Reading and writing CSV files in R

Before loading a file in R is often useful change directory in the environment, this is done by:

setwd('pathname')

If you have a doubt about which is your current working directory, just print it:

getwd()

Reading from a CSV file to a data frame is pretty simple:

df = read.csv('path/to/file.csv')

Now we can get the structure of the dataframe:

str(df)

It gives us information on the number of observations (rows) and variables (columns); names of variables, a few of their values and, when they are detected as 'factors', also the number of 'level'on which that variable is structured.
Another useful function is:

summary(df)

It tries to provide us useful summary for each variable, giving the levels in case of factor, or a few statistic measures otherwise (min, max, mean, median, first and third quartile).

We can create a subset from a dataframe selecting a specific value for a variable, like this:

sub = subset(df, MyVariable = 'a value')

Then we can save this subset to a CSV file:

write.csv(sub, 'path/to/subFile.csv')