On 13-02-01 8:47 AM, John Sorkin wrote:
Windows 7, R 2.12.1
Colleagues,
I am trying to understand the n.for.2means function. The code below is a copy
of the function (renamed to n.for.2means.js). I have inserted a single line of
code towards the bottom of the function which uses the cat function to print
the value of n1. You will note the value (preceded by stars) is printed as 1.
The function (1) prints a lot of output without any instructions in the
function to print anything (other than the cat statement I added), and when it
prints (2) reports the value of n1 to be 2!.
I have two questions, (i) how is the function printing when there is no code to
print and (ii) how is n1 which equals 1 being reported as 2? I suspect there is
something fundamental about R that I don't know.
I haven't run the code, but presumably it's just the usual auto
printing. Unless a function sets the result to be invisible, results
of functions are printed by calling print() after they are returned.
Since your function returns something with class
c("n.for.2means", "list")
R will look for a print method for those classes, and use it to print
the result. There's no print.list method in standard R; it just uses
the default. But there's probably a print.n.for.2means function in the
package (possibly not exported; you might need
epicalc:::print.n.for.2means to see it). There might be a print.list
function there instead or as well.
Duncan Murdoch
Thank you for the help.
John
library(epicalc)
n.for.2means.js <- function (mu1, mu2, sd1, sd2, ratio = 1, alpha = 0.05, power
= 0.8)
{
n1 <- (sd1^2 + sd2^2/ratio) * (qnorm(1 - alpha/2) - qnorm(1 -
power))^2/(mu1 - mu2)^2
n1 <- round(n1)
n2 <- ratio * n1
if (length(alpha) == 1) {
alpha1 <- NULL
}
else {
alpha1 <- alpha
}
if (length(power) == 1) {
power1 <- NULL
}
else {
power1 <- power
}
if (length(ratio) == 1) {
ratio1 <- NULL
}
else {
ratio1 <- ratio
}
table1 <- cbind(mu1, mu2, sd1, sd2, n1, n2, alpha1, power1,
ratio1)
colnames(table1)[colnames(table1) == "alpha1"] <- "alpha"
colnames(table1)[colnames(table1) == "power1"] <- "power"
colnames(table1)[colnames(table1) == "ratio1"] <- "n2/n1"
table1 <- as.data.frame(table1)
cat("******n1*******=",n1,"\n")
returns <- list(mu1 = mu1, mu2 = mu2, sd1 = sd1, sd2 = sd2,
alpha = alpha, n1 = n1, n2 = n2, power = power, ratio =
ratio,
table = table1)
class(returns) <- c("n.for.2means", "list")
returns
}
n.for.2means.js(0,8,10/6,10/6)
John David Sorkin M.D., Ph.D.
Chief, Biostatistics and Informatics
University of Maryland School of Medicine Division of Gerontology
Baltimore VA Medical Center
10 North Greene Street
GRECC (BT/18/GR)
Baltimore, MD 21201-1524
(Phone) 410-605-7119
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