209507097@ucis.vill.edu (MCREE, JAMES F) (04/09/91)
I been following the thread of calling procedures by pointers with a
lot of interest recently. Mostly because I have an application that is just
begging for a solution like that. However, the discussion and samples that I
have seen (including those in the manual) deal with hardcoding procedure names
into the program. (ie. Proc[i] := Do_It_Procedure) Is there a way of doing
what my syntactically incorrect program below is trying to do?
Program Call_A_Variable_Procedure (Input,Output);
.
Var
Proc_Name : Procedure;
.
.
Assorted necessary delcarations
.
Begin
.
.
Assorted code statements
.
.
Write ('Enter the name of a procedure to run: ');
Readln (Proc_Name);
Proc_Name;
.
.
End.
This is an example of a user-specified call to a procedure. The user
specifies the procedure directly though. Can this be done?
Happy Coding!!!
Jim.dmurdoch@watmath.waterloo.edu (Duncan Murdoch) (04/09/91)
In article <26486@adm.brl.mil> 209507097@ucis.vill.edu (MCREE, JAMES F) writes: > > I been following the thread of calling procedures by pointers with a >lot of interest recently. Mostly because I have an application that is just >begging for a solution like that. However, the discussion and samples that I >have seen (including those in the manual) deal with hardcoding procedure names >into the program. (ie. Proc[i] := Do_It_Procedure) Is there a way of doing >what my syntactically incorrect program below is trying to do? ... >Write ('Enter the name of a procedure to run: '); >Readln (Proc_Name); >Proc_Name; >. >. >End. > > >This is an example of a user-specified call to a procedure. The user >specifies the procedure directly though. Can this be done? The closest thing to what you've got there would be to create an array of procedure variables, and a corresponding array of names. Then when the user enters a name, search through the name array for a match, and call the corresponding procedure. For example: var procs : array[1..2] of procedure; names : array[1..2] of string[8]; name : string[8]; i : integer; begin { In TP 6, these assignments could all be done in a Const declaration } procs[1] := add; names[1] := 'ADD'; procs[2] := subtract; names[2] := 'SUBTRACT'; readln(name); i := 1; while (i <= 2) and (names[i] <> name) do inc(i); if i <= 2 then procs[i]; end; I hope this helps. Duncan Murdoch dmurdoch@watstat.waterloo.edu (when it gets fixed) dmurdoch@watmath.waterloo.edu (temporarily)
CDCKAB%EMUVM1.BITNET@cunyvm.cuny.edu ( Karl Brendel) (04/09/91)
In article <9104081439.aa02828@VGR.BRL.MIL>, 209507097@ucis.vill.edu (MCREE, JAMES F) wrote: > I been following the thread of calling procedures by >pointers with a lot of interest recently. Mostly because I have an >application that is just begging for a solution like that. However, >the discussion and samples that I have seen (including those in the >manual) deal with hardcoding procedure names into the program. (ie. >Proc[i] := Do_It_Procedure) Is there a way of doing what my >syntactically incorrect program below is trying to do? > >Program Call_A_Variable_Procedure (Input,Output); >. >Var > Proc_Name : Procedure; >. >. >Assorted necessary delcarations >. >Begin >. >. >Assorted code statements >. >. >Write ('Enter the name of a procedure to run: '); >Readln (Proc_Name); >Proc_Name; >. >. >End. > > >This is an example of a user-specified call to a procedure. The >user specifies the procedure directly though. Can this be done? It isn't really apparent (to me) that your application is truly "begging for a solution like [pointers to procedures]". Why can you not just use a series of IF statements for your purpose? Proc_Name := stUpCase(Proc_Name); IF Proc_Name = 'PROC_A' THEN Proc_A ELSE IF Proc_Name = 'PROC_B' THEN ProcB etc.... Note that you may want to let the user specify arguments prior to calling the routine: Enter the name of a procedure to run (with arguments): proc_a(first_var, second_var)_ where the number of arguments may be different from routine to routine. I don't think there's any way of doing that with proceure types (or pointers thereto) which is really any more elegant or compact than the series of IF statements. (But that's probably shortsightedness on my part :) ) Cheers-- --Karl +--------------------------------------------------------------------+ | Karl Brendel Centers for Disease Control | | Internet: CDCKAB@EMUVM1.BITNET Epidemiology Program Office | | Bitnet: CDCKAB@EMUVM1 Atlanta GA 30093 USA | | Home of Epi Info 5.0 | +--------------------------------------------------------------------+
bg11+@andrew.cmu.edu (Brian E. Gallew) (04/10/91)
I don't know much about Procedure Pointers in PASCAL, but I can see one
way in which this becomes truly elegant.
Define an array of Procedure Pointers. This, in conjuction with a
numerical or key-letter input reduces to a mapping function (for
key-letters) and a single call ProcPtrVar[Choice];
While I have never done this in PASCAL, I have done exactly this in C.
