[solved] looping direction controlled by flag

[Joanna from IRC]

hi I'm trying to make a function that can iterate through an array with direction controlled by a boolean flag. so far i have this but it doesn't compile

Code: Pascal  [Select][+][-]

1. FUNCTION TCASCADING_FLOWPANEL.GET_THE_CONTROL(CONST IS_FIRST:BOOLEAN):TCONTROL;
2.          VAR X:INTEGER;
3. BEGIN
4. IF IS_FIRST
5.    THEN FOR X:= LOW(AR_CONTROLS) TO HIGH(AR_CONTROLS) DO
6.             IF PROVIDES_SQL(AR_CONTROLS[X])THEN EXIT(AR_CONTROLS[X])
7.    ELSE FOR X:= HIGH(AR_CONTROLS) DOWNTO LOW(AR_CONTROLS) DO
8.             IF PROVIDES_SQL(AR_CONTROLS[X])THEN EXIT(AR_CONTROLS[X]);
9. ERROR_HALT('FUNCTION TCASCADING_FLOWPANEL.GET_THE_CONTROL');
10. END;

does anyone have any idea of how to do this?

[MarkMLl]

What error message are you getting? Also note that you've got a dangling else in the middle of it.

MarkMLl

[Thaddy]

@Joanna
I am pretty sure that is just your capslock being stuck

[Joanna from IRC]

The error is illegal assignment to a loop variable on the line with the else. In other words it seems to think the second loop is the else condition for the if statement inside the loop.. Definitely wrong

[MarkMLl]

The error is illegal assignment to a loop variable on the line with the else. In other words it seems to think the second loop is the else condition for the if statement inside the loop.. Definitely wrong

Let's try to rearrange it a bit.

Code: Pascal  [Select][+][-]

1. FUNCTION TCASCADING_FLOWPANEL.GET_THE_CONTROL(CONST IS_FIRST:BOOLEAN):TCONTROL; VAR X:INTEGER;
2.  
3. BEGIN
4.     IF IS_FIRST THEN
5.         FOR X:= LOW(AR_CONTROLS) TO HIGH(AR_CONTROLS) DO
6.             IF PROVIDES_SQL(AR_CONTROLS[X]) THEN
7.                 EXIT(AR_CONTROLS[X])
8.             ELSE
9.                 FOR X:= HIGH(AR_CONTROLS) DOWNTO LOW(AR_CONTROLS) DO
10.                     IF PROVIDES_SQL(AR_CONTROLS[X]) THEN
11.                         EXIT(AR_CONTROLS[X]);
12.     ERROR_HALT('FUNCTION TCASCADING_FLOWPANEL.GET_THE_CONTROL');
13. END;
14.  

Can you see it now?

MarkMLl

[Josh]

your trying to use same x variable in a loop that isalreadyin an x loop
reformatting inyour CAPS style, hopefully helps

Code: Pascal  [Select][+][-]

1. IF IS_FIRST THEN
2.   FOR X:= LOW(AR_CONTROLS) TO HIGH(AR_CONTROLS) DO
3.     IF PROVIDES_SQL(AR_CONTROLS[X])THEN EXIT(AR_CONTROLS[X])
4.     ELSE
5.       FOR X:= HIGH(AR_CONTROLS) DOWNTO LOW(AR_CONTROLS) DO  // <<- You creating a new x foor loop,while still in thef irst x loop
6.         IF PROVIDES_SQL(AR_CONTROLS[X])THEN EXIT(AR_CONTROLS[X]);        

If the below code is what your after,by adding a ;

Code: Pascal  [Select][+][-]

1. IF IS_FIRST THEN FOR X:= LOW(AR_CONTROLS) TO HIGH(AR_CONTROLS) DO IF PROVIDES_SQL(AR_CONTROLS[X])THEN EXIT(AR_CONTROLS[X]);       // added;
2. ELSE FOR X:= HIGH(AR_CONTROLS) DOWNTO LOW(AR_CONTROLS) DO IF PROVIDES_SQL(AR_CONTROLS[X])THEN EXIT(AR_CONTROLS[X]);
3.          

