For a function to return a value in Pascal the assignment FunctionName := SomeVal; is used. I assume it doesn't stop the function execution in that exact place as return in C does. Is there something similar to C return in Pascal? (I'm using FreePascal compiler)
You can use the Exit procedure.
function Foo (Value : integer) : Integer;
begin
Exit(Value*2);
DoSomethingElse(); // This will never execute
end;
The return code of the end of every program is stored in the EAX register. So you can use Assembly inline on Pascal to return wherever you want to end the program running using!
asm
mov eax,%ERROLEVEL%
end;
I think you can use either the function name itself, "result", or Exit(), but I've only used the result identifier, so don't know if the others will work for you:
function Foo(a, b: Integer): Integer;
begin
result := a + b;
end;
Hope that helps ^^
In canonical pascal (without keyword Exit) you can emulate return via goto:
function Foo (Value : integer) : boolean;
label return;
begin
if Value < 0 then
begin
Foo := false;
goto return;
end;
Calc();
Foo := true;
return:
end;
Related
I have a small terminal program which displays a menu. I want to have a function which takes the user's input and an array of procedure names and be able to call the procedure the user chose. I know that I could do that using if or case statements in the main program, but I want to make a unit with the above procedure and a couple of other menu functions, and if this was possible, I could make it call arbitrary procedures. Here's more or less how I would like it to work (I know it's wrong, but so you get a general idea).
program menu;
uses crt;
type procs = array [0..1] of procedure;
procedure call_procs(inp: int; procsy: procs);
begin
writeln(procsy[ord(inp)]); {And call it here.}
end;
var procsx : procs;
begin
procsx[0] := readkey; {I would like to somehow store the procedure here.}
procsx[1] := clrscr;
call_procs(0, procsx);
end.
Is there any way to do something like this? Thank you in advance.
There are a few things wrong with your original code which are not cited in your answer.
You have an array of procedure but you are calling writeln with these procedure calls as arguments as if they were function, which they are not.
readkey is a function, not a procedure, so its type doesn't match the element type of your array
Your assignment of the procedures to the array need to use # to reference the procedure pointer and not actually call the procedure
Not sure what compiler or options you're using, but int isn't the standard Pascal integer type, rather integer is.
As a niggle, since you're already using the integer index of the array, you don't need to use ord.
So the minimal changes to your code to make it basically work would be:
program menu;
uses crt;
type procs = array [0..1] of procedure;
procedure call_procs(inp: integer; procsy: procs);
begin
procsy[inp]; { call the procedure here - removed 'ord' since it's superfluous }
end;
var procsx : procs;
begin
{ procsx[0] := readkey; {- 'readkey' is a function and won't work here }
procsx[1] := #clrscr;
call_procs(1, procsx);
end.
You can create an array of functions that return char which matches the type for readkey:
program menu;
uses crt;
type procs = array [0..1] of function: char;
procedure call_procs(inp: integer; procsy: procs);
begin
writeln(procsy[inp]); { call the function; write out the returned char }
end;
function foo: char;
begin
foo := 'X';
end;
var procsx : procs;
begin
procsx[0] := #readkey;
procsx[1] := #foo;
call_procs(0, procsx);
call_procs(1, procsx);
end.
I figured out how to do this. One can use pointers to a procedure, then create an array of those pointers, and pass them to the procedure I wanted to use. Also, for some reason, it doesn't seem to work with the functions that come with Pascal (such as readkey or clrscr). For this example, one could do this:
program menu;
type
Tprocptr = procedure; {This creates a pointer to procedures.}
Tprocarray = array of Tprocptr;
procedure writeHi;
begin
writeln('Hi!');
end;
procedure writeHello;
begin
writeln('Hello!');
end;
procedure call_proc(inp: integer; procsy: Tprocarray);
{This now calls functions like one would expect.}
begin
procsy[ord(inp)];
end;
var
proclist : Tprocarray;
begin
setlength(proclist, 2);
proclist[0] := #writeHi; {The '#' creates a pointer to those procedures.}
proclist[1] := #writeHello;
call_proc(0, proclist);
end.
