Cannot dynamically modify external variables in a subroutine in R while use apply on a matix? -

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this problem has confused me couple of days. let have 2 matrices: 

matrix_a <- matrix(0, nrow = 3, ncol = 3,                     dimnames = list(c("r1", "r2", "r3"), c("c1", "c2", "c3"))) matrix_b <- matrix(c("r1", "r2", "c1", "c2"), nrow = 2, ncol = 2)

i want dynamically modify matrix_a in function:

change_var <- function(x, matrix_a) {     if(any(rownames(matrix_a) == x[1])  && any(colnames(matrix_a) == x[2])) {         matrix_a[x[1], x[2]] <- 1         return (matrix_a)     } } apply(matrix_b, 1, change_var, matrix_a)

however, seems code cannot change matrix_a @ all. intended result of matrix_a should 

   c1 c2 c3 r1  1  0  0 r2  0  2  0 r3  0  0  0

how achieve goal of dynamically modification of matrix_a? please provide me not-for-loop solution. in advance.

as know, <<- or assign can used modify "global" variable. <<- similar calling assign inherits=true.

...but seems you're trying create matrix counts number of occurrences of cell coordinates given matrix_b? more efficiently done (without loops!) table function. added duplicate row in matrix_b show works:

matrix_a <- matrix(0, nrow = 3, ncol = 3,                     dimnames = list(c("r1", "r2", "r3"), c("c1", "c2", "c3"))) matrix_b <- matrix(c("r1", "r2", "r2", "c1", "c2", "c2"), nrow = 3, ncol = 2) # convert matrix_b 2-d 1-d indices row <- match(matrix_b[,1], rownames(matrix_a)) col <- match(matrix_b[,2], colnames(matrix_a)) idx <- row+(col-1)*nrow(matrix_a) # count number of identical indices using table matrix_a[sort(unique(idx))] <- table(idx)

which updates matrix_a to:

   c1 c2 c3 r1  1  0  0 r2  0  2  0 r3  0  0  0

...of course, if need put 1 in there, don't need call table:

row <- match(matrix_b[,1], rownames(matrix_a)) col <- match(matrix_b[,2], colnames(matrix_a)) idx <- row+(col-1)*nrow(matrix_a) matrix_a[idx] <- 1

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