c(), sum(), mean(), and var(). Inside the parentheses are the arguments. In this context, "argument" doesn't mean "disagreement," "confrontation," or anything like that. It's just the math term for whatever a function operates on.
Even if a function takes no arguments, you still include the parentheses.
These four R functions are pretty simple in terms of their arguments and their output. As you work with R, however, you encounter functions that take more than one argument.R provides a couple of ways for you to deal with multiargument functions. One way is to list the arguments in the order in which they appear in the function's definition. R calls this positional matching.
For example, the function substr() takes three arguments. The first is a string of characters like "abcdefg", which R refers to as a character vector. The second argument is a start position within the string (1 is the first position, 2 is the second position, and so on). The third is a stop position within the string (a number greater than or equal to the start position). In fact, if you type substr into the Scripts pane, you see a helpful pop-up message that looks like this:
substr(x, start, stop)
Extract or replace substrings in a character vector
where x stands for the character vector.
This function returns the substring, which consists of the characters between the start and stop positions.
Here's an example:
> substr("abcdefg",2,4)
[1] "bcd"
What happens if you interchange the 2 and the 4?
> substr("abcdefg",4,2)
[1] ""
This result is completely understandable: No substring can start at the fourth position and stop at the second position.
But if you name the arguments, it doesn't matter how you order them:
> substr("abcdefg",stop=4,start=2)
[1] "bcd"
Even this works:
> substr(stop=4, start=2,"abcdefg")
[1] "bcd"
So when you use a function, you can place its arguments out of order, if you name them. R calls this keyword matching, which comes in handy when you use an R function that has many arguments. If you can't remember their order, just use their names and the function works.
If you ever need help for a particular function — substr(), for example — type ?substr and watch helpful information appear on the Help tab.