如何在visual studio 2010中创建空Qt项目(How to create empty Qt project in visual studio 2010)

 需要在内存中过度分配data.table，以便通过引用工作来添加列。 加载后并非如此： 
 
load("ttt")
length(a)
#[1] 1
truelength(a)
#[1] 0

b <- data.table(x=1:2)
length(b)
#[1] 1
truelength(b)
#[1] 100
 
 来自help(truelength) ： 
 
 
 
 但是对于从磁盘加载的表，在R 2.14.0中truelength为0，在R <= 2.13.2中随机为0; 也就是说，在这两种情 data.table检测此状态，并在下一列添加或删除发生时过度分配已加载的data.table。 
 
 
 但似乎你将一个（新加载的）data.table传递给一个函数，然后在函数内部通过引用添加过度分配，但是在全局环境中没有到达符号（只有函数内部的本地符号） ）。 如果您直接在全局环境中执行此操作，或者未将data.table作为函数参数传递，则它可以正常工作。 
 
 如果data.table已经过度分配（通常就是这种情况，除了从磁盘上新加载的那个），那么有一些备用插槽可以通过引用添加到列中而没有浅拷贝（以实现过度分配） ）需要通过以下方式完成:=函数内部。 
 
 这可能值得一个错误报告（但我还没有检查是否已有）。 
 
R version 3.0.1 (2013-05-16)
Platform: x86_64-apple-darwin10.8.0 (64-bit)

locale:
[1] de_DE.UTF-8/de_DE.UTF-8/de_DE.UTF-8/C/de_DE.UTF-8/de_DE.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] microbenchmark_1.3-0 data.table_1.8.8    

loaded via a namespace (and not attached):
[1] tools_3.0.1

A data.table needs to be over-allocated in memory for adding columns by reference to work. After loading it that's not the case:
 
load("ttt")
length(a)
#[1] 1
truelength(a)
#[1] 0

b <- data.table(x=1:2)
length(b)
#[1] 1
truelength(b)
#[1] 100
 
From help(truelength):
 
 
 
For tables loaded from disk however, truelength is 0 in R 2.14.0 and random in R <= 2.13.2; i.e., in both cases perhaps unexpected. data.table detects this state and over-allocates the loaded data.table when the next column addition or deletion occurs.
 
 
But it seems like if you pass a (freshly loaded) data.table to a function and then add by reference inside the function over-allocation happens but doesn't reach the symbol up in the global environment (only the local symbol inside the function). If you do it in the global environment directly or don't pass the data.table as function parameter, it works.
 
If the data.table is over-allocated already (as is normally the case, other than when freshly loaded from disk), then there are spare slots for the column to be added into by reference and no shallow copy (to achieve over-allocation) needs to be done by := inside the function.
 
This might be worth a bug report (but I haven't checked if there is already one).
 
R version 3.0.1 (2013-05-16)
Platform: x86_64-apple-darwin10.8.0 (64-bit)

locale:
[1] de_DE.UTF-8/de_DE.UTF-8/de_DE.UTF-8/C/de_DE.UTF-8/de_DE.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] microbenchmark_1.3-0 data.table_1.8.8    

loaded via a namespace (and not attached):
[1] tools_3.0.1