Equivalence of IF and WHERE

Question

We all know that the DO loop is more powerful than the FORALL statement in Fortran. That is, you can always substitute a FORALL by a DO, but not vice versa.

What about the WHERE statement and block?

Can I always substitute the IF by a WHERE? Is it always possible to code the conditionals and bifurcations with a WHERE, thus avoiding the IF?

Answer 1

WHERE statements are reserved for arrays assignments and nothing else, e.g.:

INTEGER, DIMENSION(100,100) :: a, b
... define a ...
WHERE(a < 0)
   b = 1
ELSEWHERE
   b = 0
ENDWHERE

If you tried adding in something, say a WRITE statement, inside the WHERE block, you would see something like the following compiling error (compiler dependent):

Error: Unexpected WRITE statement in WHERE block at (1)

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EDIT

Note that nested WHERE blocks are legal:

WHERE(a < 0)
   WHERE( ABS(a) > 2)
      b = 2
   ELSEWHERE
      b = 1
   ENDWHERE
ELSEWHERE
   b = 0
ENDWHERE