Or easier to read IMO, as ZORAN suggests

Code: Pascal  [Select][+][-]

1. IF IS_FIRST THEN
2. BEGIN // GOING UP
3.   FOR X:= LOW(AR_CONTROLS) TO HIGH(AR_CONTROLS) DO
4.   BEGIN
5.     IF PROVIDES_SQL(AR_CONTROLS[X])THEN EXIT(AR_CONTROLS[X]);
6.   END;
7. END
8. ELSE
9. BEGIN  // GOING DOWN
10.   FOR X:= HIGH(AR_CONTROLS) DOWNTO LOW(AR_CONTROLS) DO
11.   BEGIN
12.     IF PROVIDES_SQL(AR_CONTROLS[X])THEN EXIT(AR_CONTROLS[X]);
13.   END;
14. END;

[Zoran]

does anyone have any idea of how to do this?

This is not Python, the "else" is always connected to the last "if" before it. See Mark's post, it should make it very clear for you.
Putting begin-end where appropriate certainly solves this.

Then, you are lucky that you used the same variable in both loops (as Josh spotted) which turned it to compile time error, not only semantic error which could go unnoticed.

Make a habit of using begin-end with if statement.

[MarkMLl]

This is not Python, the "else" is always connected to the last "if" before it. See Mark's post, it should make it very clear for you.
Putting begin-end where appropriate certainly solves this.

Steady on: I don't think Joanna knows Python (and if she does, she likes it less than any of us). I'm hoping that she gives me an opportunity to work through what she's done (when compared with what she /thinks/) she's done in some detail, so that she doesn't fall into this trap again.

MarkMLl

[Joanna from IRC]

thanks for the replies. it was indeed getting confused by the THEN inside the loop. the redundancy still bothers me but how about this?

Code: Pascal  [Select][+][-]

1. FUNCTION TCASCADING_FLOWPANEL.GET_THE_CONTROL(CONST IS_FIRST:BOOLEAN):TCONTROL;
2.          VAR X:INTEGER;
3. BEGIN
4. IF IS_FIRST
5.    THEN FOR X:= LOW(AR_CONTROLS) TO HIGH(AR_CONTROLS) DO
6.             IF PROVIDES_SQL(AR_CONTROLS[X])THEN EXIT(AR_CONTROLS[X]);
7. IF NOT IS_FIRST
8.    THEN FOR X:= HIGH(AR_CONTROLS) DOWNTO LOW(AR_CONTROLS) DO
9.             IF PROVIDES_SQL(AR_CONTROLS[X])THEN EXIT(AR_CONTROLS[X]);
10. ERROR_HALT('FUNCTION TCASCADING_FLOWPANEL.GET_THE_CONTROL');
11. END;    

it seems that there is no way to choose which loop to execute for the same line of code that follows...

[Josh]

Add a Y var for main loop, assign x value according to direction, and check in loop to inc or dec x

Note this harder to follow, and will be slower..... Y for loop will be optimized as value likely store in register,but the inc/dec of x may not be, plus addition of check code inside loop

Something like

Code: Pascal  [Select][+][-]

1. if IS_FIRST then x:=low(AR_CONTROLS) else x:=high(AR_CONTROLS);    // set x initial value
2. for y:=low(AR_CONTROLS) to high(AR_CONTROLS) do
3. begin
4.   if PROVIDES_SQL(AR_CONTROLS[X])THEN Exit(AR_CONTROLS[X]);
5.   if IS_FIRST then inc(x) else dec(x); // adjust x based on direction
6. end;
7. ERROR_HALT('FUNCTION TCASCADING_FLOWPANEL.GET_THE_CONTROL');      

[TBMan]

I would do something like this (in semi psuedo code):
(I prefer to use begins and ends even though you don't always have to. It makes for an easier read. And I try to remember
to label the "ends" to match the "begins"

Code: Pascal  [Select][+][-]

1.  
2. var
3.  X,GotIt:integer;
4.  
5. if IsFirst then
6.   begin
7.      for x := low to high do
8.        begin
9.          if condition then
10.             begin
11.               Gotit := X;
12.               break;
13.             end; // end if
14.        end; // for
15.   end
16.   else // IsFirst
17.   begin
18.       for x := high to low do
19.       begin
20.          if condition then
21.             begin
22.                 GotIt := x;
23.                 break;
24.            end; // endIf
25.      end; // for
26.   end; // else  
27. result := Whatever(GotIt);

[TRon]

Add a Y var for main loop, assign x value according to direction, and check in loop to inc or dec x

That was a nudge in the good direction but did you know that there exist delta airlines and they use planes that can fly in different directions and make use of planes that can have other ranges ? While that is good to know that you could also choose for another airline ?

feeling a bit cryptic today... 

