>I already tried like that. The problem is when the parameter is not an array, like a character type, then when it come to the command myarray[1] <> "U" it gave me an error, myarray.prg doest not exist.

If it's not an array, you should return .f. from your function. You can also display a message to the user, say,

=messagebox('Invalid parameter passed. Myarray should be an array',48,'Invalid parameter')
return .f.
endif