-Brian
You drop the bomb -more-
It goes off... -more-
-------------------------------------------------------------------------
I am *NOT* as think as you dumb I am!! | This space for rent (241-6939)
-------------------------------------------------------------------------mwizard@eecs.cs.pdx.edu (Craig Nelson) (04/10/91)
CDCKAB%EMUVM1.BITNET@cunyvm.cuny.edu ( Karl Brendel) writes: >In article <9104081439.aa02828@VGR.BRL.MIL>, 209507097@ucis.vill.edu > (MCREE, JAMES F) wrote: >> I been following the thread of calling procedures by >>pointers with a lot of interest recently. Mostly because I have an >>application that is just begging for a solution like that. However, >>the discussion and samples that I have seen (including those in the >>manual) deal with hardcoding procedure names into the program. (ie. >>Proc[i] := Do_It_Procedure) Is there a way of doing what my >>syntactically incorrect program below is trying to do? >> >>Program Call_A_Variable_Procedure (Input,Output); >>. >>Var >> Proc_Name : Procedure; >>. >>. >>Assorted necessary delcarations >>. >>Begin >>. >>. >>Assorted code statements >>. >>. >>Write ('Enter the name of a procedure to run: '); >>Readln (Proc_Name); >>Proc_Name; >>. >>. >>End. >> >> >>This is an example of a user-specified call to a procedure. The >>user specifies the procedure directly though. Can this be done? >It isn't really apparent (to me) that your application is truly >"begging for a solution like [pointers to procedures]". Why can you >not just use a series of IF statements for your purpose? > Proc_Name := stUpCase(Proc_Name); > IF Proc_Name = 'PROC_A' THEN Proc_A > ELSE IF Proc_Name = 'PROC_B' THEN ProcB > etc.... How about this: {$F+} procedure proc1; begin writeln('In Proc1'); end; procedure proc2; begin writeln('In Proc2'); end; procedure proc3; begin writeln('In Proc3'); end; Const NumProcedures = 3; Type ProcString = String[10]; ProcRec = Record Name:PString; Proc:Procedure; end; Const MyProcedures : Array[1..NumProcedures] of ProcRec = ( (Name:'PROC1';Proc:proc1), (Name:'PROC2';Proc:proc2), (Name:'PROC3';Proc:proc3)); With this kind of setup you should be able to have a function that runs the procedure desired if found from the input line. For instance, if the function returned and index if the proper procedure was found, and 0 on an error, then i:= FindProcedure(ReadString); if (i>0) then MyProcedures[i].Proc else writeln('No such procedure'); Am I missing something or will this work ? Should.... Looks like it will. Cheers! []====================================================================[] || Craig R. Nelson | CCSofD Software Inc. || || Programmer | Beaverton, OR, 97005 || || mwizard@eecs.ee.pdx.edu | (unlisted on the net) ||
John G. Spragge <SPRAGGEJ@QUCDN.QueensU.CA> (04/10/91)
To read the name of a procedure and map it to a procedure
address, the program would have to look up the procedure in
the symbol table. Unfortunately, most programs do not include
the symbol table in their production versions, since it takes
up space. So the user can not enter the name of the program
to execute directly.
disclaimer: Queen's University supplies me with computer services, not
my opinions.
John G. SpraggeCDCKAB%EMUVM1.BITNET@cunyvm.cuny.edu ( Karl Brendel) (04/11/91)
In article <Ec0TSOS00WCZAYM0Q3@andrew.cmu.edu>, bg11+@andrew.cmu.edu (Brian E. Gallew) wrote: >I don't know much about Procedure Pointers in PASCAL, but I can see >one way in which this becomes truly elegant. > >Define an array of Procedure Pointers. This, in conjuction with a >numerical or key-letter input reduces to a mapping function (for >key-letters) and a single call ProcPtrVar[Choice]; True, but the original question involved accepting a character _string_ naming the procedure (although e-mail has revealed that what the poster really _wants_ is a menu pick). Also this solution doesn't allow for passing parameters to procedures unless they all take the same sequence of parameters (same quantity, order and type). (E-mail has also revealed that the poster does want varying sequences of parameters.) (Of course, an input character string can be compared to an array of character strings to determine an index to an array of pointers. I think this has previously been proposed in this discussion. IMO, that is only slightly more elegant than IFs, and again does not allow varying sequences of parameters.) Cheers-- --Karl +--------------------------------------------------------------------+ | Karl Brendel Centers for Disease Control | | Internet: CDCKAB@EMUVM1.BITNET Epidemiology Program Office | | Bitnet: CDCKAB@EMUVM1 Atlanta GA 30093 USA | | Home of Epi Info 5.0 | +--------------------------------------------------------------------+
209507097@ucis.vill.edu (MCREE, JAMES F) (04/11/91)
Karl Brendel <CDCKAB%EMUVM1.BITNET@cunyvm.cuny.edu> writes... >>I don't know much about Procedure Pointers in PASCAL, but I can see >>one way in which this becomes truly elegant. >> >>Define an array of Procedure Pointers. This, in conjuction with a >>numerical or key-letter input reduces to a mapping function (for >>key-letters) and a single call ProcPtrVar[Choice]; > >True, but the original question involved accepting a character >_string_ naming the procedure (although e-mail has revealed that >what the poster really _wants_ is a menu pick). Also this solution >doesn't allow for passing parameters to procedures unless they all >take the same sequence of parameters (same quantity, order and >type). (E-mail has also revealed that the poster does want varying >sequences of parameters.) That's not quite what I had in mind. The user picks a menu choice which calls a procedure whose name is specified in an input file. Consider the following file: 1 Do_Operation_One (Some params) 2 Do_A_Different_Op (A few other params) 3 The_Final_Solution (All params) My goal was to read in a menu file (use the above as an example). If the user entered a 1 to a prompt, procedure "Do_Operation_One (Some params)" would be called. Likewise for a 2 and 3. Initially, this does not present a problem. Here's the problem. Let's say we want to make this a generic menu unit which will have the capability of calling any procedure so long as its name is specified in an input file. The name of the procedures would be read and if its number came up, it would be called. All of this ideally should be done without the need to recompile the menu unit. The solution that I have ended up falling back to was a simple giant case statement because the procedures will have params that will not all be the same. I'm still interested in the idea of calling a procedure whose name is specified by an input file entry without parameters though. A series of IF statements might work equally as well, but would probably end up being longer if I were to stick to structured programming. Thanks for all of the ideas. There are still a bunch of them that I would like to play with and see how useful they will be in other areas. Have a good day, Jim.