Using csv column names as parameter in R function

Question

I'm trying to write a function that will open some .csv files and preform several calculations on specific parts of those dataframes. I'm having trouble passing a column name as a parameter in the function, and I'm not sure why. So something like this (minimal working example, my function is more complex than this):

MyFunction <- function(file, columnname){
    data <- read.table(file,sep=",",header=TRUE);
    mean(data$columnname);
    }

I have a .csv in my desktop called "test.csv", and all it has in it is a column called "numbers" and the numbers 1:10. If I run either of these:

MyFunction("~/Desktop/test.csv",numbers)
MyFunction("~/Desktop/test.csv","numbers")

I get this error:

[1] NA
Warning message:
In mean.default(data$columnname) :
   argument is not numeric or logical: returning NA

However, if I run this:

data <- read.table("~/Desktop/test.csv",sep=",",header=TRUE);
mean(data$numbers);

I get this:

[1] 5.5

... which is what I want.

I'm not sure how my function is different from doing it by hand here. I can use function parameters to find and open the file, but using the function parameter in the data$parameter seems to be causing an error. Why is this? What workaround is there?

You could just use bracket indexing instead of indexing by $. For example, get the column number with something like Idx <- match(columnname,names(data)) and then mean(data[,Idx]). — nrussell
– nrussell, Commented Oct 16, 2014 at 21:52
I actually ended up using this, although both answers were very helpful! — wugology
– wugology, Commented Oct 16, 2014 at 22:27

G. Grothendieck · Accepted Answer · 2014-10-16 22:21:42Z

4

Try this:

MyFunction <- function(file, columnname) {
    data <- read.csv(file)
    mean(data[[columnname]])
}

Note:

b <- "a"
DF <- data.frame(a = 1, b = 2)
DF$b
## [1] 2
DF[[b]]
## [1] 1

edited Oct 16, 2014 at 22:21

answered Oct 16, 2014 at 21:54

G. Grothendieck

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2 Comments

wugology Over a year ago

Awesome, thank you! Was my problem using read.table or $ or both?

G. Grothendieck Over a year ago

$ is the problem. See clarification just added.

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