Transform from Wide to Long without sorting columns

Question

I want to convert a dataframe from wide format to long format.

Here it is a toy example:

mydata <- data.frame(ID=1:5, ZA_1=1:5, 
            ZA_2=         


        

Answer 1

An alternative approach with data.table:

melt(mydata, id = 'ID')[, c("variable", "measure") := tstrsplit(variable, '_')
                        ][, variable := factor(variable, levels = unique(variable))
                          ][, dcast(.SD, ID + measure ~ variable, value.var = 'value')]

which gives:

    ID measure ZA BB CC
 1:  1       1  1  3 NA
 2:  1       2  5  6 NA
 3:  1       7 NA NA  6
 4:  2       1  2  3 NA
 5:  2       2  4  6 NA
 6:  2       7 NA NA  5
 7:  3       1  3  3 NA
 8:  3       2  3  6 NA
 9:  3       7 NA NA  4
10:  4       1  4  3 NA
11:  4       2  2  6 NA
12:  4       7 NA NA  3
13:  5       1  5  3 NA
14:  5       2  1  6 NA
15:  5       7 NA NA  2