Expressions whose values do not change in a loop should be moved outside the loop. In the following loop:

FOR I = 0, N - 1 DO ARR(I, 2 * J - 1) = ...

the expression (2 * J - 1) is invariant and should be evaluated only once before the loop is entered:

TEMP = 2 * J - 1

FOR I = 0, N - 1 DO ARR(I, TEMP) = ...