检查NSNumber是否为空 [英] check if NSNumber is empty

问题描述

如何检查NSNumber对象是否为空或为空？

OK nil很容易：

  NSNumber * myNumber; 
 if（myNumber == nil）
 doSomething 
  

已经创建，但是没有值，因为赋值失败，我如何检查这个？使用这样的东西？

  if（[myNumber intValue] == 0）
 doSomething 
  

有空测试对象的一般方法，如NSString可用（参见 post ）？

示例1

  NSMutableDictionary * dict = [[NSMutableDictionary alloc] init]; 
 [dict setValue：@forKey：@emptyValue]; 
 NSNumber * emptyNumber = [dict objectForKey：@emptyValue]; 
  

emptyNumber 包含哪个值？如何检查 emptyNumber 是否为空？

示例2

  NSMutableDictionary * dict = [[NSMutableDictionary alloc] init]; 
 [dict setValue：@forKey：@emptyValue]; 
 NSString * myString = [dict objectForKey：@emptyValue]; 
 if（myString == nil || [myString length] == 0）
 //得到一个空值
 NSNumber * emptyNumber = nil; 
 

如果我在 emptyNumber 设为nil？

  [emptyNumber intValue] 
  

我得零吗？

示例3

  NSMutableDictionary * dict = [[NSMutableDictionary alloc] init]; 
 [dict setValue：@forKey：@emptyValue]; 
 NSNumber * myEmptyValue = [dict objectForKey：@emptyValue]; 
 if（myEmptyValue == nil）
 // NSLog永远不会调用
 NSLog（@It is empty！ 
  

像这样NSLog不会被调用。 myEmptyValue 不是 nil ，而不是 NSNull 。那么它包含一个任意数字？

解决方案

NSValue ， NSNumber ，...应该从一个值创建并始终保持一个。测试 0 等特定值只有在不在您使用的有效值范围内时才有效。

在极少数情况下，如果代码具有代表无效或未设置的值，代码更为直接， t使用 nil （例如使用标准容器），您可以使用 NSNull 。 p>

在您的第一个示例中，可以是：

  [dict setValue：[NSNull null] forKey：@emptyValue]; 
 
 if（[dict objectForKey：@emptyValue] == [NSNull null]）{
 // ... 
} 
  nil （即不在容器中），例如无效：
  if （[dict objectForKey：@nonExistent] == nil）{
 // ... 
} 
  
对于第二个示例， -intValue 为您提供 0   - 但是因为发送消息到 nil 返回 0 。您还可以获得 0 。对于 intValue 设置为 0 之前的 NSNumber  
 
如上所述，如果 0  不是有效值，则只能执行类似的操作为你。 
          ：
 
 
 
 选项＃1： 
 
 
 
如果不需要所有值从数字范围，您可以使用一个（ 0 或 -1 或...）和 -intValue  / ...以具体表示空。这显然不适合你。
 
 
 
 选项＃2： 
 
 
 
   
：
  // add if not empty：
 [dict setObject：someNumber forKey：someKey]; 
 // remove if empty：
 [dict removeObjectForKey：someKey]; 
 //检索数字：
 NSNumber * num = [dict objectForKey：someKey]; 
 if（num == nil）{
 // ...不在字典中，代表空
} else {
 // ...不为空
} 
  
然而，这意味着空的键和永远不存在的键之间没有区别
   
 
 
 
在某些罕见的情况下更方便地将所有键保存在字典中，并用不同的值表示空。如果你不能使用一个从数字范围，我们必须把不同的东西在 NSNumber 没有一个概念空 。对于这种情况，Cocoa已经有 NSNull ：
  //设置为number如果不为空：
 [dict setObject：someNumber forKey：someKey]; 
 //如果为空，设置为NSNull：
 [dict setObject：[NSNull null] forKey：someKey]; 
 //检索数字：
 id obj = [dict objectForKey：someKey]; 
 if（obj == [NSNumber null]）{
 // ...空
} else {
 // ...不为空
 NSNumber * num = obj; 
 // ... 
} 
  
此选项现在允许您区分之间 （例如非法键）
 $ b $  b
How do I check if a NSNumber object is nil or empty?

