R Under development (unstable) (2023-12-09 r85665 ucrt) -- "Unsuffered Consequences"
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> library("matrixStats")
> 
> indexByRow_R1 <- function(dim, idxs = NULL, ...) {
+   n <- prod(dim)
+   x <- matrix(seq_len(n), nrow = dim[2L], ncol = dim[1L], byrow = TRUE)
+   if (!is.null(idxs))
+     x <- x[idxs]
+   as.vector(x)
+ }
> 
> 
> indexByRow_R2 <- function(dim, idxs = NULL, ...) {
+   n <- prod(dim)
+   if (is.null(idxs)) {
+     x <- matrix(seq_len(n), nrow = dim[2L], ncol = dim[1L], byrow = TRUE)
+     as.vector(x)
+   } else {
+     idxs <- idxs - 1L
+     cols <- idxs %/% dim[2L]
+     rows <- idxs  %% dim[2L]
+     cols + dim[1L] * rows + 1L
+   }
+ }
> 
> 
> dim <- c(5L, 4L)
> x <- matrix(NA_integer_, nrow = dim[1L], ncol = dim[2L])
> y <- t(x)
> idxs_by_cols <- seq_along(x)
> 
> # Assign by columns
> x[idxs_by_cols] <- idxs_by_cols
> print(x)
     [,1] [,2] [,3] [,4]
[1,]    1    6   11   16
[2,]    2    7   12   17
[3,]    3    8   13   18
[4,]    4    9   14   19
[5,]    5   10   15   20
> 
> # Truth
> y0 <- t(x)
> idxs_by_rows <- as.vector(y0)
> 
> # Assert
> idxs <- indexByRow(dim)
> stopifnot(all.equal(idxs, idxs_by_rows))
> y <- x
> y[idxs_by_rows] <- idxs
> print(y)
     [,1] [,2] [,3] [,4]
[1,]    1    6   11   16
[2,]    2    7   12   17
[3,]    3    8   13   18
[4,]    4    9   14   19
[5,]    5   10   15   20
> stopifnot(all(as.vector(y) == as.vector(x)))
> 
> idxs_R1 <- indexByRow_R1(dim)
> stopifnot(all.equal(idxs_R1, idxs_by_rows))
> 
> idxs_R2 <- indexByRow_R2(dim)
> stopifnot(all.equal(idxs_R2, idxs_by_rows))
> 
> # Assert
> idxs_by_cols <- seq(from = 1, to = length(x), by = 3L)
> idxs_by_rows <- as.vector(t(x)[idxs_by_cols])
> 
> idxs <- indexByRow(dim, idxs = idxs_by_cols)
> stopifnot(all(idxs == idxs_by_rows))
> 
> idxs_R1 <- indexByRow_R1(dim, idxs = idxs_by_cols)
> stopifnot(all(idxs_R1 == idxs_by_rows))
> 
> idxs_R2 <- indexByRow_R2(dim, idxs = idxs_by_cols)
> stopifnot(all(idxs_R2 == idxs_by_rows))
> 
> 
> ## DEFUNCT: Backward compatibility
> res <- tryCatch({
+   idxs1 <- indexByRow(x)
+ }, error = identity)
> stopifnot(inherits(res, "error"))
> 
> 
> ## Exceptions:
> ## Too large matrices are not supported, which happens
> ## when prod(dim) > .Machine$integer.max
> dim_too_large <- c(.Machine$integer.max, 2L)
> res <- tryCatch({
+   idxs <- indexByRow(dim_too_large, idxs = 1L)
+ }, error = identity)
> stopifnot(inherits(res, "error"))
> 
> ## Non-positive indices are not supported
> res <- tryCatch({
+   idxs <- indexByRow(c(1,1), idxs = 0L)
+ }, error = identity)
> stopifnot(inherits(res, "error"))
> 
> res <- tryCatch({
+   idxs <- indexByRow(c(1,1), idxs = -1L)
+ }, error = identity)
> stopifnot(inherits(res, "error"))
> 
> proc.time()
   user  system elapsed 
   0.17    0.09    0.20