# [R] mapply for matrices

From: Tamas K Papp <tpapp_at_princeton.edu>
Date: Thu 06 Oct 2005 - 07:36:43 EST

Hi,

I have a matrix A and a vector b, and would like to apply a function f(a,b) to the rows of A and the elements of b. Eg

```A <- matrix(1:4,2,2)
b <- c(5,7)
f <- function(a,b) {sum(a)*b}

```

myapply(f,A=A,b=b)

would give

(1+3)*5 = 20
(2+4)*7 = 42

I found mapply, but it does not work for matrices. How could I do this without loops? The above is just a toy example, the problem I am using this for has larger matrices, and f is a computation that does not handle vectors.

One thing I thought of is

sapply(seq(along=b),function(i,A,b){f(A[i,],b[i])},A=A,b=b)

but this is not very elegant. I checked the archives and found nothing.

Thank you,

Tamas

```--
Bayesian statistics is difficult in the sense that thinking is difficult.
--Donald A. Berry, American Statistician 51:242 (1997)

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