VMS Help FORTRAN, Statements, IF  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  Conditionally transfers control or executes a statement or block of
  statements.

  For each type of IF statement, the decision to transfer control or
  to execute the statement or block of statements is based on the
  evaluation of an expression within the IF statement.

1 - Arithmetic

  Executes the statement at the first label if the arithmetic
  expression evaluates to a value less than 0; the statement at the
  second label if the arithmetic expression evaluates to 0; or the
  statement at the third label if the arithmetic expression evaluates
  to a value greater than 0.  Statement format:

     IF (e) s1,s2,s3

     e         Is an arithmetic expression.

     s1,s2,s3  Are labels of executable statements in the same
               program unit.  All three labels are required,
               but they need not refer to different statements.

  Executes the statement at the first label ("s1") if the arithmetic
  expression evaluates to a value less than 0; the statement at the
  second label ("s2") if the arithmetic expression evaluates to 0; or
  the statement at the third label ("s3") if the arithmetic
  expression evaluates to a value greater than 0.

2 - Logical

  Executes the statement if the logical expression is true.
  Statement format:

     IF (e) st

     e   Is a logical expression.

     st  Is a complete Fortran statement. The statement can
         be any statement except DO, END DO, END, block IF,
         or another logical IF statement.

3 - Block

  Executes a block of statements if the logical expression is true.
  The block of statements starts immediately following the IF
  statement.  The block of statements can be followed by optional
  ELSE IF statements (any number) and one optional ELSE statement.
  The entire block IF construct must be terminated by an END IF
  statement.  Format:

     IF (e) THEN
       block
     ELSE IF (e1) THEN
       block
     ELSE
       block
     END IF

     e,e1   Are logical expressions.

     block  Is a series of zero or more Fortran statements
            (called a statement block).

  NOTE:  No additional statement can be placed after the IF THEN
  statement in a block IF construct.  For example, the following
  statement is invalid in the block IF construct:

     IF (e) THEN I = J

  This statement is translated as the following logical IF statement:

     IF (e) THENI = J