Split data frame into n parts, compute and recombine

Question

I would like to perform some computation on a large dataframe. To do so I need to

1. split my dataframe into n equal parts (the last chunk will have its own size)
2. Do my computation (I add the result in a new column)
3. Recombine the dataframe

How can I do that?

Many thanks in advance for your help!

 dataframe <- MyDataFrame
 nb_obs <- nrow(dataframe) # in my dataframe I have 153 036 rows
 nb_chunk <- ceiling(dataframe / 250) # I thus need 613 chunks if I want 250 obs per sub-dataframe
 for(i in 1:nb_chunk) {
         # my computation here, I want to add a new columns to the chunk to store my results..
 }  

 # then I want to recombine the final dataset (equals to original dataset with a new column added)

EDITED PART

Thank you for your comments, please find below my proposal in a reproducible example using iris dataset.

I have 2 additional questions at this stage:

1. Can I initiate 'final.df' without hard coding the column names?

2. Is there a better way to proceed with dplyr?

   df <- iris                                        # using iris as an example (my real dataframe is 153036 rows and 17 columns)
   nb_obs <- nrow(df)                                # nb of observations in the dataframe (thus nb of operations to be performed)
   nb_obs_in_chunk <- 13                             # nb of rows per chunk
   nb_chunk <- ceiling(nb_obs / nb_obs_in_chunk)     # total nb of chunks to be created
   nb_chunk_full <- floor(nb_obs / nb_obs_in_chunk)  # nb of chunks to be created with nb_obs_in_chunk rows
   nb_obs_last_chunk <- nb_obs - nb_obs_in_chunk*nb_chunk_full # nb of rows in final chunks
   df$split.factor <- as.factor(c(rep(1:nb_chunk_full, each = nb_obs_in_chunk), rep(nb_chunk_full + 1, nb_obs_last_chunk))) # create factor to split dataframe into equal parts
   final.df <- data.frame(Sepal.Length = numeric(), Sepal.Width = numeric(), Petal.Length = numeric(), Petal.Width = numeric(), Species = factor(), split.factor = factor()) # initiate final dataframe (desired output)
   for(i in 1:nb_chunk) {
     temp_i <- df[df$split.factor == i, ]
     temp_i$NEW <- temp_i$Sepal.Length + temp_i$Sepal.Width
     final.df <- rbind(final.df, temp_i)
   }
   

Answer 1

To answer your initial questions:

df <- iris
do.call(rbind,
        lapply(split(df, rep(seq(13), length.out = 150, each = 13)),
               function(chunk) {
                 chunk$NEW = chunk$Sepal.Length + chunk$Sepal.Width
                 chunk
               })
       )

rep(seq(13), length.out = 150, each = 13) gives you the same as the split.factor column in your example and the split() function turns this into a factor.

This also answers the first of the questions in your edit: yes, you don't need to initialize the final data frame.

Note that the row names are no longer a sequence from 1 to 150 as they now contain the chunk number.

Answer 2

I think this does it:

library("dplyr")
ddf <- iris                                        
nb_obs_in_chunk <- 13
ddf %>% mutate(id=seq(nrow(ddf)),
               chunk=cut(id,id %/% nb_obs_in_chunk)) %>%
    group_by(chunk) %>%
        mutate(NEW=Sepal.Length+Sepal.Width)

but haven't tested yet ... @mjkallen's rep(seq(13), length.out = 150, each = 13)) might be better than my %/%-based solution to getting the chunk numbers.