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rng(s) takes previous seed instead of current one

Asked by YT
on 10 Dec 2018
Latest activity Commented on by YT
on 10 Dec 2018
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?

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Products


Release

R2018b

2 Answers

Answer by Walter Roberson
on 10 Dec 2018
 Accepted 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)

  1 Comment

Ah didn't read the documentation good enough then. Thanks.
I fixed it by doing the following instead (for future readers)
if exist('seeder','var') == 1
rng(seeder);
else
rng('shuffle');
end
s = rng;
disp(['Seed = ' num2str(s.Seed)]);

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Answer by Steven Lord
on 10 Dec 2018

This is the expected and documented behavior of rng. From the documentation: "sprev = rng(...) returns the previous settings of the random number generator used by rand, randi, and randn before changing the settings."
This allows you to "remember" the previous state of the generator and change it to a different state in one call then (when you're done generating random numbers) restore the remembered state with a second rng call.
What exactly are you trying to do with that code? If you describe in words what you want to happen, we may be able to help you make that happen.

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