如何按不同列分组并根据一列查找百分比？

Question

我想将下面的数据表按var1和var2分组，然后在var2中求百分比：

data <- as.data.table(list(var1 = c("x1","x1","x2","x1","x2"), 
                           var2 = c("y1","y1","y1","y2","y2"))) 

data[, .(count = .N), by=.(var1, var2)]

#    var1 var2 count
#1:   x1   y1     2
#2:   x2   y1     1
#3:   x1   y2     1
#4:   x2   y2     1

这是我感兴趣的结果：

#    var1 var2 count  ratio in var2
#1:   x1   y1     2         0.66
#2:   x2   y1     1         0.33
#3:   x1   y2     1         0.5
#4:   x2   y2     1         0.5

如何更改代码来实现这一点？

Answer 1

这应该会给你你想要的：

data <- data[, .N, by = .(var1, var2)][, ratio:=N/sum(N), by = var2]

导致：

> data
   var1 var2 N     ratio
1:   x1   y1 2 0.6666667
2:   x2   y1 1 0.3333333
3:   x1   y2 1 0.5000000
4:   x2   y2 1 0.5000000