Transform data / repeat based on previous row

I was wondering if there was a way to update data in a column such that a null value in a cell would take on the preceeding value until a non null value was reached at which point that value would be used to replce the next null value.  The data I have comes from a speadsheet, only has a maximum of 50 rows and its format can't be changed...  I was wondering if a custom function would work?  Sadly I cant use Python. Thanks.