On Feb 1, 2013, at 7:47 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.
1) on my machine the output from the cat() call is:
******n1*******= 1
2) All of the "output without any instructions in the function to
print anything" is just the value of the list object from the
function. Unless you return values using the `invisible` function, any
user define function executed at the console will print its value.
That is standard interactive session behavior. So one gets after the
the cat output:
$mu1
[1] 0
$mu2
[1] 8
$sd1
[1] 1.666667
$sd2
[1] 1.666667
$alpha
[1] 0.05
$n1
[1] 1
$n2
[1] 1
$power
[1] 0.8
$ratio
[1] 1
$table
mu1 mu2 sd1 sd2 n1 n2
1 0 8 1.666667 1.666667 1 1
attr(,"class")
[1] "n.for.2means" "list"
--
David.
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
(Fax) 410-605-7913 (Please call phone number above prior to faxing)
Confidentiality Statement:
This email message, including any attachments, is for ...{{dropped:15}}
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