R Language Missing values Reading and writing data with NA values


When reading tabular datasets with the read.* functions, R automatically looks for missing values that look like "NA". However, missing values are not always represented by NA. Sometimes a dot (.), a hyphen(-) or a character-value (e.g.: empty) indicates that a value is NA. The na.strings parameter of the read.* function can be used to tell R which symbols/characters need to be treated as NA values:

read.csv("name_of_csv_file.csv", na.strings = "-")

It is also possible to indicate that more than one symbol needs to be read as NA:

read.csv('missing.csv', na.strings = c('.','-'))

Similarly, NAs can be written with customized strings using the na argument to write.csv. Other tools for reading and writing tables have similar options.