r - From long to wide data with multiple columns

Question

Suggestions for how to smoothly get from foo to foo2 (preferably with tidyr or reshape2 packages)?

This is kind of like this question, but not exactly I think, because I don't want to auto-number columns, just widen multiple columns. It's also kind of like this question, but again, I don't think I want the columns to vary with a row value as in that answer. Or, a valid answer to this question is to convince me it's exactly like one of the others. The solution in the second question of "two dcasts plus a merge" is the most attractive right now, because it is comprehensible to me.

foo:

foo = data.frame(group=c('a', 'a', 'b', 'b', 'c', 'c'),
                  times=c('before', 'after', 'before', 'after', 'before', 'after'),
                  action_rate=c(0.1,0.15, 0.2, 0.18,0.3, 0.35),
                  num_users=c(100, 100, 200, 200, 300, 300))
foo <- transform(foo,
                 action_rate_c95 = 1.95 * sqrt(action_rate*(1-action_rate)/num_users))

> foo
  group  times action_rate num_users action_rate_c95
1     a before        0.10       100      0.05850000
2     a  after        0.15       100      0.06962893
3     b before        0.20       200      0.05515433
4     b  after        0.18       200      0.05297400
5     c before        0.30       300      0.05159215
6     c  after        0.35       300      0.05369881

foo2:

foo2 <- data.frame(group=c('a', 'b', 'c'),
                   action_rate_before=c(0.1,0.2, 0.3),
                   action_rate_after=c(0.15, 0.18,0.35),
                   action_rate_c95_before=c(0.0585,0.055, 0.05159),
                   action_rate_c95_after=c(0.069, 0.0530,0.0537),
                   num_users=c(100, 200, 300))

> foo2
  group action_rate_before action_rate_after action_rate_c95_before
1     a                0.1              0.15                 0.0585
2     b                0.2              0.18                 0.0550
3     c                0.3              0.35                 0.05159
  action_rate_c95_after num_users
1                 0.0690       100
2                 0.0530       200
3                 0.0537       300

EDIT: Now I'd probably try to do it with pivot_wider from tidyr.

Answer 1

You can use data.table instead of reshape2, because its dcast() function accepts several variables, and is faster too:

require(data.table)
setDT(foo)
dcast(foo,group+num_users~times,value.var=c("action_rate","action_rate_c95"))

   group num_users action_rate_after action_rate_before action_rate_c95_after action_rate_c95_before
1:     a       100              0.15                0.1            0.06962893             0.05850000
2:     b       200              0.18                0.2            0.05297400             0.05515433
3:     c       300              0.35                0.3            0.05369881             0.05159215