坑瀑布

 placeholderCopy# no error, even the subscript is out of range.
julia> sub2ind((3,3), 3, 4)
12

通过比较索引，无法确定下标是否在数组范围内：

 placeholderCopyjulia> sub2ind((3,3), -1, 2)
2

julia> 0 < sub2ind((3,3), -1, 2) <= 9
true