Matrix function in R - r

Question

I'm trying to create a function that will transform a matrix (MATR1). It should multiply every element that is a prime by 2 and return the new matrix (MATR2). This is what I've done so far:
MATR1 <- matrix()
My.Func <- function(MATR1)
MATR2 <- matrix(nrow(MATR1), ncol(MATR1))
for (i in 1:nrow(MATR1)) {
for(j in 2:ncol(MATR1)) {
if (MATR1[i,j]%%2 == 0) {
MATR2[i,j] <- MATR1[i,j]/2
} else {MATR2[i,j] <- MATR1[i,j]}
}
}
return(matrix(MATR2, nrow(MATR1)))
But I can't get it to work. Can anyone see where I've made a mistake? Appreciate all help!

Answer 1

Use vectorized operations instead of for loops. You'll also need a function to determine whether numbers are prime. There are several possible algorithms, but none built into base R. I'll use numbers::isPrime here, though you can write your own prime sieve or other algorithm from scratch if you like.
First, some setup:
library(numbers)
set.seed(47) # for reproducibility
mat <- matrix(rpois(100, 10), 10) # sample matrix
mat
## [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
## [1,] 16 6 8 7 12 14 10 12 9 14
## [2,] 12 4 7 8 13 9 7 5 14 15
## [3,] 10 12 4 15 15 14 12 10 10 12
## [4,] 9 8 3 9 16 13 12 10 9 9
## [5,] 10 7 9 9 14 14 10 13 8 11
## [6,] 6 7 9 13 12 6 7 10 8 11
## [7,] 10 11 8 8 9 5 10 14 12 11
## [8,] 14 8 10 7 10 12 8 5 12 14
## [9,] 4 14 6 5 10 13 7 12 10 6
## [10,] 7 7 10 13 15 16 13 7 9 9
To define the function, assign to the subset that meets the criteria:
double_primes <- function(m){
m[isPrime(m)] <- m[isPrime(m)] * 2;
m
}
double_primes(mat)
## [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
## [1,] 16 6 8 14 12 14 10 12 9 14
## [2,] 12 4 14 8 26 9 14 10 14 15
## [3,] 10 12 4 15 15 14 12 10 10 12
## [4,] 9 8 6 9 16 26 12 10 9 9
## [5,] 10 14 9 9 14 14 10 26 8 22
## [6,] 6 14 9 26 12 6 14 10 8 22
## [7,] 10 22 8 8 9 10 10 14 12 22
## [8,] 14 8 10 14 10 12 8 10 12 14
## [9,] 4 14 6 10 10 26 14 12 10 6
## [10,] 14 14 10 26 15 16 26 14 9 9