Substitution / collation of rows and columns in R

Question

Ok, so I need a very specific formula for a subset. From the matrix x, I only want to save the elements defined by rows and columns. Then unwanted elements must be replaced with zeros. Example below:

> x <- matrix(c(65,46,52,76,34,345,65,87,12,53),nrow = 5,ncol = 2)
> x
     [,1] [,2]
[1,]   65  345
[2,]   46   65
[3,]   52   87
[4,]   76   12
[5,]   34   53

> rows <- c(1,1,2,3,3,4,5)
> cols <- c(1,2,2,1,2,1,2)

magic

> x
     [,1] [,2]
[1,]   65  345
[2,]    0   65
[3,]   52   87
[4,]   76    0
[5,]    0   53

Many thanks

Answer 1

This kind of magic is called the matrix index. If you have rowsand colsthose you do not need, or if the matrix indexing admit a negative, it would be even easier.

y <- matrix(0, nrow=5, ncol=2)
y[cbind(rows,cols)] <- x[cbind(rows,cols)]
y
##      [,1] [,2]
## [1,]   65  345
## [2,]    0   65
## [3,]   52   87
## [4,]   76    0
## [5,]    0   53

Alternatively, you can do the same thing “manually” and be able to use a negative subscriber, knowing that the matrix can be considered as a vector with indexing going down the columns.

k <- (cols-1)*nrow(x) + rows
x[-k] <- 0
x
##      [,1] [,2]
## [1,]   65  345
## [2,]    0   65
## [3,]   52   87
## [4,]   76    0
## [5,]    0   53

Answer 2

, . , , 0.

x <- matrix(c(65,46,52,76,34,345,65,87,12,53),nrow = 5,ncol = 2)
rows <- c(1,1,2,3,3,4,5)
cols <- c(1,2,2,1,2,1,2)
coords <- paste(rows,cols)
numRows <- 5
numCols <- 2
for (i in 1:numRows){
  for (j in 1:numCols){
    if (!(paste(i,j) %in% coords)){
      x[i,j] <- 0
    }
  }
}

Answer 3

Baroque view:

    indicies <- NULL
    for (iter in 1:length(rows)) {
      indicies <- rbind(indicies, c(rows[iter], cols[iter]))
    }

    indicies
     [,1] [,2]
[1,]    1    1
[2,]    1    2
[3,]    2    2
[4,]    3    1
[5,]    3    2
[6,]    4    1
[7,]    5    2

    y <- matrix(rep(0, 10), nrow=5, ncol=2)
    for (k in 1:nrow(indicies)) {
      y[indicies[k,1], indicies[k,2]] <- x[indicies[k,1], indicies[k,2]]
    }
    y
         [,1] [,2]
    [1,]   65  345
    [2,]    0   65
    [3,]   52   87
    [4,]   76    0
    [5,]    0   53