如何使用向量元素作为r中矩阵位置的坐标？

Question

I'd like to use elements of a vector that consists strictly of double digit figures as a mapping to specific matrix positions. For instance, consider the following:

mat1 <- rbind(c(12,31,22,13),c(3,1,5,7))
mat <- matrix(rep( 0, len=25), nrow = 5)

What I'd like to do is use mat1[1,1] to access position 1 2 of mat , and at that position, I'd like the corresponding value of 3 to be stored. I've tried converting mat1's elements to strings, splitting them, storing them, calling the as.numeric() function on the vector in which they're stored and finally running a loop, and while I ultimately got it to work, I now have close to a million loops in my program. I'd appreciate any ideas!

Answer 1

We can strsplit the first row of 'mat', convert it to a row/column index, and use that to assign the corresponding elements of 'mat' with the second rows of 'mat1'

i1 <- do.call(rbind, lapply(strsplit(as.character(mat1[1,]),""), as.numeric))
mat[i1] <- mat1[2,]
mat
#     [,1] [,2] [,3] [,4] [,5]
#[1,]    0    3    7    0    0
#[2,]    0    5    0    0    0
#[3,]    1    0    0    0    0
#[4,]    0    0    0    0    0
#[5,]    0    0    0    0    0

---

Or with substr

m1 <- mat1[1,]
i1 <- cbind(as.integer(substr(m1, 1, 1)), as.integer(substr(m1, 2, 2)))
mat[i1] <- mat1[2,]