Using rng: Why does changing the seed still give the same random sequence?

Dear reader

When I run the following simple script the output displayed in the Matlab prompt shows two times the same random sequence while the seed has clearly changed. I'm wondering how this is possible? I know that by replacing rng(state) with rng(15) the seed can also be changed. (I tried it and the sequences are different as expected). However I would like to know why changing the seed directly in the state and then writing the state back, doesn't have the same effect.

Kind regards

Lex

script:

    close all
    clear 
    clc

    rng('default')
    state=rng; rng()
    randn(1,5)
    state.Seed=15;
    rng(state); rng()
    randn(1,5)

Output:

    ans = 
         Type: 'twister'
         Seed: 0
        State: [625x1 uint32]
    ans =
        0.5377    1.8339   -2.2588    0.8622    0.3188
    ans = 
         Type: 'twister'
         Seed: 15
        State: [625x1 uint32]
    ans =
        0.5377    1.8339   -2.2588    0.8622    0.3188

Matlab version: R2011b (7.13.0.564) 64-bit (win64)

Answer 1

[Walter Roberson]

The obvious postulate would be that the random number generator uses the State without checking whether the Seed field has changed. Changing the Seed field is not a defined mechanism for changing the state (or State) of the random number generator. Using the defined mechanisms for changing the seed changes the State.

Modern random number generators use a lot more than 32 or 64 bits of state information.

[Lex]

Your right, the seed only determines the starting state of the RandStream. Later only the state is used to determine the next random number. By changing the seed and not the state the same sequence will come up. It is only when the RandStream gets constructed or reset that the seed influences the state.

Thanks for clarifying.