OK nil is easy:
NSNumber *myNumber;
if (myNumber == nil)
    doSomething
But if the object has been created, but there is no value in it because an assignment failed, how can I check this? Use something like this?
if ([myNumber intValue]==0)
   doSomething
Is there a general method for testing objects on emptiness like for NSString available (see this post)?

Example 1
NSMutableDictionary *dict = [[NSMutableDictionary alloc] init];
[dict setValue:@"" forKey:@"emptyValue"];
NSNumber *emptyNumber = [dict objectForKey:@"emptyValue"];
Which value does emptyNumber contain? How can I check if emptyNumber is empty?

Example 2
NSMutableDictionary *dict = [[NSMutableDictionary alloc] init];
[dict setValue:@"" forKey:@"emptyValue"];
NSString *myString = [dict objectForKey:@"emptyValue"];
if (myString == nil || [myString length] == 0)
    // got an empty value
    NSNumber *emptyNumber=nil;
What happens if I use this after emptyNumber was set to nil?
[emptyNumber intValue]
Do I get zero?

Example 3
NSMutableDictionary *dict = [[NSMutableDictionary alloc] init];
[dict setValue:@"" forKey:@"emptyValue"];
NSNumber *myEmptyValue = [dict objectForKey:@"emptyValue"];
if (myEmptyValue == nil)
    // NSLog is never called
    NSLog(@"It is empty!");
Like this way NSLog is never called. myEmptyValue is not nil and not NSNull. So it contains an arbitrary number?
解决方案
NSValue, NSNumber, ... are supposed to be created from a value and to always hold one. Testing for a specific value like 0 only works if it isn't in the range of valid values you are working with.

In the rare case where code is more straight-forward to work with if you have a value that represents "invalid" or "not set" and you can't use nil (e.g. with the standard containers) you can use NSNull instead.

In your first example this could be:
[dict setValue:[NSNull null] forKey:@"emptyValue"];

if ([dict objectForKey:@"emptyValue"] == [NSNull null]) {
    // ...
}
But note that you can simply not insert (or remove) that value unless you need to differentiate nil (i.e. not in the container) and, say, "invalid":
if ([dict objectForKey:@"nonExistent"] == nil) {
    // ...
}
As for the second example, -intValue gives you 0 - but simply because sending messages to nil returns 0. You could also get 0 e.g. for a NSNumber whose intValue was set to 0 before, which could be a valid value.

As i already wrote above, you can only do something like this if 0 is not a valid value for you. Note the for you, what works best completely depends on what your requirements are.

Let me try to summarize:

Option #1: 

If you don't need all values from the numbers range, you could use one (0 or -1 or ...) and -intValue / ... to specifically represent "empty". This is apparently not the case for you.

Option #2:

You simply don't store or remove the values from the container if they are "empty":
// add if not empty:
[dict setObject:someNumber forKey:someKey];    
// remove if empty:
[dict removeObjectForKey:someKey];
// retrieve number:
NSNumber *num = [dict objectForKey:someKey];
if (num == nil) {
    // ... wasn't in dictionary, which represents empty
} else {
    // ... not empty
}
This however means that there is no difference between keys that are empty and keys that never exist or are illegal.

Option #3:

In some rare cases its more convenient to keep all keys in the dictionary and represent "empty" with a different value. If you can't use one from the number range we have to put something differently in as NSNumber doesn't have a concept of "empty". Cocoa already has NSNull for such cases:
// set to number if not empty:
[dict setObject:someNumber forKey:someKey];
// set to NSNull if empty:
[dict setObject:[NSNull null] forKey:someKey];
// retrieve number:
id obj = [dict objectForKey:someKey];
if (obj == [NSNumber null]) {
    // ... empty
} else { 
    // ... not empty
    NSNumber *num = obj;
    // ...
}
This option now allows you to differentiate between "empty", "not empty" and "not in the container" (e.g. illegal key).

                        
这篇关于检查NSNumber是否为空的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！