Open Watcom FORTRAN 77 Language Reference

Language Reference
Transfer of control into the range of a DO-loop from outside the range is not permitted.
A DO-loop may be executed 0 or more times. The following sequence occurs when a DO statement is
encountered.
(i)
An initial value, m1, is calculated by evaluating expression e1. A terminal value, m2, is
calculated by evaluating expression e2. An incrementation value, m3, is calculated by
evaluating expression e3 if it is present; otherwise m3 has the value one. If e3 is specified,
m3 must not be zero. The type of m1, m2, and m3 is determined from the DO-variable
and any conversions of type are done as required.
(ii) The DO-variable is defined with the initial value m1.
(iii)
The iteration count (i.e., the maximum number of times that the DO-loop will be executed)
is calculated as follows:
MAX( INT((m2 - m1 + m3)/m3), 0 )
The iteration count will be zero whenever:
m1 > m2 and m3 > 0, or
m1 < m2 and m3 < 0.
The number of times that the DO-loop is executed may be reduced if control is transferred
outside the range of the DO-loop, or if a RETURN or STOP statement is executed.
The steps involved in each iteration of the DO-loop are as follows:
(i)
(ii)
(iii)
(iv)
Check the iteration count. If it is not zero then start execution of the first executable
statement of the DO-loop. If the count is zero then iteration of the DO-loop is complete.
Execute statements until the terminal statement is encountered. During this time, the
DO-variable may not be redefined.
Execute the terminal statement. Unless execution of the terminal statement causes a
transfer of control, proceed with the next step which is "incrementation" processing.
The DO-variable is incremented by the value m3. The iteration count is decremented by
one. Go back to step (i).
Example:
DO 10 I = -5, 5
PRINT *, I, I*I
10 CONTINUE
In this example, the initial value is -5, the terminal value is 5, and the incrementation value is 1 (the
default). The DO-variable is I. The DO-loop is executed
MAX( INT((5 - (-5) + 1)/1), 0 )
or 11 times. The successive values of I, inside the range of the DO-loop, are -5, -4, -3, ..., 0, 1, ..., 4, 5.
When the DO-loop is terminated, the value of I will be 6. It should be noted that when a DO-loop variable
is of type real, the iteration count may be one less than expected. Because of rounding errors, the value of
m2 - m1 + m3 may be slightly less than the exact value and when the INT function is applied, the
resulting iteration count is one less than expected.
46 DO Statement