r - reshaping a data frame duplicating IDs -

this question has answer here:

• reshaping multiple sets of measurement columns (wide format) single columns (long format) 4 answers

this dataframe:

df <- data.frame(id = c(1,2,3), a1 = c("a1","a3","a5"),  b1 = c("b1","b3","b5"), a2 = c("a2","a4","a6"), b2 = c("b2","b4","b6"))     

and result want this:

  id   b 1  1 a1 b1 2  1 a2 b2 3  2 a3 b3 4  2 a4 b4 5  3 a5 b5 6  3 a6 b6 

i tried approach solution, had no luck.

we can use melt data.table can take multiple measure patterns convert 'wide' 'long' format.

library(data.table) melt(setdt(df), measure = patterns("^a", "^b"),            value.name = c("a", "b"))[, variable := null][order(id)] #  id   b #1:  1 a1 b1 #2:  1 a2 b2 #3:  2 a3 b3 #4:  2 a4 b4 #5:  3 a5 b5 #6:  3 a6 b6