[R] Fastest way to extract rows of smaller matrix many times by index to make larger matrix? and multiply columsn of matrix by vector

[Leonard Mada]

Hello Nevil,


you could test something like:


# the Matrix
m = matrix(1:1000, ncol=10)
m = t(m)

# Extract Data
idcol = sample(seq(100), 100, TRUE); # now columns
for(i in 1:100) {
     m2 = m[ , idcol];
}
m2 = t(m2); # transpose back


It may be faster, although I did not benchmark it.


There may be more complex variants. Maybe it is warranted to try for 
10^7 extractions:

- e.g. extracting one row and replacing all occurrences of that row;


Sincerely,


Leonard


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