Apply a function to every row of a matrix or a data frame

Question

Suppose I have a n by 2 matrix and a function that takes a 2-vector as one of its arguments. I would like to apply the function to each row of the matrix and get a n-vector. How to do this in R?

For example, I would like to compute the density of a 2D standard Normal distribution on three points:

bivariate.density(x = c(0, 0), mu = c(0, 0), sigma = c(1, 1), rho = 0){

    exp(-1/(2*(1-rho^2))*(x[1]^2/sigma[1]^2+x[2]^2/sigma[2]^2-2*rho*x[1]*x[2]/(sigma[1]*sigma[2]))) * 1/(2*pi*sigma[1]*sigma[2]*sqrt(1-rho^2))

}

out <- rbind(c(1, 2), c(3, 4), c(5, 6))

How to apply the function to each row of out?

How to pass values for the other arguments besides the points to the function in the way you specify?

Answer 1

You can directly use the apply() function to apply a function to each row of a matrix.

For example:

M <- matrix(1:20, nrow=4, byrow=TRUE)

 M

     [,1] [,2] [,3] [,4] [,5]

[1,]    1    2    3    4    5

[2,]    6    7    8    9   10

[3,]   11   12   13   14   15

[4,]   16   17   18   19   20

apply(M, 1, function(x) x[1]+x[2]+x[3]+x[4]+x[5])

[1] 15 40 65 90

The apply function above applies the function(x) i.e, to print the sum of elements in each row.