[R] How does this function print, why is n1 which equals 1 printed as 2?

[Duncan Murdoch]

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
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