Infinite Loop with Label
lazarus / 61-Beginners

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OlivenHandcreme · 2022-01-27 16:35

Hey Lazarus-Free Pascal forum,

im a totally beginner in Pascal/Lazarus IDE.

I got a task to say which output this programm will have in the first
picture.
My first guess was, that it had to be an infinite loop.
But the outcome is not infite, in fact it just works like the Label is
not there?

After trying out, it somehow works to be infinite if i put a writeln
there, as you can see in the second picture. Funny thing is, the
writeln doesnt seem to be in the console,
rather just the infinite loop now??

Why does the infinite loop works, if i put a writeln there?

Thanks in advance!

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Thaddy · 2022-01-27 16:57

Read up on modulo 
What you need is a comparison to <= or equal or below the limt.
Instead of = the limit. That may never be true and hence you enter an
infinite loop. Exactness can bite.....

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wp · 2022-01-27 17:34

Your indentation suggests that several begin-end pairs are missing.

First picture:
The "for" loop executes "if ((i mod 3) = 1) or (i = 9) then
writeln(...)". The next instruction is NOT part of the loop although
you indented it (Pascal is not Python!). (You must use "begin"/"end"
pairs to enclose multiple instructions inside the loop - see below).

Therefore, your code executes only the WriteLn instructions. Then the
loop is finished. It is not clear which value i has after the loop,
maybe 15 or 16, certainly not 2. Therefore the condition "if i = 2" is
false and the program does not jump back to L1, there is no infinite
loop.

If you want to have the "goto" inside the loop you must add a begin
after the "do" of the "for" loop, and an "end" after the "goto". Like
this:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
Versuche;label   L1;var  i: Integer;begin  L1:  for i := 1 to 15 do 
begin    if ((i mod 3) = 1) or (i = 9) then       writeln('i was: ',
i);    if i = 2 then      goto L1;  end;end; 
In this case the program jumps out of the loop back to the label L1
each time when the counter i reaches the value 2. A very bad
programming technique, by the way. Shame on your teacher for even
showing you something like this!

Second picture:
Here again the program is behaving not as suggested by the indentation
due to the missing begin/end pairs. Like before, it iterates through
15 i values in the "for" loop and prints out the "i was.." lines.
After the loop we have the instruction "if i = 2 then goto L1" - but
since i is not 2 after the loop the program does not jump the L1. But
the next line has an unconditional goto which brings you back to the
start and into the endless loop. I want to show you what your original
code looks like with standard indentation:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
Versuche;Label   L1;var  i: Integer;beginL1:  for i := 1 to 15 do   
if ((i mod 3) = 1) or (i = 9) then      writeln('i was: ', i);  if i =
2 then    writeLn('again');  goto L1;end;
When I interpret your indentation correctly you probably mean the code
to be like this:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
Versuche;Label  L1:var  i: Integer;beginL1:  for i := 1 to 15 do 
begin    if ((i mod 3) = 1) or (i = 9) then      writeLn('i was: ',
i);    if i = 2 then begin      Writeln('again');      goto L1;   
end;  end;end;

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Zvoni · 2022-01-28 07:22

The first picture without Label and Goto (untested):
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i program
Project1;Var  i:Integer;Begin  i:=1;  Repeat    If ((i mod 3=1) or
(i=9)) Then       Writeln('i was: ',i);    If i=2 Then i:=1 Else
Inc(i);  Until i>15;end.

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OlivenHandcreme · 2022-01-28 19:22

First of all, thank you all for you very detailed help!

This helped me really alot, and more that my professor could possibly
do haha.
And for wp, i wish i could give you an award  you really took sometime
to explain it to me.

Intresting to see that someone is criticizing my professor just by
seeing his code, makes me really ask myself if  i learn something
right.
One point to defend him, i guess his goal was to confuse us, and see
if we understood whats really happening there. But then again, i dont
know if this is a good way to do.
Wish you all a nice weekend!

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Josh · 2022-01-28 21:06

Hi

I think your tutor was trying to demonstrate to you the flaws of badly
indented coding.
As you can assume a program flow by how its laid out, badly
indentented formatted code is a nightmare to follow and debug, I have
spent many hours just re-formatting such code even before I attempt to
debug it.

If the code you have was reformatted the flow does become more
obvious. Also adding in begin end blocks in the if then and for loops
again helps the flow ie,
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i L1:  for
i:= 1 to 15 do  begin    if ((i mod 3) = 1) or (i =9 ) then    begin 
   writeln('i was: ',i);    end;  end; // end of loop  if i = 2 then 
begin    goto L1;  end; L1:  for i:= 1 to 15 do  begin    if ((i mod
3) = 1) or (i =9 ) then    begin      writeln('i was: ',i);    end; 
end; // end of loop  if i = 2 then  begin    writeln('again');  end; 
goto L1;     
Note the value of a for loop variable cannot be guaranteed to be what
you think it is after the loop has finished.

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MarkMLl · 2022-01-28 21:43

Quote from: OlivenHandcreme on January 28, 2022, 08:22:28 pmIntresting
to see that someone is criticizing my professor just by seeing his
code, makes me really ask myself if  i learn something right.
One point to defend him, i guess his goal was to confuse us, and see
if we understood whats really happening there. But then again, i dont
know if this is a good way to do.

If you think /you've/ got problems, or for that matter if you think
/he's/ got problems, I suggest that the section on "Whitespace" in
https://hackaday.com/2022/01/26/strange-computer-languages-a-hackers-
field-guide/ might interest (or at least amuse).

To put a slightly different spin on things, it's worth emphasising
that in Pascal if...then controls a single adjacent statement where
that statement can be a begin...end block.

The subtlety there is that begin...end can itself be a container of
multiple statements.

MarkMLl