Question

Given the following matrix with weights in ls in the first column and heihts in the second colum:

> wgt.hgt.matrix
     [,1] [,2]
[1,]  180   70
[2,]  156   67
[3,]  128   64
[4,]  118   66
[5,]  202   72

I am looking for a concise way to apply this a binary function like

function(lb, inch) { (lb/inch**2)*703 } -> bmi

to each row of the matrix, resulting in an array, list or vector of with the 5 resulting BMI values. One way I found uses the apply function:

apply(wgt.hgt.matrix, 1, function(row) bmi(row[1], row[2]))

But a splat operator as in Ruby (*) would help making the call more concise and clear:

apply(wgt.hgt.matrix, 1, function(row) bmi(*row))

Does an equivalent to the splat operator exist, i.e. a syntax element telling R to split all vector-like objects to populate argument lists? Are there other, simpler or more concise suggestion for the apply call?

Answer 1

Using the bmi() function as a vectorized solution is preferable since it has all vectorized operators, as was illustrated in Joshua’s answer. You can also do this with:

colnames(wgt.hgt.matrix) <- c("lb", "inch")
with( as.data.frame(wgt.hgt.matrix), bmi(lb,inch) )
# [1] 25.82449 24.43039 21.96875 19.04362 27.39313

Unfortunately matrices are not good substrate for constructing environments using ‘with’ so coercing to a dataframe was needed above. You could get an apply solution (which will be less time-efficient than a vectorized approach) to work with a version of bmi() re-ritten to take a vector with named elements (as created above):

 bmi <- function(vec) { (vec['lb']/vec['inch']**2)*703 }
 apply(wgt.hgt.matrix, 1, function(row)  bmi(row ) )
# [1] 25.82449 24.43039 21.96875 19.04362 27.39313