This works as expected, calling (in this case) the procedure writeHi, so if you run this program it will output Hi! to the terminal. If you change call_proc(0,proclist) to call_proc(1, proclist), it will call writeHello instead.
I'm currently working on an exercise of testing Boolean value from user input, as being presented below:
function ReadBoolean(prompt: String): Boolean;
var
choice: String;
exit: boolean;
begin
repeat
begin
WriteLn(prompt);
ReadLn(choice);
case choice of
'yes','y','t','true': exit := true;
'no','n','f','false': exit := false;
else
WriteLn('Not a boolean input. Enter again: ');
end;
end;
until exit=true or exit=false;
result := exit;
end;
It is expected to keep looping asking for value until it receives the input from specified string, however at my first attempt when I try to input 'fred' the boolean variable is automatically assigned as TRUE and exit the function.
Any helps would be very appreciated.
As my understanding, you only want the loop to end when user enters some specific strings.
It can be achieved by modifying the until condition like this:
choice='yes' or choice='y' or choice='t' or choice='true' or choice='no' or choice='n' or choice='f' or choice='false'
Or alternatively, create an indefinite loop and break it when user enters an expected string:
while true do
...
'yes','y','t','true':
begin
exit := true;
break;
end;
'no','n','f','false':
begin
exit := false;
break;
end;
...
end;
Your loop terminates when exit=true or exit=false. As exit can only ever be one of those two values, it will always meet that condition, so it will never run your loop.
But also, consider explicitly setting the value of exit := false before starting your loop.
What you're asking here is "nullable" boolean thing (value is true, value is false, value is not provided). As far as I know it's not implemented in any Pascal dialect. So that, you have to split your indication into two separate flags: a) is there any well-formed input provided by user; b) is the input is recognized as true or false
function ReadBoolean(prompt: String): Boolean;
var
choice: String;
exit: boolean;
recognized: boolean; { this is our termination flag }
begin
recognized := false; { we place it to false initially as no user input recognized yet }
repeat
begin
WriteLn(prompt);
ReadLn(choice);
case choice of
'yes','y','t','true': begin exit := true; recognized := true; end; { we mark it as recognized }
'no','n','f','false': begin exit := false; recognized := true; end; { we mark it as recognized }
else
WriteLn('Not a boolean input. Enter again: ');
end;
end;
until not recognized; { we keep asking for user input until known input provided }
result := exit;
end;
I'm following an internet course on the basics of programming. After making a diagram I convert it to code, right now this is PASCAL language.
I'm having a problem with procedures and can't find an answer, nor in the course, nor with some google-ing.
I want to get a variavble back form a procedure. Right now iIhave a working piece of code but I think this is not the good way of working. Here's an extract of the code:
program WELKEWAGEN;
// declare your variables here
var T, N, KM, vari, prijsDW, prijsBW, jrenGEBR, taksDW, taksBW, prijsB, verbrBW, prijsD, verbrDW : real;
procedure OPHALEN(para : string);
begin
repeat
writeln('geef de ', para , ' op');
readln(vari);
until (vari > 0);
end;
begin
//this is the main program but there is more code ofcourse
OPHALEN('prijs benzinewagen');
prijsBW := vari;
//...
end.
Now the internet course says I should program it like this:
begin
//...
prijsBW := OPHALEN('prijs benzinewagen');
//...
end.
But this is not working.
I get following errors:
WELKEWAGEN.pas(24,14) Error: Incompatible types: got "untyped" expected "Real"
WELKEWAGEN.pas(50) Fatal: There were 1 errors compiling module, stopping
pas(24,14) is this line: prijsBW := OPHALEN('prijs benzinewagen');
Procedures don't return values, so the syntax
prijsBW := OPHALEN('prijs benzinewagen');
is invalid.
