data.table按组分配向量

install.packages("data.table")
library(data.table)

# 创建示例数据表
dt <- data.table(group = c(1, 1, 2, 2, 3, 3),
                 value = c(10, 20, 30, 40, 50, 60))

# 按group列分配一个新的向量
dt[, new_vector := .(seq_len(.N)), by = group]

# 查看结果
print(dt)

   group value new_vector
1:     1     10          1
2:     1     20          2
3:     2     30          1
4:     2     40          2
5:     3     50          1
6:     3     60          2