Restructure Dataset

Question

I've been working for 4 hours to try to format a dataset.

Here is the structure of the original dataset.

Subject_ID | A1 | A2 | A3 | A4 | A5 | B1 | B2 | B3 | B4 | B5 

1. 

2. 

3. 

4. 

5. 

I'd like to make it the following format

Subject_ID | Task | A | B | 

1. (under subject column)           1 (under Task Column)

1.  (under subject column)          2 (under Task Column)

1.   (under subject column)         3 (under Task Column)

1.   (under subject column)         4 (under Task Column)

1.   (under subject column)         5 (under Task Column)

2.   (under subject column)         1 (under Task Column)

2.    (under subject column)        2 (under Task Column)

2.    (under subject column)        3 (under Task Column)

2.    (under subject column)        4 (under Task Column)

2.    (under subject column)        5 (under Task Column)

I have tried the following code, which formats the data correctly, but does not create the Task column that is based off of the numbers in the original datasets column names.

setDT(test)
test <- melt(test, id.vars = c("Subject_ID"),  
         measure = patterns('A', 'B'), 
         value.name = c('a', 'b', 'skin_conductance'))[, variable := NULL] 
[order(Subject_ID)]