MATLAB: Rng(s) takes previous seed instead of current one

MATLABrngseeduserinput

I've encountered this problem when using functions in combination with user input and rng(s). To clarify my problem, I've got the following 2 scripts:
%main.m
user_seed = "1739995225";
disp(['Main input seed = ' char(user_seed)]);
checkSeed(user_seed)
and
%checkSeed.m
function checkSeed(user_seed)
    %clear s seeder;
    seeder = double(user_seed);
    %check if seed isset
    if exist('seeder','var') == 1
        s = rng(seeder);
        disp(['Function seed = ' num2str(s.Seed)]);
    else
        s = rng('shuffle');
        disp(['random seed = ' num2str(s.Seed)]);
    end
end
The issue is that everytime the previous user seed is being used instead of the current one. For example
%first run
user_seed = "1739995225";
%Main input seed = 1739995225
%Function seed = 1739995225

%second run
user_seed = "1739995226";
%Main input seed = 1739995226
%Function seed = 1739995225
%third run
user_seed = "1739995227";
%Main input seed = 1739995227
%Function seed = 1739995226
I tried putting the checkSeed.m contents in the main script, but that gave me the same result. Also tried adding clear s seeder; to the function but that didnt help either. So any ideas?

Best Answer

This is not a bug. This is how rng is defined. The documentation says:
"sprev = rng(...) returns the previous settings of the random number generator used by rand, randi, and randn before changing the settings."
The reason for this is that it permits you to code
prev = rng(new)
...
rng(prev)
instead of having to code
prev = rng
rng(new)
...
rng(prev)
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