Example: CSV I/O

Readading data from csv files

Reading csv files is realized using the read.table function from R’s utils library. The function will return a data frame which contains the information of the csv file.

df <- read.table(paste0(envrmt$path_data_csv, "/AI001_gebiet_flaeche.csv"),
                 skip = 4, header = TRUE, sep = ";", dec = ",")

As you can see, the read.table function gets several arguments (which is common for many functions). The first one gives the filename inclducing the path to the file.

  • skip = 4 tells the function to skip the first four lines (which are plain text lines in this case and not tabulated values)
  • header = TRUE tells the function, that the csv file has a header line which is used by read.table to name the columns of the returning data frame
  • sep = ";" defines the separator of the individual columns in the data frame
  • dec = "," defines the decimal separator used in the dataset

A note on the sequence of the arguments: the sequence of the arguments does not matter as long as you name them explicetly. If you do not use the argument identfier as it is the case for the first argument, the filename, in the example then the sequence matters. To get information on the default sequence and of course the general application of the each R function, type ?<function name> (e.g. ?read.table) in an R console.

After you executing the read.table function above, the content of the csv file is stored into a two dimensional data frame called df in the example above.

A quick way to check if everything is fine is to display the first few lines of the data file using the head function (without the 2, it will print 5 lines as a standard setting).

head(df,2)

##      X X.1                  X.2
## 1 1996  DG          Deutschland
## 2 1996  01   Schleswig-Holstein
##   Anteil.Siedlungs..und.Verkehrsfläche.an.Gesamtfl.
## 1                                              11,8
## 2                                              10,8
##   Anteil.Erholungsfläche.an.Gesamtfläche
## 1                                    0,7
## 2                                    0,7
##   Anteil.Landwirtschaftsfläche.an.Gesamtfläche
## 1                                         54,1
## 2                                         73,0
##   Anteil.Waldfläche.an.Gesamtfläche
## 1                              29,4
## 2                               9,3

Writing data to csv files

Writing data is as easy as reading it. Just use the write.table function.

write.table(df, file = paste0(envrmt$path_data_tmp, "new.csv"), 
            sep = ",", dec = ".")

As you can see, you can define any column or decimal separator.

For more information have a look at e.g. the respective data importing and data exporting site at Quick R, have a look into the package documentation or search the web.

Alternative data I/O using RDS

If you stay within R for reading and writing R objects from and to data files, you could also use the serialization of readRDS and saveRDS.

saveRDS(df, file = paste0(envrmt$path_data_tmp, "new.rds"))

# Read data to different variable
df2 = readRDS(paste0(envrmt$path_data_tmp, "new.rds"))

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