Fortran array assignment with two indices

Question

I need to initialize a two dimensional array with x*x-y*y, where x and y are indices.

Relevant code is

REAL, DIMENSION(1:XSIZE,1:YSIZE) :: PHI
PHI(1:XSIZE,1:YSIZE) = reshape((/ (i*i,i=1,XSIZE) /),shape(PHI))

But what I really want is something like

PHI(1:XSIZE,1:YSIZE) = reshape((/ (i*i-j*j,i=1,XSIZE,j=1,YSIZE) /),shape(PHI))

But this does not work due to bad syntax.

Can you verbally describe what you are trying to do with the arrays in both cases? — Tim Biegeleisen
– Tim Biegeleisen, Commented Feb 10, 2016 at 6:02
@TimBiegeleisen The first sentence of my question says that. See edit for more clarity — Cheeku
– Cheeku, Commented Feb 10, 2016 at 6:15

IanH · Accepted Answer · 2016-02-10 07:02:50Z

2

Initialization in Fortran has a specific meaning - it is the process by which an object acquires a value before the program starts executing. Your examples show assignment, which is one of the many actions that can happen during the execution of a program.

For initialization proper in Fortran 90, you could do something like:

INTEGER :: ix
INTEGER :: iy
REAL, DIMENSION(XSIZE,YSIZE) :: PHI = RESHAPE(  &
    (/ ( (ix * ix - iy * iy, ix = 1, XSIZE), iy = 1, YSIZE) /),  &
    SHAPE=[XSIZE, YSIZE] )

You could also use the initializer (the expression after the =) in the above as the right hand side in an assignment statement.

Other options for execution time assignment of the value include use of do constructs or, with later standards, use of FORALL.

FORALL (INTEGER :: ix = 1:XSIZE, iy = 1:YSIZE) PHI(ix,iy) = ix*ix - iy*iy

answered Feb 10, 2016 at 7:02

IanH

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