Short-circuit of compound Boolean expressions

Short-circuit evaluation (aka. minimal evaluation or McCarthy evaluation) is when the remaining arguments in a Boolean operation are executed or evaluated only if the previous operations are true. Fortran does not provide short-circuit evaluations. This allows for optimizations such as parallel evaluation. Some compilers do provide an extension to provide short-circuit evaluation but to keep your code standard-conforming you should assume all sections of an expression may or may not be evaluated and executed, including function calls in the expression.

The solution is to use nested conditionals. For example:

val=0.0
if(val.ne.0 .and. (top/val.gt.10.0) )then ! DO NOT DO THIS
   write(*,*)'met conditions'
endif

Should be written as

val=0.0
if(val.ne.0 )then
   if (top/val.gt.10.0) )then
      write(*,*)'met conditions'
   endif
endif

So any time you have a compound Boolean expression (anywhere, not just in an IF(3f) expression) make sure all subexpressions are safe to evaluate or change the expression to conditionally executed sections.

Common errors …

IF ( INDX .GT.0 .AND. ARRAY(INDX) .EQ. 'xxx' ) THEN ...

If INDX can be out of range for array ARRAY this could cause such problems as getting a run-time out-of-bounds error. This is true not just in IF statements but anywhere a compound Boolean expression is used …

I=MERGE (A(ii), 0, ii .GE. 1)

By the way …

Some new forms of short-circuiting conditional expressions are planned for Fortran 202x.