Expand each group to the max n of rows

Question

How can I expand a group to length of the max group:

df <- structure(list(ID = c(1L, 1L, 2L, 3L, 3L, 3L), col1 = c("A", 
"B", "O", "U", "L", "R")), class = "data.frame", row.names = c(NA, 
-6L))


ID col1
1 A
1 B
2 O
3 U
3 L
3 R

Desired Output:

1 A
1 B
NA NA
2 O
NA NA
NA NA
3 U
3 L
3 R

Answer 1

You can take advantage of the fact that df[n_bigger_than_nrow,] gives a row of NAs

dplyr

max_n <- max(count(df, ID)$n)

df %>% 
  group_by(ID) %>% 
  summarise(cur_data()[seq(max_n),])
#> `summarise()` has grouped output by 'ID'. You can override using the `.groups`
#> argument.
#> # A tibble: 9 × 2
#> # Groups:   ID [3]
#>      ID col1 
#>   <int> <chr>
#> 1     1 A    
#> 2     1 B    
#> 3     1 <NA> 
#> 4     2 O    
#> 5     2 <NA> 
#> 6     2 <NA> 
#> 7     3 U    
#> 8     3 L    
#> 9     3 R

base R

n <- tapply(df$ID, df$ID, length)
max_n <- max(n)
i <- lapply(n, \(x) c(seq(x), rep(Inf, max_n - x)))
i <- Map(`+`, i, c(0, cumsum(head(n, -1))))
df <- df[unlist(i),]
rownames(df) <- NULL
df$ID <- rep(as.numeric(names(i)), each = max_n)

df
#>   ID col1
#> 1  1    A
#> 2  1    B
#> 3  1 <NA>
#> 4  2    O
#> 5  2 <NA>
#> 6  2 <NA>
#> 7  3    U
#> 8  3    L
#> 9  3    R