A pre-defined function in R is the substring() function, which returns parts of a given character string, like so: substring("ABCDE", 2, 4) # Returns "BCD" If you look at the signature of this function, it has three inputs: text, first and last Look at the function's signature or help page to answer this question: Which of these inputs are arguments (that must be specified) and which are options (that might not)