Finding different max between specified rows of different fields

Question

I'm looking to find the max values of different columns based on specified rows of each column.

My actual data frame is 50K columns and 1K+ rows so I can't use a loop without greatly increasing run time.

Data Frame:

row	V1	V2	V3	V4
1	5	2	4	5
2	3	5	1	6
3	7	3	2	6
4	2	5	3	10
5	6	9	1	2

beg_row <- c(2, 1, 2, 3)
end_row <- c(4, 3, 3, 5)

output:

c(7, 5, 2, 10)

Answer 1

You can try mapply (but I suspect that it won't speed up the runtime if you have massive columns)

> mapply(function(x, y, z) max(x[y:z]), df[-1], beg_row, end_row)
V1 V2 V3 V4
 7  5  2 10

Data

df <- structure(list(row = 1:5, V1 = c(5L, 3L, 7L, 2L, 6L), V2 = c(
  2L,
  5L, 3L, 5L, 9L
), V3 = c(4L, 1L, 2L, 3L, 1L), V4 = c(
  5L, 6L, 6L,
  10L, 2L
)), class = "data.frame", row.names = c(NA, -5L))

beg_row <- c(2, 1, 2, 3)

end_row <- c(4, 3, 3, 5)