"I have many functions. so instead of use the variables as function arguments to transfer values between functions. I set many variables as global variables."
I don't see why having many functions requires using global variables. You would likely be much better off passing the data as proper input/output arguments, rather than wasting time using slow and buggy globals.
"on the top of main function, I did use the following to clear the space. clc clear all; close all; "
Why? When a function is first called its workspace is empty (apart from any input arguments), so calling clear all serves no purpose whatsoever and is in fact counter-productive: it will clear all loaded functions and classes, which will then just have to be loaded again when the code is run, thus slowing down your code. It is highly unlikely that clc or close all serve any purpose actually related to the functionality of your functions, so what is the point of calling them? Of course apart from the usual reason why people call those everywhere:
On to your your questions:
"1. How to reduce memory space with many global variables? How to only assign space when it is used?"
Don't use globals.
Pass data properly between workspaces and trust the MATLAB engine to deal with memory and garbage collection.
"2. I have few toolboxes, like statistics, which are only linked to 2014b version. Is it possible to use these toolbox for 2017 version?"
Are these MATLAB toolboxes or third-party toolboxes?
