[R] Trying to understand a function passed to lapply

John Sorkin jsorkin at grecc.umaryland.edu
Tue Mar 31 03:48:45 CEST 2015

I am trying to understand the syntax of a function passed to apply. The code below generates a matrix, and passes the matrix to a function that is called by apply. I don't understand the syntax of the function. In some way the function computes data[,"delta"]/data[,"SE"]. I can't understand how the body of the function, x[c1]/x[c2] refers to the columns "data" and "SE" of the matrix data. Can someone help me understand the syntax? 
Thank you,

myfun <- function(x, c1, c2) x[c1]/x[c2]


data<-matrix(data=c(-0.70 ,-0.90, -0.50, 20, 20,
                    -0.30 ,-0.43, -0.17, 43, 43,
                    -0.50 ,-1.05,  0.05, 16, 18,
                     0.00 ,-0.21,  0.21, 22, 23,
                    -1.30 ,-1.48, -1.12, 28, 32,
                    -0.90 ,-1.01, -0.79, 18, 15,
                    -0.20 ,-0.47,  0.07, 39, 39,
                    -0.30 ,-0.83,  0.23, 27, 27),
dimnames(data) <- list(NULL,c("delta","low","high","n1","n2"))
CI <- data[,"high"]-data[,"low"]
data <- cbind(data,CI)
data <- cbind(data,SE=data[,"CI"]/(4*1.96))
data <- cbind(data,SD=data[,"SE"]*sqrt(data[,"n1"]+data[,"n2"]))
myfun <- function(x, c1, c2) x[c1]/x[c2]

John David Sorkin M.D., Ph.D.
Professor of Medicine
Chief, Biostatistics and Informatics
University of Maryland School of Medicine Division of Gerontology and Geriatric Medicine
Baltimore VA Medical Center
10 North Greene Street
Baltimore, MD 21201-1524
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