Role of square brackets in R

Question

I got this code from elsewhere and I wondering if someone can explain what the square brackets are doing.

     matrix1[i,] <- df[[1]][] 

I am using this to assign values to a matrix and it works but I am not sure what exactly it's doing. What does the initial set of [[]] mean followed by another []?

Thank you in advance, Drashti

Answer 1

The general function of [ is that of subsetting, which is well documented both in help (as suggested in comments), and in this piece . The rest of of my answer is heavily based on that source.

In fact, there are operators for subsetting in R; [[ , [ , and $ . The [ and $ are useful for returning the index and named position, respectfully, for example the first three elements of vector a = 1:10 may be subsetted with a[c(1,2,3)] . You can also negatively subset to remove elements, as a[-1] will remove the first index.

To answer more specifically, what does df[[1]][] do? First, the [[ -operator will return the 1st element from df , and the following empty [ -operator will pull everything from that output.

Answer 2

This might help you understand a bit. You can copy and paste this code and see the differences between different ways of indexing using [] and $. The only thing I can't answer for you is the second empty set of square brackets, from my understanding that does nothing, unless a value is within those brackets.

#Retreives the first column as a data frame
mtcars[1]

#Retrieves the first column values only (three different methods of doing the same thing)
mtcars[,1]
mtcars[[1]]
mtcars$mpg

#Retrieves the first row as a data frame
mtcars[1,]

#I can use a second set of brackets to get the 4th value within the first column
mtcars[[1]][4]
mtcars$mpg[4]