Why does it takes two calls to rng(seed) to see that the seed was updated?

Why does it takes two calls to rng(seed) to see that the seed was updated? Is it a bug?

s = rng(100)
Type: 'twister'
     Seed: 0
    State: [625x1 uint32]

s = rng(100)
    Type: 'twister'
     Seed: 100
    State: [625x1 uint32]

s = rng(101)
    Type: 'twister'
     Seed: 100
    State: [625x1 uint32]

s = rng(101)
    Type: 'twister'
     Seed: 101
    State: [625x1 uint32]

Answer 1

[Titus Edelhofer]

Hi,

no, there is a misunderstanding: the call s=rng(100) does two things, namely, setting the seed to 100 and returning the current state before setting the seed. The reason for this is the following "workflow":

% save the current state, and set to seed=100
s = rng(100);
% do some random numbers
x = rand(100, 1);
% restore the state as it was before
rng(s);

You might use RandStream, if you want to create a random number stream object to use.

Titus

Answer 2

[Amit]

Got it

Thx

Answer 3

[Chris MacMinn]

What is the motivation for this confusing behavior? It seems quite non-standard... Do any other MATLAB functions behave like this? Surely the command

state = rng(seed);

should always return the new state, not the previous one ?!?

The reason cited above by Titus is a bit silly. The following example achieves the same result, is much less confusing, and is only one line longer:

 % save the current state
 s = rng();
 % set to seed=100
 rng(100);
 % do some random numbers
 x = rand(100, 1);
 % restore the state as it was before
 rng(s);