Open Watcom FORTRAN 77 Language Reference

Program Structure Control Statements
the block is completed. A block started within a DO-loop must be terminated before the
DO-loop is completed. Indenting the statements of each new block, as shown in the
examples, is helpful in avoiding invalid nesting and helps to make the structure of the
program visually obvious.
(iii)
The normal flow of control of the new programming constructs described earlier may be
altered with standard FORTRAN control statements. For example, the program may exit
from a block using a GO TO, STOP, RETURN or arithmetic IF statement. However, a
block may not be entered in the middle through use of any control statement such as GO
TO or the arithmetic IF.
Consider the following example.
Example:
GO TO 20
10 IF( X .GT. Y )THEN
CALL REDUCE( X, Y )
20 X = X - 1
ELSE
CALL SCALE( X )
END IF
This is an example of an illegal attempt to transfer execution into the middle of an IF-block.
The statement X = X - 1 is contained within the IF-block and may only be transferred
to from within the block.
Example:
IF( X .GT. Y )THEN
20 CALL REDUCE( X, Y )
X = X - 1
IF( X .GT. 0 )GO TO 20
ELSE
CALL SCALE( X )
END IF
This last example demonstrates a legal transfer of control within an IF-block. However, we
have seen better ways to express the loop with this IF-block.
Example:
IF( X .GT. Y )THEN
LOOP
CALL REDUCE( X, Y )
X = X - 1
UNTIL( X .LE. 0 )
ELSE
CALL SCALE( X )
END IF
(iv)
(v)
Many control structure statements cannot be branched to using a GO TO statement. For a
list of these statements, see the section entitled "Classifying Statements" on page 9 in the
chapter entitled "FORTRAN Statements"
Many control structure statements cannot be the object statement of a logical IF statement,
or be the last statement of a DO-loop. For a list of these statements, see the section entitled
"Classifying Statements" on page 9 in the chapter entitled "FORTRAN Statements"
Notes on Structured Programming Statements 213