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How to convert a dataframe in long format into a list of an appropriate format?

I have a dataframe in the following long format:

1

I need to convert it into a list which should look something like this: 2

Wherein, each of the main element of the list would be the "Instance No." and its sub-elements should contain all its corresponding Parameter & Value pairs - in the format of "Parameter X" = "abc" as you can see in the second picture, listed one after the other.

Is there any existing function which can do this? I wasn't really able to find any. Any help would be really appreciated.

Thank you.

Solution

  • A dplyr solution

    require(dplyr)
df_original <- data.frame("Instance No." = c(3,3,3,3,5,5,5,2,2,2,2),
                      "Parameter" = c("age", "workclass", "education", "occupation", 
                                      "age", "workclass", "education", 
                                      "age", "workclass", "education", "income"),
                      "Value" = c("Senior", "Private", "HS-grad", "Sales",
                                  "Middle-aged", "Gov", "Hs-grad",
                                  "Middle-aged", "Private", "Masters", "Large"),
                      check.names = FALSE)
    
# the split function requires a factor to use as the grouping variable.
# Param_Value will be the properly formated vector
df_modified <- mutate(df_original,
                      Param_Value = paste0(Parameter, "=", Value))
# drop the parameter and value columns now that the data is contained in Param_Value
df_modified <- select(df_modified,
                      `Instance No.`,
                      Param_Value)

# there is now a list containing dataframes with rows grouped by Instance No.
list_format <- split(df_modified, 
                     df_modified$`Instance No.`)

# The Instance No. is still in each dataframe. Loop through each and strip the column.
list_simplified <- lapply(list_format, 
                          select, -`Instance No.`)

# unlist the remaining Param_Value column and drop the names.                      
list_out <- lapply(list_simplified , 
                   unlist, use.names = F)

    There should now be a list of vectors formatted as requested.

    $`2`
[1] "age=Middle-aged"   "workclass=Private" "education=Masters" "income=Large"     

$`3`
[1] "age=Senior"        "workclass=Private" "education=HS-grad" "occupation=Sales" 

$`5`
[1] "age=Middle-aged"   "workclass=Gov"     "education=Hs-grad"

    The posted data.table solution is faster, but I think this is a bit more understandable.