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

You can melt several columns simultaneously, if you pass a list of column names to the argument measure =. One approach to do this in a scalable manner would be to:

1. Extract the column names and the corresponding first two letters:

   measurevars <- names(mydata)[grepl("_[1-9]$",names(mydata))]
   groups <- gsub("_[1-9]$","",measurevars)
   

2. Turn groups into a factor object and make sure levels aren't ordered alphabetically. We'll use this in the next step to create a list object with the correct structure.

   split_on <- factor(groups, levels = unique(groups))
   

3. Create a list using measurevars with split(), and create vector for the value.name = argument in melt().

   measure_list <- split(measurevars, split_on)
   measurenames <- unique(groups)
   

Bringing it all together:

melt(setDT(mydata), 
     measure = measure_list, 
     value.name = measurenames,
     variable.name = "measure")
#    ID measure ZA BB
# 1:  1       1  1  3
# 2:  2       1  2  3
# 3:  3       1  3  3
# 4:  4       1  4  3
# 5:  5       1  5  3
# 6:  1       2  5  6
# 7:  2       2  4  6
# 8:  3       2  3  6
# 9:  4       2  2  6
#10:  5       2  1  6