[R] Trying to understand a function passed to lapply

[William Dunlap]

> 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.

If you put the line 'str(x)' at the start of myfun(), as in
   myfun <- function(x, c1, c2) {
      str(x)
      x[c1]/x[c2]
   }
you would start to see why it works - extracting a row from a matrix gives
you a vector with names copied from the column names of the matrix.
Thus subscripting with a single name (or number) works.
  > apply(data,1,myfun,c1="delta",c2="SE")
   Named num [1:8] -0.7 -0.9 -0.5 20 20 ...
   - attr(*, "names")= chr [1:8] "delta" "low" "high" "n1" ...
   Named num [1:8] -0.3 -0.43 -0.17 43 43 ...
   - attr(*, "names")= chr [1:8] "delta" "low" "high" "n1" ...
   Named num [1:8] -0.5 -1.05 0.05 16 18 ...
   ...
   Named num [1:8] -0.3 -0.83 0.23 27 27 ...
   - attr(*, "names")= chr [1:8] "delta" "low" "high" "n1" ...
  [1] -13.720000  -9.046154  -3.563636   0.000000 -28.311111 -32.072727
 -2.903704  -2.218868

By the way, that call to apply is just a slow way of dividing two columns
of the matrix
  > data[, "delta"]/data[, "SE"]
  [1] -13.720000  -9.046154  -3.563636   0.000000 -28.311111 -32.072727
 -2.903704  -2.218868


Bill Dunlap
TIBCO Software
wdunlap tibco.com

On Mon, Mar 30, 2015 at 6:48 PM, John Sorkin <jsorkin at grecc.umaryland.edu>
wrote:

> Colleagues,
> 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,
> John
>
> myfun <- function(x, c1, c2) x[c1]/x[c2]
> apply(data,1,myfun,c1="delta",c2="SE")
>
> CODE:
>
> 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),
>                      nrow=8,ncol=5,byrow=TRUE)
> dimnames(data) <- list(NULL,c("delta","low","high","n1","n2"))
> data
> CI <- data[,"high"]-data[,"low"]
> data <- cbind(data,CI)
> data
> data <- cbind(data,SE=data[,"CI"]/(4*1.96))
> data
> data <- cbind(data,SD=data[,"SE"]*sqrt(data[,"n1"]+data[,"n2"]))
> data
> myfun <- function(x, c1, c2) x[c1]/x[c2]
> apply(data,1,myfun,c1="delta",c2="SE")
>
> 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
> GRECC (BT/18/GR)
> Baltimore, MD 21201-1524
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