[cdbc]

Hi
Another possibility would be to utilize 'case', like this:

Code: Pascal  [Select][+][-]

1. var x: integer;
2. begin
3.   case IsFirst of
4.     false: begin
5.              for x:= ControlCount-1 downto 0 do
6.                if Controls[x].Name = 'Memo1' then exit(Controls[x]);
7.            end;
8.     true: begin
9.             for x:= 0 to ControlCount-1 do
10.               if Controls[x].Name = 'Edit1' then exit(Controls[x]);
11.           end
12.   end;
13.   Result:= nil;
14.   fPresenter.Provider.NotifySubscribers(prStatus,nil,Str2Pch('(!) Attention: Control was not found! '));
15. end;
16.  

I hope you get the gist of the algorithm, 'cause I really don't wanna mess with your caps-locked-code...
Regards Benny

[Josh]

Using Delta Airlines. Avoids the comparison check in loop

add A DeltaAirlines Var

Code: Pascal  [Select][+][-]

1. DeltaAirlines:=1; // DeltaAirlines is Going to its Destination
2. x:=low(AR_CONTROLS); // set x initial value
3. if Not IS_FIRST then
4. begin
5.   DeltaAirlines:=-1; // DeltaAirlines is going its Origin
6.   x:=high(AR_CONTROLS); // set x initial value
7. end;
8. for y:=low(AR_CONTROLS) to high(AR_CONTROLS) do
9. begin
10.   if PROVIDES_SQL(AR_CONTROLS[X])THEN Exit(AR_CONTROLS[X]);
11.   x:=x+DeltaAirlines; // adjust x based on DeltaAirlines direction
12. end;
13. ERROR_HALT('FUNCTION TCASCADING_FLOWPANEL.GET_THE_CONTROL');

And as function

Code: Pascal  [Select][+][-]

1. FUNCTION TCASCADING_FLOWPANEL.GET_THE_CONTROL(CONST IS_FIRST:BOOLEAN):TCONTROL;
2. VAR X,Y,DELTA_AIRLINES:INTEGER; // ADD EXTRA VARS
3. BEGIN
4.   DELTA_AIRLINES:=1; // DELTA_AIRLINES IS GOING TO ITS DESTINATION
5.   X:=LOW(AR_CONTROLS); // SET X INITIAL VALUE
6.   IF NOT IS_FIRST THEN
7.   BEGIN
8.     DELTA_AIRLINES:=-1; // DELTA_AIRLINES IS GOING TO ITS ORIGIN
9.     X:=HIGH(AR_CONTROLS); // SET X INITIAL VALUE
10.   END;
11.   FOR Y:=LOW(AR_CONTROLS) TO HIGH(AR_CONTROLS) DO
12.   BEGIN
13.     IF PROVIDES_SQL(AR_CONTROLS[X])THEN EXIT(AR_CONTROLS[X]);
14.     X:=X+DELTA_AIRLINES; // ADJUST X BASED ON DELTA_AIRLINES DIRECTION
15.   END;
16.   ERROR_HALT('FUNCTION TCASCADING_FLOWPANEL.GET_THE_CONTROL');
17. END;    

[TRon]

Bravo Josh.

With a bit of abstraction it might even end up something like

Code: Pascal  [Select][+][-]

1. var
2.   Flight  : TIterate;
3.   Airport : sizeint;
4. begin
5.   if isNormalRoute
6.    // The following can be abstracted as well, f.e. by adding a direction (delta) parameter
7.    then Flight := TIterate.Create(low(Destinations), high(Destinations))
8.   else Flight := TIterate.Create(high(Destinations), low(Destinations));
9.  
10.   while Flight.Next(Airport) do
11.     if NeedToLand(Destinations[Airport])
12.       then exit(Destinations[Airport]);
13. end;
14.  

Not the fastest solution to use a self made iterator but certainly pleasant to read/understand. The omitted iterator record uses the same mechanism as per Josh's example.