~exist(xxx) in IF condition not working

I have observed a, for me strange behaviour, of MATLAB.

I setup a new workstation with a new MATLAB 2018b installation. I try to use a MATLAB script (which I use very often on my laptop without issues - same MATLAB version installed) which is crashing in an IF condition.

Condition is:

if ~exist('data')
    data.data = [];
    data.time = [];
end

On my laptop this code is checking the workspace for 'data'. If 'data' is existing it is doing nothing, if 'data' is not there 'data' will created with empty elements.

On the workstation with the new MATLAB installation this code is doing nothing if 'data' is not existing?!?!? It seems that the if condition is ignored.

If I use

not(exist('data'))

ans = logical
   0

or

~exist('data','var')

ans = logical
   1

instead of

~exist('data')

Code is working.

How could it be that same MATLAB version is reacting different on different machines? Input data (measurement file) is always the same.

I have also checked Matlab 2021b on my laptop...same behaviour...if condition is not working if 'data' is not existing...

Regards,

Timo

[Stephen23]

"~exist(xxx) in IF condition not working"

IF, NOT, and EXIST are working.

"How could it be that same MATLAB version is reacting different on different machines?"

Read the EXIST documentation: does it only check for variables? (hint: it also checks for Mfiles, MEX-functions, simulink functions, classes P-code, etc). Different comupters with different versions and different toolboxes and different code and different Mfiles and different search paths and different folder structures.... that EXIST returns different results does not seem at all surprising.

"On my laptop this code is checking the workspace for 'data'"

No, it is not: it is checking for all of the different things that are listed in the documentation (and a variable is just one of those things).

This is a good example of how to write fragile and inefficient code: robust code does not change what variables are in the workspace.

[Bruno Luong]

Something is NOT right on your post

not(exist('data'))

is stricly equivalent to

~exist('data')

Both should NOT work on your workstation.

Check value returned by exist before negate it

exist('data')

What is the value?

[Timo Breker]

First I have to correct myself

not(exist('data'))

Is creating the same issue, sorry for that.

I have tested now the same script with the same measurement files on my laptop with MATLAB 2018b and 2021b.

2018b is running smooth through the script. 2021b is crashing directly after the IF condition because the used function has no output (data would be the output of the function).

As I mentioned earlier the script is analysis measurment data and in the case a signal is not existing, 'data' will be created with empty elements.

In the situation where the signal is missing, MATLAB 2018b is creating a logical ZERO for

exist('data')

which is absolute clear to me. MATLAB 2021b is giving back the value 7 for the same code at the same position?!

Workspace has only 4 elements, a struct, a variable of thy char, and 2 variable of type double and none of them are called 'data'... I have no idea why this happens.

EDIT: I should start reading the documentation...7 means that there is a folder named 'data' existing.

what 'data'

is giving back a lot of results (for MATLAB 2021b - most(all) of them in the Matlab/example structure ).

If I stop the code for MATLAB 2018b at the same position

what 'data'

is giving back no results.

Many thanks for helping/understanding the problem. Code of the script/function is not robust at this position....

[Stephen23]

"As I mentioned earlier the script is analysis measurment data and in the case a signal is not existing, 'data' will be created with empty elementss.... I have no idea why this happens."

Robust code does not mix up the paradigms of (meta-)data and code.

Robust code would not use scripts and uncontrolled magical variables popping into existence, but instead encapsulate functionality within functions or classes and pass data with specific, documented interfaces.

Robust code is more efficient:

https://www.mathworks.com/help/matlab/matlab_prog/techniques-for-improving-performance.html#buwj1nn

Answer 1

[Bruno Luong]

You have a FILE or FOLDER named "data" on some of your computers, not on other

exists chech existing for file and folder as well.

Answer 2

[colordepth]

exist documentation: link

According to the documentation, exist can have 8 return values. It not only checks for variables in workspaces, but it can also check for other things like existing files/folders within subfolders of your current path. Even if you do not have a 'data' file/folder visible in your current folder, if its in one of the subfolders, exist() will return a non-zero value.

It may likely be a file or a folder thats being detected in your current path thats different on your laptop and your workstation.

Try using 'what' to see the folders causing this result:

what 'data'

For files, 'which' can help show files called 'data' or 'data.*' causing this result:

which 'data' -all

what documentation: link

which documentation: link