Re: ALTER, ASSIGN and labels-as-values (was: Unicode in strings)

Anton Ertl wrote:

"Stephen Fuld" <SFuld@alumni.cmu.edu.invalid> writes:

That feature (Alter GOTO) was also in Fortran, as the, long since
deprecated, assigned GOTO statement.  I believe they were there to
support some older computers that didn't have indexed jump/branch
instructions, so achieved the effect by modifying the branch
destination in the instruction itself.

 
Fortran's assigned goto directly corresponds to indirect branches on
modern machines, so in 1954 when Fortran was designed, the target
machines for Fortran had such instructions.
 
The ALTER statement looks somewhat stranger, judging from the example
given at
>

<https://www.microfocus.com/documentation/cobol-it/4-11/user-guide/compiler-suite/procedure-division-statements-alter.html>:

 
           ...
              ALTER ALTER-1-EXIT TO PROCEED TO ALTER-2-EXIT
           END-IF.
 
           DISPLAY "GO TO ALTER-1-EXIT...." LINE 4 COL 10.
           GOTO ALTER-1-EXIT.
 
       ALTER-1-EXIT.
           GOTO LABEL-1.
 
       LABEL-1.
           DISPLAY "ALTER-1 EXIT!" LINE 10 COL 10.
           ACCEPT DUMMY LINE 10 COL 30.
           STOP RUN.
 
       ALTER-2-EXIT.
           DISPLAY "ALTER-2 EXIT!" LINE 10 COL 10.
           ACCEPT DUMMY LINE 10 COL 30.
           STOP RUN.
 
What confuses me is that there is a GOTO right in the ALTER-1-EXIT
section (or is it a procedure?).

It is called, consistant with the desire to make it "natural language
like", a paragraph.

So does the ALTER change the "GOTO
ALTER-1-EXIT" and every other occurence of "GOTO ALTER-1-EXIT" that
occurs in the program (as I would expect from the general rule 1. on
the web page linked above; if so, why have the "GOTO LABEL-1" which is
followed right by the label "LABEL-1."?

One caveat.  It has been over 40 years since I wrote any COBOL
programs, and when I was taught it, we were taught to avoid the ALTER
statement, so my experience should be taken with a grain of salt.


This is a rather poor example.  As best as I can tell from the full
listing on the web site you linked to, the LABEL-1 paragraph is never
executed at all!  Your expectations are correct.  After the Alter,
every time control is transferred to ALTER-1-Exit it will then go to
ALTER-2-Exit.  The only reason to have LABEL-1 immediately after the
paragraph is if the Alter was conditional.  A better example usage is
some code that is executed at say the first time though,
i.e.initialization code, that execute the ALTER to skip that code on
subsequent trips through the code.

Of is it required that the
given section or procedure starts with a GOTO, and the ALTER statement
changes that GOTO?

The syntax requires a paragraph that contains only the single GOTO
statement that is the target of the ALTER statement.  Prior to the
ALTER statement being executed, when that paragraph is executed,
control is transferred to the target initially specified in the source
code.  After the ALTER is executed, any time control is transferred to
the paragraph containing the GOTO, control will then be transferred to
the location specified in the ALTER statement.  Of course, execution of
a subsequent ALTER statement can change that again.

 
Let's compare this to FORTRAN's assigned goto.
<https://docs.oracle.com/cd/E19957-01/805-4939/6j4m0vn9j/index.html>
gives the following example:
 
       ASSIGN 10 TO N
       ...
       GO TO N ( 10, 20, 30, 40 )
       ...
10     CONTINUE
       ...
40     STOP
 
Here N could be a register, the ASSIGN statment loads a constant (the
code address of the label 10) into that register, and the GOTO N
performs a branch to that code address.  The labels given in
parentheses limits the possible targets, allowing the compiler to
perform more accurate data-flow analysis and especially limit the
variables which are considered to be alive at the GOTO

While not as convenient, the COBOL compiler could do the equivalent by
creating a list of all of the target paragraph names of all the ALTER
statement that specify the GOO being altered.




--
 - Stephen Fuld
(e-mail address disguised to prevent spam)