How do I choose the max sum of one column while the sum of another column is less than constant in R?

Question

Say I have a data frame with 2 columns x and y.

How do I choose 3 elements from x so that it's sum is the largest possible while the sum of y stays under a certain amount.

    x  y
1  50  5
2  25  6
3  35  3
4  45  7
5  12  9

e.g. I want the sum of y to be <= 20 and the sum of x to be as large as possible. Then the largest sum of 3 elements from x would be entries 1,3,4 (50+35+45). What function can I use to do this?

Answer 1

You can use combn() to get all possible combinations of 3 rows, then test each set of rows:

df <- data.frame(
  x = c(50, 25, 35, 45, 12),
  y = c(5, 6, 3, 7, 9)
)

rowsets <- combn(1:nrow(df), m = 3, simplify = FALSE)

x_sums <- vector("double", length = length(rowsets))
for (i in seq_along(rowsets)) {
  df_rows <- df[rowsets[[i]], ]
  x_sums[[i]] <- if (sum(df_rows$y) <= 20) sum(df_rows$x) else NA_real_
}

df[rowsets[[which.max(x_sums)]], ]
#>    x y
#> 1 50 5
#> 3 35 3
#> 4 45 7

^{Created on 2022-03-01 by the reprex package (v2.0.1)}