Hi all,
I apologize for the ignorance implicit in this question, but I'm
having a hard time figuring out how R functions work. For example, if
I wanted to write a function to compute a variance, I would do
something like
>my.var <- function(x) (sum(((x-mean(x)))^2))/(length(((x-mean(x)))
^2)-1)
And this seems to work, e.g.,
> my.var(V1)
[1] 116.1
> var(V1)
[1] 116.1
But when I try to see what the built-in var function does I get
> var
function (x, y = NULL, na.rm = FALSE, use)
{
if (missing(use))
use <- if (na.rm)
"complete.obs"
else "all.obs"
na.method <- pmatch(use, c("all.obs",
"complete.obs",
"pairwise.complete.obs"))
if (is.data.frame(x))
x <- as.matrix(x)
else stopifnot(is.atomic(x))
if (is.data.frame(y))
y <- as.matrix(y)
else stopifnot(is.atomic(y))
.Internal(cov(x, y, na.method, FALSE))
}
<environment: namespace:stats>
Being a novice, I can't understand what this means. I only have one
variable, yet the code seems to be based on the covariance between x
and y. What is y? Sorry for such a stupid question. I am just trying
to figure out how R does things, and I can't seem to get my head
around it. Thank you for your patience.
-Ista Zahn
http://izahn.homedns.org
'y' has a default value of NULL, so its value is NULL. Check ?cov to see what happens when it is NULL. On 10/24/07, Ista Zahn <istazahn at gmail.com> wrote:> Hi all, > I apologize for the ignorance implicit in this question, but I'm > having a hard time figuring out how R functions work. For example, if > I wanted to write a function to compute a variance, I would do > something like > > >my.var <- function(x) (sum(((x-mean(x)))^2))/(length(((x-mean(x))) > ^2)-1) > > And this seems to work, e.g., > > > my.var(V1) > [1] 116.1 > > var(V1) > [1] 116.1 > > But when I try to see what the built-in var function does I get > > > var > function (x, y = NULL, na.rm = FALSE, use) > { > if (missing(use)) > use <- if (na.rm) > "complete.obs" > else "all.obs" > na.method <- pmatch(use, c("all.obs", "complete.obs", > "pairwise.complete.obs")) > if (is.data.frame(x)) > x <- as.matrix(x) > else stopifnot(is.atomic(x)) > if (is.data.frame(y)) > y <- as.matrix(y) > else stopifnot(is.atomic(y)) > .Internal(cov(x, y, na.method, FALSE)) > } > <environment: namespace:stats> > > Being a novice, I can't understand what this means. I only have one > variable, yet the code seems to be based on the covariance between x > and y. What is y? Sorry for such a stupid question. I am just trying > to figure out how R does things, and I can't seem to get my head > around it. Thank you for your patience. > > -Ista Zahn > http://izahn.homedns.org > > ______________________________________________ > R-help at r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. >-- Jim Holtman Cincinnati, OH +1 513 646 9390 What is the problem you are trying to solve?
?var points out that "var is another interface to cov", i.e. it calculates (possibly part of) the "variance" matrix. In the simplest special case y = x and you get the entire variance matrix, or just the sample variance if x is a simple vector, as in your case. BTW you little function is a bit more complicated than it need be, isn't it?> my.var <- function(x) sum((x-mean(x))^2)/(length(x)-1)would do the same job with many fewer parentheses.... Bill Venables CSIRO Laboratories PO Box 120, Cleveland, 4163 AUSTRALIA Office Phone (email preferred): +61 7 3826 7251 Fax (if absolutely necessary): +61 7 3826 7304 Mobile: +61 4 8819 4402 Home Phone: +61 7 3286 7700 mailto:Bill.Venables at csiro.au http://www.cmis.csiro.au/bill.venables/ -----Original Message----- From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf Of Ista Zahn Sent: Thursday, 25 October 2007 11:35 AM To: r-help at r-project.org Subject: [R] Novice programing question Hi all, I apologize for the ignorance implicit in this question, but I'm having a hard time figuring out how R functions work. For example, if I wanted to write a function to compute a variance, I would do something like >my.var <- function(x) (sum(((x-mean(x)))^2))/(length(((x-mean(x))) ^2)-1) And this seems to work, e.g., > my.var(V1) [1] 116.1 > var(V1) [1] 116.1 But when I try to see what the built-in var function does I get > var function (x, y = NULL, na.rm = FALSE, use) { if (missing(use)) use <- if (na.rm) "complete.obs" else "all.obs" na.method <- pmatch(use, c("all.obs", "complete.obs", "pairwise.complete.obs")) if (is.data.frame(x)) x <- as.matrix(x) else stopifnot(is.atomic(x)) if (is.data.frame(y)) y <- as.matrix(y) else stopifnot(is.atomic(y)) .Internal(cov(x, y, na.method, FALSE)) } <environment: namespace:stats> Being a novice, I can't understand what this means. I only have one variable, yet the code seems to be based on the covariance between x and y. What is y? Sorry for such a stupid question. I am just trying to figure out how R does things, and I can't seem to get my head around it. Thank you for your patience. -Ista Zahn http://izahn.homedns.org ______________________________________________ R-help at r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.