Operations can be restricted to a set of array elements with the WHERE construct. It has the effect of a parallel IF statement. Note that nested WHERE's are not allowed within Fortran 90. The salient features of WHERE are:

• Its syntax involves the use of a logical mask; in other words...
• It is equivalent to doing an `IF' on all elements of an array of LOGICAL type
• There can also be an ELSE WHERE clause
• <, <=, ==, /=, > and => in place of .LT., .LE., .EQ., .NE., .GE. and .GT.

 REAL, DIMENSION(7,7) :: a, b
 ...
 WHERE (a /= 0) b = 1/a
 ...
 WHERE (a > 2.0)
  b = a * 9.0 
 ELSE WHERE
  b = a / 3.0
 END WHERE