Convert data from long format to wide format with multiple measure columns

Question

I am having trouble figuring out the most elegant and flexible way to switch data from long format to wide format when I have more than one measure variable I want to bring

Answer 1

Note -Sept 2019: within tidyr, the gather()+spread() approach (described in this answer) has more or less been replaced by the pivot_wider() approach (described in `this newer tidyr answer). For current info about the transition, see the pivoting vignette.

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Here's a solution with the tidyr package, which has essentially replaced reshape and reshape2. As with those two packages, the strategy it to make the dataset longer first, and then wider.

library(magrittr); requireNamespace("tidyr"); requireNamespace("dplyr")
my.df %>%
  tidyr::gather(key=variable, value=value, c(X, Y)) %>%   # Make it even longer.
  dplyr::mutate(                                          # Create the spread key.
    time_by_variable   = paste0(variable, "_", TIME)
  ) %>%
  dplyr::select(ID, time_by_variable, value) %>%          # Retain these three.
  tidyr::spread(key=time_by_variable, value=value)        # Spread/widen.

After the tidyr::gather() call, the intermediate dataset is:

ID TIME variable value
1   A    1        X     1
2   B    1        X     2
3   C    1        X     3
...
28  A    5        Y    28
29  B    5        Y    29
30  C    5        Y    30

The eventual result is:

  ID X_1 X_2 X_3 X_4 X_5 Y_1 Y_2 Y_3 Y_4 Y_5
1  A   1   4   7  10  13  16  19  22  25  28
2  B   2   5   8  11  14  17  20  23  26  29
3  C   3   6   9  12  15  18  21  24  27  30

tidyr::unite() is an alternative, suggested by @JWilliman. This is functionally equivalent to the dplyr::mutate() and dplyr::select() combination above, when the remove parameter is true (which is the default).

If you're not accustomed to this type of manipulation, the tidyr::unite() may be a small obstacle because it's one more function you have to learn & remember. However, it's benefits include (a) more concise code (ie, four lines are replaced by one) and (b) fewer places to repeat variable names (ie, you don't have to repeat/modify variables in the dplyr::select() clause).

my.df %>%
  tidyr::gather(key=variable, value=value, c(X, Y)) %>%           # Make it even longer.
  tidyr::unite("time_by_variable", variable, TIME, remove=T) %>%  # Create the spread key `time_by_variable` while simultaneously dropping `variable` and `TIME`.
  tidyr::spread(key=time_by_variable, value=value)                # Spread/widen.