[R] Accesing data frame members from within functions

From: David Schwab <dvschwab46225_at_gmail.com>
Date: Fri, 02 May 2008 10:19:09 -0400


I am writing a simple R program to execute a t-test repeatedly on data contained in a data frame. My data looks like this:

Category                      Value1             Value2

1                                  .5                     .8

1                                  .3                     .9

. . .                               . . .                   . . .

2                                  1.4                   1.3

2                                  1.3                   1.3

. . .                               . . .                   . . .

15                                .2                     .3

15                                .5                     .1





So in all there are 15 categories, and each category contains two sets of observations which I want to compare. I only want to compare Value1 and Value2 within each category, but I need to do it 15 times (once for each category), so I wanted to write an R function to make it easier.

Right now I am using a for() loop to do the comparison. My loop looks like this:

for(i in 1:21)

{

            x <- t.test(Value1[Category == i], Value2[Category == i])

            y <- c(y, x$p.value)

}

The loop runs and everything is working well. However, I am not sure how to translate this code into a function. In particular, I'm not sure how to write a function that passes a data frame ds (containing Category, Value1, and Value2 as members) as an argument, and then accessing these members within the body of the function. I've tried the following:

repeated_test <- function(data)

{

            for(i in 1:21)

            {

              x <- t.test(ds$Value1[ds$Category == i], ds$Value2[ds$Category == i])

              y <- c(y, x$p.value)

            }

This will run, but only if the members of the data frame I am passing as an argument are in fact named Value1, Value2, and Category. This is fine for now, but in the future I will have to run this function on data where I cannot be sure this is the case. Rather than change the member names by hand, I would like to make the function generic to work with any data frame. How do I do this? Or is there a better way to do this without the for() loop (for example, using apply())?

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