R: Converting wide format to long format with multiple 3 time period variables [duplicate]

Answer 1

We can use melt from data.table which can take multiple measure columns as a regex pattern

library(data.table)
melt(setDT(sample.df), measure = patterns("^Blue", "^Red", "^Green"), 
     value.name = c("BlueTime", "RedTime", "GreenTime"), variable.name = "time")
#   Subject time BlueTime RedTime GreenTime
#1:       1    1        2       2         2
#2:       2    1        5       5         5
#3:       3    1        6       6         6
#4:       1    2        4       4         4
#5:       2    2        6       6         6
#6:       3    2        7       7         7
#7:       1    3        1       1         1
#8:       2    3        2       2         2
#9:       3    3        3       3         3

---

Or as @StevenBeaupré mentioned in the comments, if there are many patterns, one option would be to use the names of the dataset after extracting the substring as the patterns argument

melt(setDT(sample.df), measure = patterns(as.list(unique(sub("\\d+", "", 
         names(sample.df)[-1])))),value.name = c("BlueTime", "RedTime", 
          "GreenTime"), variable.name = "time")