LHS：RHS 与 data.table 中的功能

Question

为什么函数 ':=' 不使用 'by' 聚合唯一行，而 LHS:RHS 却使用 'by' 聚合？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 就可以了。我仍在努力寻找将样本数据发布到 SO 的最佳方式。我的代码的 2 个变体是：

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

---此版本不使用by聚合rowID---

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

---此版本确实使用 by 聚合 rowID ---

"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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11684,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1,0,0,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,-71.36,0,0,0,49.01,0,0,-22.35,77631,-1735052.85
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Answer 1

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，但会就地更新它。
对您的数据集进行聚合是其原始未聚合形式的副本。
您可以在 Reference semantics 小插图中阅读更多相关信息。

这是data.table中的一个设计理念，:=用于引用更新，其他形式——.()、list()或直接表达式用于查询数据。并且查询数据不是 by reference 操作。 by reference 操作不能聚合行，它只能计算聚合并将其就地放入数据集中。查询能够聚合数据集，因为查询结果与原始data.table在内存中的对象不同。