Open Watcom FORTRAN 77 Language Reference
Open Watcom FORTRAN 77 Language Reference
Open Watcom FORTRAN 77 Language Reference
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Program Structure Control Statements<br />
Example:<br />
LOOP : PRLOOP<br />
GUESS<br />
LINE = LINE + 1<br />
READ *, ICODE, X<br />
AT END, QUIT :PRLOOP<br />
IF( ICODE .NE. 1 )QUIT<br />
LINE = LINE + 1<br />
READ *, ICODE, Y<br />
AT END, QUIT<br />
IF( ICODE .NE. 2 )QUIT<br />
PRINT *, X, Y<br />
CALL PROCES(X, Y)<br />
ADMIT<br />
PRINT *, ’INVALID SEQUENCE: LINE =’, LINE<br />
QUIT :PRLOOP<br />
END GUESS<br />
END LOOP<br />
The above example attempts to read a code and number. If an end of file occurs then the loop is terminated<br />
by the QUIT statement.<br />
If the code is not 1 then we did not get what we expected and an error situation has arisen. Control is<br />
passed to the statement following the ADMIT statement. An error message is printed and the loop is<br />
terminated by the QUIT statement.<br />
If the code is 1 then a second code and number are read. If an end of file occurs then we are missing a set<br />
of data and an error situation has arisen. Control is passed to the statement following the ADMIT statement.<br />
An error message is printed and the loop is terminated by the QUIT statement. Similarly if the expected<br />
code is not 2 an error situation has arisen. Control is passed to the statement following the ADMIT<br />
statement. An error message is printed and the loop is terminated by the QUIT statement.<br />
If the second code is 2, the values of variables X and Y are printed. A subroutine is then called to process<br />
the data. Control resumes at the statement following the END GUESS statement. Since this statement is an<br />
END LOOP, control is transferred to the beginning of the loop.<br />
The above example illustrates the point that all the information required to make a choice (in this case<br />
between a valid set of data and an invalid set) is not available from the beginning. In this case we make an<br />
assumption that the data values are correct (our hypothesis) and then test the assumption at various points in<br />
the algorithm. If any of the tests fail we reject the hypothesis (and, perhaps, select a new hypothesis).<br />
It should be noted that no alternative selection need be coded (i.e., we need not use any ADMIT-blocks).<br />
This is illustrated in the following example.<br />
Example:<br />
GUESS<br />
X=SQRT( X )<br />
IF( X .LT. EPS )QUIT<br />
X=Y+SQRT(Y)<br />
IF( X .LT. EPS )QUIT<br />
CALL INTGRT( X, Y )<br />
END GUESS<br />
It might be noted that the IF-ELSE-END IF construct is simply a specific instance of the more general<br />
GUESS-ADMIT-END GUESS construct wherein the data values are known beforehand (as could be<br />
illustrated using the previous example).<br />
GUESS-ADMIT-END GUESS 207
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