1.When using user_defined_function, do I have to put an 'end' at the end of code?
In a function file, either all of the functions in the file must end with an end or none of them can end with an end. Historically, prior to the introduction of nested functions (IIRC) functions didn't need to end with an end (and that behavior is still allowed) but if you only ended some of the functions in a file with an end figuring out where one ended and another began could get complicated. So it's all or none.
In a script file that contains a local function or in a class file, functions must end with an end.
2. why does not work if adding 'format compact'?
If you have a function inside a function file, the main function should have the same name as the file. If they differ, MATLAB will know the function by its file name (and will show a Code Analyzer warning for the function definition line.)
In a class file, if you have a function with the same name as the class it's the class's constructor. But classes don't need to define constructors if the "default" constructor is sufficient.
If you have a function inside a script file, which is the case for your example, then as stated in the first Note on this documentation page none of the functions in the script are allowed to have the same name as the script file. If you had a script named myscript.m and a function inside myscript.m named myscript, then this line of code inside the script would be ambiguous. Is it trying to run the script or call the local function with no input arguments? 
