What does declaring function at the start of a line do?

Question

I encountered this code:

    res <- lapply(strsplit(s, "\n")[[1]],
    (function (str) paste(rev(strsplit(str, "")[[1]]), collapse = "")))

The secodnd line reverses each of the splitted strings at the first line. How does it do that? Namely, what does calling 'function' at the start do?

Answer 1

Calling lapply takes and performs some function on each list element. It takes the form lapply(list_data, some_function). So, for instance, if I have a list of integers and want to find out how many integers are in each list element, I would run:

list_data <- list(list1 = 1:5,
                  list2 = 6:10,
                  list3 = 11:30)
lapply(list_data, length)

The function here is length, which is a function that is inherent in R. Some functions aren't defined in R, say if I want define my own formula for each value in the list, I could define my own function. Calling a function allows users to define a function that is not already in R or an R library. Like so:

lapply(list_data, function(x) x^2+4-x^3)

The function here is x^2+4-x^3, which is not defined in R programming itself.

So in your example, your data is strsplit(s, "\n")[[1]] and it is taking that data and applying the function paste(rev(strsplit(str, "")[[1]]), collapse = "")) to each element in the data.

Note that in my example, I put function(x) - your example puts function(str) - what's in the parentheses doesn't matter and is user defined. For example lapply(list_data, function(str) str^2+4-str^3) will return the same thing as lapply(list_data, function(x) x^2+4-x^3)

Please note that broad "learning" style questions like this are not exactly what this site is for, and this question will likely get removed and/or receive some negative feedback. Since you are new to this site and to R, I'm providing this answer but I would not be surprised if the question is removed. Just trying to help both you and the SO community!