Fixed values not repeated over column and row

Question

I want to create a matrix in R with a set number of variables (e.g. 1 to 10). Those variables should be randomly assigned over rows and columns BUT should not be repeated in eit

Answer 1

This seems almost like generating a Sudoku grid. The code below works pretty fast, but some minor R optimizations could be done:

backtrack = function(n = 10){
  x = matrix(NA, ncol = n, nrow = n)
  cells = list()
  k = 1
  for (i in 1:n){
    for (j in 1:n){
      cells[[k]] = sample(1:n)
      k = k + 1
    }
  }

  i = 0
  while (i < n*n){
    candidates = cells[[i + 1]]
    idx = sample(1:length(candidates), 1)
    val = candidates[idx]

    if (length(candidates) == 0){
      cells[[i + 1]] = sample(1:n)
      i = i - 1
      x[as.integer(i/n) + 1,  i %% n + 1] = NA
    }
    else {
      rr = as.integer(i/n) + 1
      cc = i %% n + 1
      if ((val %in% x[rr, ]) || (val %in% x[, cc])){
        candidates = candidates[-idx]
        cells[[i + 1]] = candidates
      }
      else{
        x[as.integer(i/n) + 1, i %% n + 1] = val
        candidates = candidates[-idx]
        cells[[i + 1]] = candidates
        i = i + 1
      }
    }
  }
  x
}

Testing:

set.seed(1) # Please change this
x = backtrack(10)
print(x)

      [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
 [1,]    8   10    4    6    9    7    1    2    3     5
 [2,]    5    6    9    8    1   10    4    3    2     7
 [3,]   10    7    1    2    8    9    5    4    6     3
 [4,]    3    9    8   10    6    5    7    1    4     2
 [5,]    9    1    6    4    7    3    2    5   10     8
 [6,]    1    4   10    3    2    6    8    7    5     9
 [7,]    2    8    5    9   10    1    3    6    7     4
 [8,]    6    5    2    7    3    4   10    9    8     1
 [9,]    4    3    7    1    5    2    6    8    9    10
[10,]    7    2    3    5    4    8    9   10    1     6


any(apply(x, 1, function(r)any(duplicated(r)))) # FALSE
any(apply(x, 2, function(r)any(duplicated(r)))) # FALSE