R：按列名列表分组

Question

我有一个数据表，它是另外两个表（a，b）

之间的连接结果

joinedTable   = a[b, on = columnName]

我想计算基于组合名称的行组合的频率，这些名称可以变化（全部，一个在中间某处）。问题是它是硬编码的，但不是变量：

joinedTable[, list(freq =.N), by = list( COLUMN_A, COLUMN_B, ...)] # works

# what's next doesn't
columns = names(joinedTable)[-which(names(joinedTable) %in% c('COLUMN_F', 'COLUMN_G',...))]
    joinedTable[, list(freq =.N), by = as.list(columns)] # doesn't work

如果我尝试动态，我会收到此错误：

Error in `[.data.table`(joinedTable, , list(freq = .N), by = list(outColumns)) : 
  The items in the 'by' or 'keyby' list are length (13). Each must be same length as rows in x or number of rows returned by i (23).

它应该如何才能起作用？

Answer 1

已经找到了答案，错误很简单愚蠢：

这一行

joinedTable[, list(freq =.N), by = as.list(columns)]

需要更改为：

joinedTable[, list(freq =.N), by = columns]

因为列已经是一个数组。