Найти произведение положительных элементов главной диагонали матрицы - QBasic

DECLARE FUNCTION Dia! (a%(), n!)
CLS
RANDOMIZE TIMER
 
CONST n = 6
DIM SHARED p
DIM a(1 TO n, 1 TO n) AS INTEGER
p = 1
 
FOR i = 1 TO n
FOR j = 1 TO n
   a(i, j) = 40 * RND - 20
   PRINT USING " ###"; a(i, j);
NEXT j: PRINT
NEXT i: PRINT
 
PRINT Dia(a(), n)
END
 
FUNCTION Dia (a() AS INTEGER, n)
   IF n = 0 THEN EXIT FUNCTION
 
   IF a(n, n) > 0 THEN p = p * a(n, n)
   D = Dia(a(), n - 1)
   Dia = p
END FUNCTION