R operation without for loop

Question

I have an index data frame which have index$row and index$col columns which indicate row and columns of X data frame.

index data frame

row col
375  1
376  1
379  1
380  1
381  1
460  4
461  4
462  4
463  4

I want to take the minimum values of each observation in X data frame, which is indicated by index data frame.

How can I do it without using a for loop? I need this because i have more than one index data frame and this takes very much time with for loop.

Answer 1

We can convert the 'index' to matrix (assuming 'index' is data.frame) and extract the values of the minimum values from the 'X' dataset using the row/column indexing provided by 'index'. The code is below

X{as.matrix(index)]