为什么不将值存储到NSDictionary中？

Question

请参阅代码片段末尾的日志语句注释... acceleration.x是一个小数，但是当我将它存储到dict时，它似乎是0。

NSDictionary *entry = [NSDictionary dictionaryWithObjectsAndKeys:
  [NSNumber numberWithDouble:acceleration.x], @"x",
  [NSNumber numberWithDouble:acceleration.y], @"y",
  [NSNumber numberWithDouble:acceleration.z], @"z",
  [NSDate date], @"date",
   nil];
[self.accelerometerArray addObject: entry];
//this logs a decimal number as expected
NSLog(@"Acceleration.x: %g", acceleration.x);
//this logs a 0 :(
NSLog(@"Accel Array X: %g", [entry objectForKey: @"x"]);

Answer 1

致电时：

[entry objectForKey: @"x"]

您返回NSNumber实例，而不是double值。将该行更改为：

NSLog(@"Accel Array X: %g", [[entry objectForKey: @"x"] doubleValue]);

您将无法直接从NSDictionary检索double，因为键和值只能是对象，而不是基本类型。