Extract indecies corresponding to the largest elements

Extracts the indecies of the \(n\) largest elements of the input. This procedure is equivalent to order(x, decreasing = TRUE)[1:n_elements], but is much faster and avoids the overhead of sorting discarded elements. This function is useful for extracting the rows in a data frame having the largest values in one of the columns.

partial_argsort(x, n_elements)

Arguments

x

Numeric vector, the vector containing the numbers to sort.

n_elements

Integer scalar, the number of indecies to return.

Value

Numeric vector, the indecies of the largest elements (in sorted order) in x.

Examples

x <- c(10L,5L,-2L,12L,15L)
max_indecies <- partial_argsort(x,3L)
max_indecies
#> [1] 5 4 1
x[max_indecies]
#> [1] 15 12 10
order(x)[1:3]
#> [1] 3 2 1
mtcars[partial_argsort(mtcars$hp,5L),]
#>                    mpg cyl disp  hp drat    wt  qsec vs am gear carb
#> Maserati Bora     15.0   8  301 335 3.54 3.570 14.60  0  1    5    8
#> Ford Pantera L    15.8   8  351 264 4.22 3.170 14.50  0  1    5    4
#> Duster 360        14.3   8  360 245 3.21 3.570 15.84  0  0    3    4
#> Camaro Z28        13.3   8  350 245 3.73 3.840 15.41  0  0    3    4
#> Chrysler Imperial 14.7   8  440 230 3.23 5.345 17.42  0  0    3    4