I couldn't find the error in my binary search function - binary-search

Question

I don't know why this function I made in pascal only gives the index of the number I'm searching for (n) and doesn't give -1 when it doesnt find it..
(i checked theres no problem with other functions the only problem is that is doesnt print the message 'num is not here' when it doesnt exist) i would also appreciate it if someone points out where my code could have been more efficient.
`
Function binary_search(L : Array Of Integer; n : Integer) : Integer;
Var
i, p, middle, first, last : Integer;
Begin
first := 0;
binary_search := -1;
last := Sizeof(L) Div Sizeof(L[0]);
While (first <= last) Do
Begin
middle := (first + last) Div 2;
If (middle = n) Then
Begin
binary_search := middle;
break;
End;
If (middle < n) Then first := middle +1;
If (middle > n) Then last := middle -1;
End;
End;
Begin
Write('num of elements in array : ');
read(m);
fillup(arr, m);
For i :=0 To m-1 Do
Begin
permutarr[i] := arr[i];
End;
Write('the num youre looking for : ');
read(A);
sort(arr, 1, m);
If (binary_search(arr, A)= -1) Then Writeln('the number isnt here') //this doesnt work
Else
Begin
For i:=0 To m-1 Do
Begin
If (permutarr[i] = binary_search(arr, A)) Then
Begin
Writeln('index : ', i);
break;
End;
End;
End;
End.
`

Answer 1

I am not familiar with your dialect of Pascal (it's been 15 years since I did any serious programming), but I'm quite sure that the chief problem is that
Your code is not looking at the entries of the array L at all!
All the comparisons in the function binary_search only involve the indices, middle, first, last. The first thing I would try is to edit all the comparisons with middle to use L[middle] instead (on lines 11, 16 and 17). Then your code will, at least, actually be looking at the array :-)
Other things:
I'm not 100 per cent sure that your code handles arrays of all lengths correctly (can't test right now). It might happen that first and last never meet. I'm probably wrong about this, because then your code would get stuck in an endless loop.
When I was coding I took care never to assign a value to the function_name, here binary_search prematurely. In other words, I would not be surprised to learn that your function always returns $-1$, because I fully expect the execution of a function to end the instant anything is assigned to it, so here at the line binary_search:=-1;. As I said, my dialect was different, and the recollection is "dated" at best.
Anyway, the problem in bold fits your description of the problematic behavior. You can test my theory by giving A a value that exceeds the length of the array. Then it should be unable to find it with your current code.