如何传递R函数参数来子集列

Question

First I am new here, this is my first post so my apologies in advance if I am not doing everything correct. I did take the time to search around first but couldn't find what I am looking for. Second, I am pretty sure I am breaking a rule in that this question is related to a 'coursera.org' R programming course I am taking (this was part of an assignment) but the due date has lapsed and I have failed for now, I will repeat the subject next month and try again but I am kind of now in damage control trying to find out what went wrong.

Basically below is my code: What I am trying to do is read in data from a series of files. These files are four columns wide with the titles: Date, nitrate, sulfate and id and contain various rows of data.

The function I am trying to write should take the arguments of the directory of the files, the pollutant (so either nitrate or sulfate), and the set of numbered files, eg files 1 and 2, files 1 through to 4 etc. The return of the function should be the average value of the selected pollutant across the selected files.

I would call the function using a call like this

pollutantmean("datafolder", "nitrate", 1:3)

and the return should just be a number which is the average in this case of nitrate across data files 1 through to 3

OK, I hope I have provided enough information. Other stuff that may be useful is:

• Operating system :Ubuntu
• Language: R

• Error message received:

  Warning message: In is.na(x) : is:na() applied to non(list or vector) of type 'NULL'

As I say, the data files are a series of files located in a folder and are four columns wide and vary as to the number of rows.

My function code is a follows:

pollutantmean <- function(directory, pollutant, id = 1:5) { #content of the function
#create a list of files, a vector I think
files_list <- dir(directory, full.names = TRUE) 
# Now create an empty data frame
dat <- data.frame()

# Next step is to execute a loop to read all the selected data files into the dataframe
for (i in 1:5) { 
    dat <- rbind(dat, read.csv(files_list[i]))
}

#subsets the rows matching the selected monitor numbers
dat_subset <- dat[dat[, "ID"] == id, ]

#identify the median of the pollutant and ignore the NA values
median(dat_subset$pollutant, na.rm = TRUE)

ok, that is it, through trial and error I am pretty sure the final line of code, the " median(dat_subset$pollutant, na.rm = TRUE) " appears to be the problem. I pass an argument to the function of pollutant which should be either sulfate or nitrate but it seems the dat_subset$pollutant bit of code is what is not working. Somehow I am getting the passed pollutant argument to not come into the function body. the dat_subset$pollutant bit should ideally be equivalent to either dat_subset$nitrate or dat_subset$sulfate depending on the argument fed to the function.

Answer 1

You cannot subset with $ operator if you pass the column name in an object like in your example (where it is stored in pollutant ). So try to subset using [] , in your case that would be:

median(dat_subset[,pollutant], na.rm = TRUE)

or

median(dat_subset[[pollutant]], na.rm = TRUE)

Does that work?