Use filter multiple times for average calculation

Question

I need to use the filter function (or maybe some other alternatives) multiple times to calculate the average based on the conditions specified.

Here is the dataset:

df <- data.frame(id = c(1,2,3,4,5,6,7), 
                 cond = c("Y", "Y", "N", "Y", "N", "Y", "N"), score = c(3,4,5,2,1,2,9))

I need to calculate the average separately for cond=Y and cond=N and later append this average column to the original dataset like this:

  id cond score  average
1  1    Y     3   2.75
2  2    Y     4   2.75
3  3    N     5   5
4  4    Y     2   2.75
5  5    N     1   5
6  6    Y     2   2.75
7  7    N     9   5

Answer 1

We may do

library(data.table)
setDT(df)[, average := mean(score), by = cond]

-output

> df
      id   cond score average
   <num> <char> <num>   <num>
1:     1      Y     3    2.75
2:     2      Y     4    2.75
3:     3      N     5    5.00
4:     4      Y     2    2.75
5:     5      N     1    5.00
6:     6      Y     2    2.75
7:     7      N     9    5.00

---

Or with collapse

library(collapse)
df$average <- fmean(df$score, df$cond, TRA = 1)