5.7.6 Initialization of Variables
  BASIC sets variables to zero or null values at the start of
  program execution. Variables initialized include the following:

  .
        Numeric variables and array elements (except those in
        MAP or COMMON statements).
  .
        String variables and array elements (except those in MAP
        or COMMON statements).
  .
        Variables in subprograms. Subprogram variables (except
        those in MAP or COMMON statements) are initialized to
        zero or the null string each time the subprogram is called.
  .
        Arrays created with an executable DIMENSION state-
        ment. The array is reinitialized each time the array is
        redimensioned.