Source code in the xpl0 programming language

    \Compare lists (rows) of integers.
    \Returns TRUE if there is an element in A that is < the corresponding
    \ element in B and all previous elements are equal, FALSE otherwise.
    \The bounds of A and B should ALB :: AUB and BLB :: BUB.
    function ILT ( A, ALB, AUB, B, BLB, BUB );
    integer  A, ALB, AUB, B, BLB, BUB;
    integer  APos, BPos, Equal;
    begin
        APos := ALB;
        BPos := BLB;
        Equal := true;
        while APos <= AUB and BPos <= BUB and Equal do begin
            Equal := A( APos ) = B( BPos );
            if Equal then begin
                APos := APos + 1;
                BPos := BPos + 1
           end \if_Equal
        end; \while_more_elements_and_Equal
        if not Equal
        then \there is an element in A and B that is not Equal
             return A( APos ) < B( BPos )
        else \all elements are Equal or one list is shorter
             \A is < B if A has fewer elements
             return APos > AUB and BPos <= BUB
    end; \ILT

    \Tests A < B has the expected result
    procedure Test ( AName, A, ALB, AUB, BName, B, BLB, BUB, Expected );
    integer   AName, A, ALB, AUB, BName, B, BLB, BUB, Expected;
    integer   IsLt;
    begin
        IsLt := ILT( A, ALB, AUB, B, BLB, BUB );
        Text(0, AName);
        Text(0, if IsLt then " <  " else " >= ");
        Text(0, BName);
        Text(0, if IsLt = Expected then " " else ", NOT as expected");
        CrLf(0);
    end; \test

    integer List1, List2, List3, List4, List5, List6, List7, List8;
begin 
    \test cases as in the BBC basic sample
    List1 := [0, 1, 2, 1, 5, 2];
    List2 := [0, 1, 2, 1, 5, 2, 2];
    List3 := [0, 1, 2, 3, 4, 5];
    List4 := [0, 1, 2, 3, 4, 5];
    Test( "List1", List1, 1, 5, "List2", List2, 1, 6, true  );
    Test( "List2", List2, 1, 6, "List3", List3, 1, 5, true  );
    Test( "List3", List3, 1, 5, "List4", List4, 1, 5, false );
    \additional test cases
    List5 := [0, 9, 0, 2, 1, 0];
    List6 := [0, 4, 0, 7, 7];
    List7 := [0, 4, 0, 7];
    List8 := [0, 0];
    Test( "List5", List5, 1, 5, "List6", List6, 1, 4, false );
    Test( "List6", List6, 1, 4, "List7", List7, 1, 3, false );
    Test( "List7", List7, 1, 3, "List8", List8, 1, 0, false );
    Test( "List8", List8, 1, 0, "List7", List7, 1, 3, true  )
end