Help with algorithm

It's very simple. Using the binary search algorithm to first find the first number, then second.
The difference of their indices will be sought. In zavimosti on the exact definition may need to clean the unit.
I suspect that the language Pascal. Then the code search will be similar to that.
\r
{foo — the desired quantity. a and b — the border of the search}
procedure Find(foo: integer; a: integer; b: integer);
var c: integer;
begin
if (b-a) > 1 then
begin
c:= (a+b) div 2;
find(foo,a,c);
find(foo,c,b);
end else
begin
if (array_[a] = foo) then Result := a;
if (array_[b] = foo) then Result := b;
end;
end;

If the language allows (C, for example), one can speed up the finding of the second number after the first is found by setting search shorter.
\r
Although modern smart processors, this will not necessarily be faster.