Add new column with name of max column in data frame

Question

So I've got this example df:

df <- dput(structure(list(arts = structure(c(1L, 1L, 3L, 4L), .Label = c("art1", "art2"), class = "character"), scr1 = c(52L, 58L, 40L, 62L), scr2 = c(25L, 23L, 55L, 26L), scr3 = c(36L, 60L, 19L, 22L)), .Names = c("art_id", "scr1", "scr2", "scr3"), row.names = c(NA, -4L), class = "data.frame"))

> df
  art_id scr1 scr2 scr3
1      1   52   25   36
2      1   58   23   60
3      3   40   55   19
4      4   62   26   22

That I'm using dplyr to summarize by art_id

df %>% 
  group_by(art_id) %>% 
  summarise_each(funs(sum))

  art_id  scr1  scr2  scr3
   <int> <int> <int> <int>
1      1   110    48    96
2      3    40    55    19
3      4    62    26    22

My question: How can I add another column called top_r that contains the column name from the maximum among src1:src3. The resultant df would look like:

  art_id  scr1  scr2  scr3  top_r
   <int> <int> <int> <int>  <char>
1      1   110    48    96   scr1  
2      3    40    55    19   scr2  
3      4    62    26    22   scr1  

I am comfortable using dplyr so if there's an answer that uses that library that's wonderful!

Answer 1

Its just this simple in base R using max.col:

df$top_r <- names(df)[-1][max.col(df[-1])]

Answer 2

This would work:

df %>%
  group_by(art_id) %>%
  summarise_each(funs(sum)) %>%
  mutate(top_r=apply(.[,2:4], 1, function(x) names(x)[which.max(x)]))

# A tibble: 3 × 5
  art_id  scr1  scr2  scr3 top_r
   <int> <int> <int> <int> <chr>
1      1   110    48    96  scr1
2      3    40    55    19  scr2
3      4    62    26    22  scr1