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# Performing array operations based on logicals

## Performing array operations based on logicals

(OP)
Hi,
I'm new to FORTRAN and only have real experience in MATLAB.

What I've got is two arrays. If the elements in one array are less than a parameter, I want to perform an operation on the corresponding elements of the other array. Otherwise, I want to perform a different operation.

Since MATLAB indices can be logical arrays, this is how I would do it in MATLAB in 5 simple lines:

a = [1 3 5 7 9];
b = [0 2 4 6 8];
c = zeros(5,1);
c(a>4) = 3*b;
c(a<=4) = 2*b

This would result in
c = [0 4 12 18 24]

How would I go about doing this operation in FORTRAN? I could use an if loop nested in a do loop, but wouldn't that be inefficient? If FORTRAN does not have this similar capability to MATLAB, what would be a good algorithm to use in this case?

Thanks!

### RE: Performing array operations based on logicals

The following won't work on F77. You have to be on at least F90.

#### CODE

program main
integer, dimension(5):: a, b, c

! a = [1 3 5 7 9];
a = (/ 1, 3, 5, 7, 9 /)
! b = [0 2 4 6 8];
b = (/ 0, 2, 4, 6, 8 /)
! c = zeros(5,1);
c = 0

! c(a>4) = 3*b;
where (a > 4)
c = 3 * b
else where
! c(a<=4) = 2*b
c = 2 * b
end where

print *, c
end 

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