Change value in matrix using defined function in R - r

Question

I am still new to R.
I was wondering how to use self-defined function to change elements in matrix.
Say, I have a matrix "test"test = matrix(c(1:9), 3, 3)
and I wrote a function "testFunction" to add each element by 20.
testFunction = function(){
for(i in rownames(test)){
for(j in colnames(test)){
test[i,j] = test[i,j] + 20
}
}
}
If I do it manually, such as test[1,1] = test[1,1] + 20, it works.
But if I use the function I wrote, the matrix remain the same.
The "test" matrix after running the "testFunction" is still
[,1] [,2] [,3]
[1,] 1 4 7
[2,] 2 5 8
[3,] 3 6 9
Does anyone know why? And how can I change the values in the matrix using functions?
Thanks.
Update
So, actually I am creating a 452*452 matrix "pairMatrix" and a function "countPair"
pairMatrix = matrix(0, 452,452)
rownames(pairMatrix) = players
colnames(pairMatrix) = players
and
I will pass a data frame to the function, and the function will loop through each row to find "Player1" and "Player2". Then, using these two values as row name and column name to find the element in the pairMatrix and add by 1.
countPair = function(someDataFrame, pairMatrix){
for (i in rownames(someDataFrame)){
w = someDataFrame[i,"Player1"]
l = someDataFrame[i,"Player2"]
pairMatrix[w,l] = pairMatrix[w,l] + 1
pairMatrix[l,w] = pairMatrix[l,w] + 1
}
return(pairMatrix)
}
However, though the simple test case now works by following the suggestion, this complicated case does not work.
Any suggestion?
Thanks.

Answer 1

If you want to add 20 to your matrix, you only need:
test <- test + 20
However, assuming that you need to make this function for another purpose; the problem is due to two reasons:
colnames and rownames are NULL for matrices, so your function is not iterating.
In your function you have to return the output of your function.
Then, the following code will work:
test <- matrix(c(1:9), 3, 3)
testFunction <- function(){
for(i in 1:dim(test)[1]){
for(j in 1:dim(test)[2]){
test[i,j] <- test[i,j] + 20
}
}
return(test)
}
test <- testFunction()
Although the previous code work, it is still not good because the idea of a function is to take an input, process it and return and output. The following code makes that:
test <- matrix(c(1:9), 3, 3)
testFunction = function(input){
for(i in 1:dim(input)[1]){
for(j in 1:dim(input)[2]){
input[i,j] <- input[i,j] + 20
}
}
return(input)
}
test_added <- testFunction(test)