首先，我们可以使用dplyr包来按组迭代数据。然后，使用mutate()函数和case_when()函数根据列值映射值。下面是代码示例：

library(dplyr)

# Example data
data <- data.frame(
  group = c("A", "A", "B", "B", "C", "C"),
  value = c(1, 2, 3, 4, 5, 6)
)

# Iterate by group and map values
mapped_data <- data %>%
  group_by(group) %>%
  mutate(mapped_value = case_when(
    value == 1 ~ "Low",
    value == 2 ~ "Medium",
    value > 2 ~ "High",
    TRUE ~ "Unknown"
  ))

# Output
mapped_data

这将输出类似以下结果的数据框：

# A tibble: 6 x 3
# Groups:   group [3]
  group value mapped_value
           
1 A         1 Low         
2 A         2 Medium      
3 B         3 High        
4 B         4 High        
5 C         5 High        
6 C         6 High