cells(x, y) output does not respect the order of a vector y argument

[florisvdh]

It seems that the cell numbers from cells(x, y) cannot be matched with the values in a provided y vector, since the cell numbers are sorted.

Shouldn't the cell numbers be sorted in correspondence with the provided vector (in case y is indeed a vector), so that both vectors can be aligned?

I have a usecase with a raster of unique values, similar to below reprex.

library(terra)
#> terra 1.7.75
r <- rast(ncols=10, nrows=10)
values(r) <- 1:ncell(r)
selected_vals <- c(85, 45, 99)
cells(r, selected_vals)
#> $lyr.1
#> [1] 45 85 99
cells(r, rev(selected_vals))
#> $lyr.1
#> [1] 45 85 99
cells(r, sort(selected_vals))
#> $lyr.1
#> [1] 45 85 99

^{Created on 2024-04-24 with reprex v2.1.0}

Session info

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[rhijmans]

That is tricky because it would only work well for a raster with unique values and all the selected values present. In other cases you can get things like

library(terra)
r <- rast(ncols=10, nrows=10)
values(r) <- round(1:ncell(r)/4)
cells(r, c(1,2))
#$lyr.1
#[1]  3  4  5  6  7  8  9 10

cells(r, c(-1,0))
#$lyr.1
#[1] 1 2

I think that what you are after would require an additional argument to indicate you want a different type of output. Something like this?

cells(r, c(1,2), pairs=TRUE)
#     value cell
#[1,]     1    3
#[2,]     1    4
#[3,]     1    5
#[4,]     1    6
#[5,]     2    7
#[6,]     2    8
#[7,]     2    9
#[8,]     2   10

[rhijmans]

Here is an implementation. Does that meet your needs effectively?

library(terra)
#terra 1.7.75
r <- rast(ncols=10, nrows=10)
values(r) <- round(1:ncell(r)/4)
cells(r, c(1,2))
#$lyr.1
#[1]  3  4  5  6  7  8  9 10

cells(r, c(1,2), pairs=T)
#$lyr.1
#     cell value
#[1,]    3     1
#[2,]    4     1
#[3,]    5     1
#[4,]    6     2
#[5,]    7     2
#[6,]    8     2
#[7,]    9     2
#[8,]   10     2

[florisvdh]

That is tricky because it would only work well for a raster with unique values and all the selected values present.

Of course, you are right!

Your solution fully covers the needs, including the usecase I had. Thank you for this quick implementation!

library(terra)
#> terra 1.7.75
r <- rast(ncols=10, nrows=10)
values(r) <- 1:ncell(r)
selected_vals <- c(85, 45, 99)
cells(r, selected_vals, pairs = TRUE)
#> $lyr.1
#>      cell value
#> [1,]   45    45
#> [2,]   85    85
#> [3,]   99    99

^{Created on 2024-04-26 with reprex v2.1.0}

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