If you want to return a value, you need to define a function instead:
function OPHALEN(para : string): Real;
var
Res: Real;
begin
Res := 0;
repeat
writeln('geef de ', para , ' op');
readln(Res);
until (Res > 0);
OPHALEN := Res;
end;
Note that the (bad) global variables you're using mean you don't have to return anything at all, because a procedure can access and change that global variable directly (but you have no way of knowing when the procedure is finished):
procedure OPHALEN(para : string);
begin
vari := 0;
repeat
writeln('geef de ', para , ' op');
readln(vari);
until (vari > 0);
end;
Modern Pascal dialects (such as Delphi and FreePascal) allow a cleaner syntax for the return value of functions by using an automatically declared function result variable of the proper type for you, named Result (because that's what it is - the result of the function):
function OPHALEN(para : string): Real;
begin
Result := 0;
repeat
writeln('geef de ', para , ' op');
readln(Result);
until (Result > 0);
end;
If you need to return multiple values, you can use var parameters, which allow them to be changed inside the function.
procedure OPHALEN(para: string; var RetVal: Real);
begin
RetVal := 0;
repeat
writeln('geef de ', para , ' op');
readln(RetVal);
until (RetVal > 0);
end;
Your original code (and the examples I've provided above) all fail to allow the user to cancel, BTW. There should be some way to exit the loop for the user; otherwise, your code just endlessly loops, writing para to the screen and then waiting for input. This has a tendency to annoy users.
I have a function in PL/SQL:
FUNCTION do_something
RETURN BOOLEAN
IS
...
BEGIN
...
END
This function can be called as such:
DECLARE
answer BOOLEAN;
BEGIN
answer := do_something();
END
Now, there are cases, where I don't need the returned boolean. If I don't want to get it and just write:
do_something();
I get PLS-00306 (wrong number of types of arguments in call to DO_SOMETHING)
Is there a way to call it without declaring and setting a new boolean, which I will never use in this case?
Very simple: create a procedure which covers this function
PROCEDURE do_something
IS
dummy boolean;
BEGIN
dummy := do_something();
END;
Sorry, but this is the only way in PL/SQL. This language is very strict in definitions of procedure and function and you cannot make a function call without handling the result. But you can make a procedure as it is shown in example above.
It will define automatically where to choose the function and where the procedure.
EDIT
As far as there are people who do not trust me (sometimes I really tell bad things so doubts are allowed :) ) this is the test:
declare
myresult boolean;
function do_something return boolean
is
begin
return true;
end;
procedure do_something
is
dummy boolean;
begin
dummy := do_something();
end;
begin
myresult := do_something();
do_something();
end;
works well.
Here is a useful alternative to assigning a dummy variable:
DECLARE
FUNCTION f_yes RETURN boolean IS
BEGIN
return true;
END f_yes;
BEGIN
IF f_yes THEN
null;
END IF;
END;
This is slightly more helpful when there are functions of different return types you want to call without using their results. It's also helpful for writing empty IF statements, which could be useful to to simplify code by eliminating possibilities for the real condition you want to match in a later ELSIF.
I'm in the same situation and the shortest solution I've come up is wrapping it in an IF statement:
if do_something then
null;
end if;
in our last class we defined a couple of functions and procedures in our unit file.
procedure WriteNap(const elo: string; const n: TNap; const uto: string);
begin
Write(elo, Nap2Str(n), uto);
end;
function PredNap(const n: TNap): TNap;
begin
case n of
hetfo: PredNap := vasarnap;
NemNap: PredNap := n;
else
PredNap := Pred(n)
end;
end;
function SuccNap(const n: TNap): TNap;
begin
case n of
NemNap: SuccNap := n;
else
SuccNap := Succ(n)
end;
end;
I have to get rid of the word 'Nap' so that i can just use Pred, Write, Succ in the main program instead of PredNap etc... i tried to create another unit file which uses this unit and contains functions like Succ Pred... didnt really work out..
thanks in advance :)
Declare in somewhere in your file:
var Pred = Function (Const n:TNap):TNap;
And before you do anything in the body of your program, write:
Pred := #PredNap;
What this code does is, it creates a new function pointer with the same signature as your PredNap() and then assign the address of PredNap() to this pointer. So, whenever you call Pred() the actual function that gets called is PredNap(). Hope this helps.