当我试图理解CAP中的“Availability”(A)和“Partition tolerance”(P)时，我发现很难理解各种文章的解释。

我有一种感觉，a和P可以同时出现(我知道事实并非如此，这就是我不能理解的原因!)

简单地解释一下，什么是A和P以及它们之间的区别?

我经常看到这样的代码:遍历数据库查询的结果，对每一行执行一些操作，然后移动到下一行。典型例子如下。

Cursor cursor = db.rawQuery(...);
cursor.moveToFirst();
while (cursor.isAfterLast() == false) 
{
    ...
    cursor.moveToNext();
}

Cursor cursor = db.rawQuery(...);
for (boolean hasItem = cursor.moveToFirst(); 
     hasItem; 
     hasItem = cursor.moveToNext()) {
    ...
}

Cursor cursor = db.rawQuery(...);
if (cursor.moveToFirst()) {
    do {
        ...                 
    } while (cursor.moveToNext());
}

在我看来，这些方法都过于冗长，每个方法都有多个Cursor方法调用。肯定有更整洁的方法吧?

我想知道粘性会话和非粘性会话之间的区别。我从网上读到的是:

粘性:只有一个会话对象将在那里。

非会话保持:每个服务器节点的会话对象

我想知道如何在ggplot上添加回归线方程和R^2。我的代码是:

library(ggplot2)

df <- data.frame(x = c(1:100))
df$y <- 2 + 3 * df$x + rnorm(100, sd = 40)
p <- ggplot(data = df, aes(x = x, y = y)) +
            geom_smooth(method = "lm", se=FALSE, color="black", formula = y ~ x) +
            geom_point()
p

任何帮助都将不胜感激。

在我的生产错误日志中，我偶尔会看到:

SQLSTATE[HY000]:一般错误:1205 超过锁等待超时;试一试 重新启动事务

我知道哪个查询在那个时刻试图访问数据库，但是是否有一种方法可以找出哪个查询在那个精确的时刻拥有锁?

我一直在研究一种方法，在多个设备(如iPad或Mac)之间同步存储在iPhone应用程序中的核心数据。在iOS上，用于core data的同步框架并不多(如果有的话)。然而，我一直在思考以下概念:

A change is made to the local core data store, and the change is saved. (a) If the device is online, it tries to send the changeset to the server, including the device ID of the device which sent the changeset. (b) If the changeset does not reach the server, or if the device is not online, the app will add the change set to a queue to send when it does come online. The server, sitting in the cloud, merges the specific change sets it receives with its master database. After a change set (or a queue of change sets) is merged on the cloud server, the server pushes all of those change sets to the other devices registered with the server using some sort of polling system. (I thought to use Apple's Push services, but apparently according to the comments this is not a workable system.)

有什么特别的需要我考虑的吗?我已经研究了REST框架，如ObjectiveResource、Core Resource和RestfulCoreData。当然，这些都是与Ruby on Rails一起工作的，我并不依赖于Ruby on Rails，但这是一个起点。我的解决方案的主要要求是:

任何更改都应该在后台发送，而不需要暂停主线程。 它应该使用尽可能少的带宽。

我想过一些挑战:

Making sure that the object IDs for the different data stores on different devices are attached on the server. That is to say, I will have a table of object IDs and device IDs, which are tied via a reference to the object stored in the database. I will have a record (DatabaseId [unique to this table], ObjectId [unique to the item in the whole database], Datafield1, Datafield2), the ObjectId field will reference another table, AllObjects: (ObjectId, DeviceId, DeviceObjectId). Then, when the device pushes up a change set, it will pass along the device Id and the objectId from the core data object in the local data store. Then my cloud server will check against the objectId and device Id in the AllObjects table, and find the record to change in the initial table. All changes should be timestamped, so that they can be merged. The device will have to poll the server, without using up too much battery. The local devices will also need to update anything held in memory if/when changes are received from the server.

我还遗漏了什么吗?我应该考虑什么样的框架来实现这一点?

你如何从给定的N个数字中测试所有可能的加法组合，使它们加起来得到给定的最终数字?

一个简单的例子:

要添加的数字集:N ={1,5,22,15,0，…} 期望结果:12345

尽管我很喜欢C和c++，但我还是忍不住对空结尾字符串的选择抓耳挠脑:

Length prefixed (i.e. Pascal) strings existed before C Length prefixed strings make several algorithms faster by allowing constant time length lookup. Length prefixed strings make it more difficult to cause buffer overrun errors. Even on a 32 bit machine, if you allow the string to be the size of available memory, a length prefixed string is only three bytes wider than a null terminated string. On 16 bit machines this is a single byte. On 64 bit machines, 4GB is a reasonable string length limit, but even if you want to expand it to the size of the machine word, 64 bit machines usually have ample memory making the extra seven bytes sort of a null argument. I know the original C standard was written for insanely poor machines (in terms of memory), but the efficiency argument doesn't sell me here. Pretty much every other language (i.e. Perl, Pascal, Python, Java, C#, etc) use length prefixed strings. These languages usually beat C in string manipulation benchmarks because they are more efficient with strings. C++ rectified this a bit with the std::basic_string template, but plain character arrays expecting null terminated strings are still pervasive. This is also imperfect because it requires heap allocation. Null terminated strings have to reserve a character (namely, null), which cannot exist in the string, while length prefixed strings can contain embedded nulls.

其中一些东西比C语言出现得更晚，所以C语言不知道它们是有道理的。然而，在C语言出现之前，有些语言就已经很简单了。为什么会选择空终止字符串，而不是明显更好的长度前缀?

编辑:因为有些人问了关于我上面提到的效率点的事实(他们不喜欢我已经提供的事实)，他们源于以下几点:

使用空结尾字符串的Concat需要O(n + m)时间复杂度。长度前缀通常只需要O(m)。 使用空结尾字符串的长度需要O(n)时间复杂度。长度前缀为O(1)。 Length和concat是迄今为止最常见的字符串操作。在一些情况下，以空结尾的字符串可能更有效，但这种情况发生的频率要低得多。

从下面的答案，这些是一些情况下，空终止字符串更有效:

When you need to cut off the start of a string and need to pass it to some method. You can't really do this in constant time with length prefixing even if you are allowed to destroy the original string, because the length prefix probably needs to follow alignment rules. In some cases where you're just looping through the string character by character you might be able to save a CPU register. Note that this works only in the case that you haven't dynamically allocated the string (Because then you'd have to free it, necessitating using that CPU register you saved to hold the pointer you originally got from malloc and friends).

上面这些词都没有length和concat常见。

下面的答案中还有一个断言:

你需要把绳子的一端剪掉

但这个是不正确的——对于以null结尾的字符串和有长度前缀的字符串，它的时间是相同的。(以Null结尾的字符串只是在你想要的新结尾的地方插入一个Null，长度前缀只是从前缀中减去。)

%列出% \微软。NET\assembly\是新的GAC。这是否意味着我们现在必须管理两个gac，一个用于。net 2.0-3.5应用程序，另一个用于。net 4.0应用程序?

问题是，为什么?

我正在通过SSH (Putty)在Linux机器上工作。我需要让一个进程在夜间运行，因此我认为可以通过在后台启动该进程(在命令末尾使用&号)并将stdout重定向到一个文件来实现这一点。

令我惊讶的是，这行不通。只要我关闭Putty窗口，进程就会停止。

我怎样才能防止这种情况发生?