Comparison of two arrays in Lazarus

Question

I have a problem with Pascal, especially Lazarus.

First of all, I created two random arrays of integer:

    procedure TForm1.b_arraycreate1Click(Sender: TObject);
begin
   randomize;
   for i := 1 to 5 do
      arr1[i] := random(10);
end;

And

procedure TForm1.b_arraycreate2Click(Sender: TObject);
begin
   randomize;
   for j := 1 to 5 do
       arr2[j] := random(10);
end; 

I know, I could put it in one procedure as well but doesn't matter now.

I want to compare these two. I wrote the following code:

procedure TForm1.b_comparisonClick(Sender: TObject);
var v:boolean;
begin

for i := 1 to 5 do begin
       for j := 1 to 5 do begin
           if   arr1[i] = arr2[j]
                then
                    begin
                         v:=true;
                    end
                else
                    begin
                         v:=false;
                    end;
       end;
end;

if v = true
     then
         begin
              ShowMessage('Yes, there is a similarity! You can find number ' +IntToStr(arr1[i])+ ' in array 1, position ' +IntToStr(i)+ ' and also in array 2, position ' +IntToStr(j)+ '.');
         end
     else
     begin
          ShowMessage('No similarities... Generate new ones!');
    end
end;  

In my own words: I want to push a button and then there should be a message window with the information if there is one number (for example 7) which exists in array 1 and array 2. If yes, it should also write the position (index) of this number.

Unfortunately, this program doesn't work and I don't know why. It always shows "No similarities" (and don't worry about the creation of the arrays. I also have a label where I can test the content of the arrays every time).

Is there a (silly) mistake in my code here?

Answer 1

As explained already by MartynA in his comment, your algorithm is wrong. Your words are:

if there is one number which exists in array 1 and array 2

To see if it is so, you must scan all array1 and, for each number, see if it exists somewhere in array2.

So yes, you need two cycles, one nested in the other. As soon as you find a correspondence, you must stop. Or, if you want more results (find multiple duplicates), show a message instead of stopping - and go ahead. Third possibility (more complicated): when found, store the couple of indexes (without overwrite old results...) and go ahead. I will only show the first option:

procedure TForm1.b_comparisonClick(Sender: TObject);
var
  i,j: integer;
  v:   boolean;

begin
  v := false;
  for i := 1 to 5 do begin
    for j := 1 to 5 do begin
      if arr1[i] = arr2[j] then begin
        v := true;
        break
      end
    end  // inner, j
  end;  // outer, i

  if v = true
    then ShowMessage(.....)
    else ShowMessage('No similarities...');
end;  // proc comparison

I tried to respect your code a bit, there are a few possible "shortcuts"; for example, if v is a boolean variable, it is better to write if v then instead of if v=true then, and some others, like

v := arr1[i]=arr[j];

...or... the outer loop does not need begin+end.

******* BEWARE (see comment below about break) To stop/exit from two nested cycle is not so simple... perhaps a goto... the code above works, but the break does little work.

******* second update, as described by comment below. IT DOES NOT WORK, because if the break does not exit BOTH loops, the outer index gets modified. The correct cycle using TWO breaks is as follows:

  for i := 1 to 5 do begin
    for j := 1 to 5 do begin
      if arr1[i] = arr2[j] then begin
        v := true;
        break
      end
    end;  // inner, j
    if v then break
  end;  // outer, i

Sorry for the mistakes... :-)

I would prefer a GOTO to exit both loops: it is faster, single instruction, and more clear ("goto found" instead of a generic break). But GOTOs are not very popular... so I've been afraid!