LHS:data.table中的RHS与功能

为什么函数“：=”不使用“by”聚合唯一行，而LHS:RHS使用“by”聚合唯一行？下面是一个包含20行数据和58个变量的.csv文件。一个简单的复制，粘贴，delim=.csv工作。我仍在努力寻找将样本数据发布到SO的最佳方式。我的代码有两种变体：

prodMatrix <- so.sample[, ':=' (Count = .N), by = eval(names(so.sample)[2:28])]  

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因为

：=

正在通过引用进行操作。这意味着它不会调用数据集的内存副本，但会就地更新数据集。
对数据集进行聚合是其原始未聚合表单的副本。
你可以在vignette中了解更多

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这是data.table中的一个设计概念，

：=

用于通过引用进行更新，其他形式-

（

）、

列表（）

或直接表达式用于查询数据。查询数据不是按引用操作。by reference操作无法聚合行，它只能计算聚合并将其放入dataset中。查询能够聚合数据集，因为查询结果在内存中不是与原始数据相同的对象。表。

为什么在“：=”周围加引号？。使用函数形式时需要加上反勾号：=。所以我保留了后面的记号，所以我试着用它们来代替。我不明白你的问题。您是否在问为什么

：=

的行为与文档中的行为相同？关于反勾号，请参阅此处：=的函数形式应产生与LHS:RHS形式相同的结果。然而，在这种情况下，它们不应该这样做，我想知道为什么。@jangorecki，所以for:=操作都不应该进行复制或聚合。这个小插曲我已经读了好几遍了。LHS:RHS和功能表的操作应相同，正确。唯一的区别是函数形式允许您发表一些评论，而且我的理解可能更容易理解。@user3067851添加了澄清，这可能会有所帮助

prodMatrix <- so.sample[, (Count = .N), by = eval(names(so.sample)[2:28])]  

"CID","NetIncome_length_Auto Advantage","NetIncome_length_Certificates","NetIncome_length_Comm. Share Draft","NetIncome_length_Escrow Shares","NetIncome_length_HE Fixed","NetIncome_length_HE Variable","NetIncome_length_Holiday Club","NetIncome_length_IRA Certificates","NetIncome_length_IRA Shares","NetIncome_length_Indirect Balloon","NetIncome_length_Indirect New","NetIncome_length_Indirect RV","NetIncome_length_Indirect Used","NetIncome_length_Loanline/CR","NetIncome_length_New Auto","NetIncome_length_Non-Owner","NetIncome_length_Personal","NetIncome_length_Preferred Plus Shares","NetIncome_length_Preferred Shares","NetIncome_length_RV","NetIncome_length_Regular Shares","NetIncome_length_S/L Fixed","NetIncome_length_S/L Variable","NetIncome_length_SBA","NetIncome_length_Share Draft","NetIncome_length_Share/CD Secured","NetIncome_length_Used Auto","NetIncome_sum_Auto Advantage","NetIncome_sum_Certificates","NetIncome_sum_Comm. Share Draft","NetIncome_sum_Escrow Shares","NetIncome_sum_HE Fixed","NetIncome_sum_HE Variable","NetIncome_sum_Holiday Club","NetIncome_sum_IRA Certificates","NetIncome_sum_IRA Shares","NetIncome_sum_Indirect Balloon","NetIncome_sum_Indirect New","NetIncome_sum_Indirect RV","NetIncome_sum_Indirect Used","NetIncome_sum_Loanline/CR","NetIncome_sum_New Auto","NetIncome_sum_Non-Owner","NetIncome_sum_Personal","NetIncome_sum_Preferred Plus Shares","NetIncome_sum_Preferred Shares","NetIncome_sum_RV","NetIncome_sum_Regular Shares","NetIncome_sum_S/L Fixed","NetIncome_sum_S/L Variable","NetIncome_sum_SBA","NetIncome_sum_Share Draft","NetIncome_sum_Share/CD Secured","NetIncome_sum_Used Auto","totNI","Count","totalNI"
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