Little useless-useful R functions – Return matrix elements in spiral order

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Another one from the Leetcode challenge. This time, get the elements (single values) from the matrix in a spiral order with a starting position of [1,1].

So, the basic idea is to retrieve a vector of elements from a matrix in the following order:

In this order, we get the vector with the following elements:

So, we need to create the boundaries, count the rows | columns and return a vector of elements. Here is the useless function:

## return elements from matrix in spiral order
matrix_spiral <- function(mat) {
  mr <- dim(mat)[1]
  nc <- dim(mat)[2]
  total_len <- mr*nc

  #path #TRUE -> visited; FALSE -> unvisited
  visit <- matrix(FALSE, nrow=mr, ncol=nc)
  #helper variables
  gor <- 1
  dol <- mr
  levo <- 1
  desno <- nc
  res <- vector()
  while (length(res) < total_len) {

    for (i in levo:nc) {
      if (!visit[gor, i]) {
          res <- c(res, mat[gor,i])
          visit[gor, i] <- TRUE
      } }
    gor <- gor + 1

    for (i in gor:mr) {
      if (!visit[i, desno]) {
          res <- c(res, mat[i,desno])
          visit[i, desno] <- TRUE
      } }
    desno <- desno - 1
    if (gor <= dol) {
      for (i in desno:levo) {
        if (!visit[dol, i]) {
          res <- c(res, mat[dol,i])
          visit[dol, i] <- TRUE
        } }
      dol <- dol - 1
    if (levo <= desno) {
      for (i in dol:gor) {
        if (!visit[i, levo]) {
            res <- c(res, mat[i,levo])
            visit[i, levo] <- TRUE
        } }
    levo <- levo + 1
    } }

As always, the complete code is available on Github in  Useless_R_function repository. The sample file in this repository is here (filename: Spiral_matrix.R). Check the repository for future updates.

Happy R-coding and stay healthy!

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