小编phe*_*nss的帖子

我有一个包含名称、日期和几个分类列的数据集。让我们说

data <- data.table(name = c('Anne', 'Ben', 'Cal', 'Anne', 'Ben', 'Cal', 'Anne', 'Ben', 'Ben', 'Ben', 'Cal'),
               period = c(1,1,1,1,1,1,2,2,2,3,3), 
               category = c("A","A","A","B","B","B","A","B","A","B","A"))

看起来像这样：

  name  period  category
  Anne       1         A
   Ben       1         A
   Cal       1         A
  Anne       1         B
   Ben       1         B
   Cal       1         B
  Anne       2         A
   Ben       2         B
   Ben       2         A
   Ben       3         A
   Cal       3         B

我想计算，对于每个时期，对于我的每组分类变量，过去时期存在多少个名字。输出应如下所示：

period  category  recurrence_count
    2         A                 2   # due to Anne and Ben being on A, period 1
    2         B                 1 …