The main function of R

Question

When reading R for programmers, I saw this function

oddcount <- function(x) { k <- 0 for (n in x) { if (n %% 2 == 1) k <- k+1 } return(k) } 

I would prefer to write it in a simpler style (i.e. in lisp)

 (defn odd-count [xs] (count (filter odd? xs))) 

I see that the length of the function is equivalent to counting, and can I write odd ones? also have built-in map / filter / remove type functions?

Answer 1

A more R-way to do this would be to avoid the for loop and use vectorization:

 oddcount <- function(x) { sum(x %% 2) } 

Comparing between x and 2 displays a vector as x is itself a vector. Sum computes the sum of the vector, where TRUE is 1 and FALSE is zero. Thus, the function calculates the number of odd numbers in the vector.

This already leads to a simpler syntax, although for non-vector oriented people, the for loop tends to be easier to read. I really prefer vector syntax as it is much shorter. I would prefer to use a more descriptive name for x , though, for example. number_vector .

Answer 2

In R, when you work with vectors, people often prefer to work with the whole vector at the same time, rather than looping it (see, for example, this discussion).

In a sense, R has a built-in filter and reduces functions: a way to select a subset of a vector. They are very convenient in R, and there are several ways to do this - I will show you a couple, but you will get more if you read about R and look at the code of other people on a site like this. I would also like to consider the options ?which and ?'[' , In which there are more examples than here.

The first way is to simply choose which items you want. You can use this if you know the indices of the elements you want:

 x <- letters[1:10] > x [1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" 

If we need only the first five letters, we can write:

 x[1:5] x[c(1,2,3,4,5)] # a more explicit version of the above 

You can also choose which items you do not want to use with the minus sign, for example:

  x[-(6:10)] 

Another way to select elements is to use a logical vector:

 x <- 1:5 selection <- c(FALSE, TRUE, FALSE, TRUE, FALSE) x[selection] # only the second and fourth elements will remain 

This is important because we can create such a vector by placing the vector in a comparison function:

 selection <- (x > 3) > selection [1] FALSE FALSE FALSE TRUE TRUE x[selection] # select all elements of x greater than 3 x[x > 3] # a shorthand version of the above 

Again, we can choose the opposite of the comparison we use (note that since it is logical, we use ! Not - ):

 x[!(x > 3)] # select all elements less than or equal to 3 

If you want to perform vector comparisons, you should consider the %in% function. For instance:

 x <- letters[1:10] > x %in% c("d", "p", "e", "f", "y") [1] FALSE FALSE FALSE TRUE TRUE TRUE FALSE FALSE FALSE FALSE # Select all elements of x that are also "d", "p", "e", "f", or "y" x[x %in% c("d", "p", "e", "f", "y")] # And to select everything not in that vector: x[!(x %in% c("d", "p", "e", "f", "y"))] 

The above examples are just a few examples; I would definitely recommend the documentation. I know this is a long post after you have already accepted the answer, but such a thing is very important and understands that it will save you a lot of time and pain in the future if you are new to R, so I thought I would share a couple of ways to do it is with you.

Answer 3

You should familiarize yourself with the funprog library, which includes map , filter , reduce , etc.