Using R for Introductory Statistics : John Verzani

Appendix E 376The logic should be clear. If the value of x is less than 0, the return value is −x. Otherwiseit is just x. This example will not work for a vector of numbers with more than oneelement as the built-in abs () function will.Suppose we wanted to improve our eda() function defined previously by making itaware of different types of x. For example, if x is a factor, we want to present oursummary differently. One simple way to do this is to have a conditional statement in thefunction, such asif(is.factor(x)) {## do factor summary} else if(is.numeric(x)) {## do numeric summary} else {## do a default summary}We could write such conditions for all the different types of data objects we have inmind. Sometimes the switch() function can help simplify the coding.There are problems with this example. If we want to add a new type of variable, thenthe original eda() function needs to be available. This is fine for functions we write, butwhat about functions written by others for the system? We wouldn’t want to modifythose. Fortunately, there are styles in R for writing functions that eliminate this concernthat are described in the section on object-oriented programming.E.2.4 LoopingComputers, unlike humans, are very happy to do things again and again and again.Repeating something is called looping. There are three functions for looping: for(),while(), and repeat (). The latter is least common and will be skipped here.for() loopsThe standard f or () loop has this basic structure:for (varname in seq) {statement(s)}The varname is the name of a variable, the seq can be any vector or list, and thestatements get executed for each value of seq. When seq is a vector, varname loops overeach value. The statements are evaluated with the given value for varname. When seq is alist, varname refers to each successive component in the list (the values seq [[i]], not seq[i]).A simple example is a countdown program:> for(i in 10:1) print(i) # say blastoff