Can someone please explain me this code? especially the role of “function x and [[x]]”?

Question

This is the code in R and I\'m having trouble understanding the role of function(x) and qdata[[x]] in this line of code. Can someone elaborate me this

Answer 1

We don't have the data object named qdata so we cannot really be sure what will happen with this code. It appears that the author of this code is trying to pass the values of components named outs from function calls to hist. If qdata is an ordinary dataframe, then I suspect that this code will fail in that goal, because the hist function does not have an out component. (Look at the output of ?hist. When I run this with a simple dataframe, I do get histogram plots that appear in my interactive plotting device but I get NULL values for the outs components. Furthermore the 12 warnings are caused by the lack of a data parameter to hte hist function.

 qdata <- data.frame(a=rnorm(10), b=rnorm(10))
 outs=lapply(names(qdata), function(x) 
         hist(qdata[[x]],data=qdata,main="Histogram of Quality Trait",
              xlab=as.character(x),las=1.5)$out)
#There were 12 warnings (use warnings() to see them)

> str(outs)
List of 2
 $ : NULL
 $ : NULL

So I think we need to be concerned about the level of R knowledge of the author of this code. It's possible I'm wrong about this presumption. The hist function is generic and it is possible that some unreferenced package has a function designed to handle a data object and retrun an outs value when delivered a vector having a particular class. In a typical starting situation with only the base packages loaded however, there are only three hist.* functions:

methods(hist)
#[1] hist.Date*   hist.default hist.POSIXt*
#see '?methods' for accessing help and source code

As far as the questions about the role of function and [[x]]: the keyword function returns a language object that can receive parameter values and then do operations and finally return results. In this case the names get passed to the anonymous function and become, each in turn, the local name, x and the that value is used by the '[['-function to look-up the column in what I am presuming is the ‘qdata’-dataframe.

Answer 2

This code generate a series of histograms, one for each of columns 12 to 35 of dataframe qdata. The lapply function iterates over the columns. At each iteraction, the name of the current column is passed as argument "x" to the anonymous function defined by "function(x)". The body of the function is a call to the hist() function, which creates the histogram. qdata[[x]] (where x is the name of a column) extracts the data from that column. I am actually confused by "data=qdata".