将因子变量的堆栈级别添加到数据帧中

# 创建一个数据框
df <- data.frame(
  id = c(1, 2, 3, 4, 5),
  category = c("A", "B", "A", "B", "C")
)

# 将category列转换为因子变量，并指定层次结构
df$category <- factor(df$category, levels = c("A", "B", "C"))

# 将因子变量添加到数据框中
df <- cbind(df, level = df$category)

# 打印结果
print(df)

  id category level
1  1        A     A
2  2        B     B
3  3        A     A
4  4        B     B
5  5        C     C