSLog([NSString stringWithFormat:@"%@", [NSNumber numberWithDouble:acceleration.x]]);
NSLog([NSString stringWithFormat:@"%@", [entry objectForKey:@"x"]]);

Answer 1

您正在交换插入对象和键的顺序:您需要首先插入对象,然后插入密钥,如以下示例所示.

NSDictionary *dict = [[NSDictionary alloc] initWithObjectsAndKeys:@"value1", @"key1", @"value2", @"key2", nil];

Answer 2

新的Objective-c支持静态初始化的这种新语法.

@{key:value}

例如:

NSDictionary* dict = @{@"x":@(acceleration.x), @"y":@(acceleration.y), @"z":@(acceleration.z), @"date":[NSDate date]};

Answer 3

NSDictionary语法:

NSDictionary *dictionaryName = [NSDictionary dictionaryWithObjectsAndKeys:@"value1",@"key1",@value2",@"key2", nil];

例:

NSDictionary *importantCapitals = [NSDictionary dictionaryWithObjectsAndKeys:
@"NewDelhi",@"India",@"Tokyo",@"Japan",@"London",@"UnitedKingdom", nil];
NSLog(@"%@", importantCapitals);

输出看起来像,

{India = NewDelhi; 日本=东京; UnitedKingdom =伦敦; }