MATLAB bug when adding new function while on breakpoint?

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My program takes a bit of time to load in variables and files, so I prefer to set a breakpoint and test run functions by pressing F9 (run line of code) while highlighting the function in the script.
Here's what happened:
Function A has the breakpoint (program paused execution)
Function B has the code I want to change/test by plotting data and changing it as needed
Function C is the new function (`edit funcC`) while breakpoint paused in function A
Function B calls for Function C to run
Function B & C successfully saved to disk (no changes made to Function A)
Function B called (highlighted and pressed F9) while on breakpoint in Function A
I subsequently got an error stating that Function C does not exist.
My Solution:
Terminate the process for Function A
Move current directory up one folder
Move back into directory with functions and rerun program
Worked fine after that.
It's a minor inconvenience with an easy solution, but could this be a bug and can it be fixed soon to improve MATLAB? I am using R2017a commercial edition.

Accepted Answer

Jan on 5 Jun 2018
Edited: Jan on 5 Jun 2018
The debugger is thought for (guess what?) debugging. It is not a powerful tool to control the program flow. Maybe the observed behavior is a but, or it is intended. Modifying code during the the program runs, can have extremely strange side-effects, e.g. if a function calls itself recursively over a certain number of other functions. There is no chance to cope with such ambiguities reliably.
In addition, enabling the debugger (e.g. by setting breakpoints) disables the JIT acceleration. This is required, because the JIT can reorder the sequence of commands to improve the speed. But then stepping through the code line by line cannot work anymore. In consequence some code can be 100 times slower compared to a disabled debugger. Therefore it is a bad idea to use the debugger to modify the code while the program is running.
There are smarter and more efficient techniques, e.g. load the data into a persistent variable:
function Data = Storage
persistent Data_P
if isempty(Data_P)
Data_P = load(...) % The long part to initialize the data
Data = Data_P;
Then you can call this function to obtain the data, but they are kept in the memory and the following calls will run much faster. This is much cleaner and more reliable than juggling with the debugger.
So my advice is not to use the debugger to control the program flow. Use code and persistently stored data to do this.

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