Random order with constraints

How can i generate random orders of 2 types of stimuli (1:n=200 and 2:n=20) with a constraint : there can't be 2 stimuli of type 2 in a row ?

[Stephan]

Cindies "answer" moved here:

What i want is a random list like:

stim1
stim1
stim2
stim1
stim1
stilm1
stim1
stim2......

With 200 stim 1 and 20 stim 2 (but never 2 stim 2 in a row in the list)

OR random positions for the 20 stim2

so like :

5, 17, 77, 99, 201, 174... 

so 20 random numbers between 1 and 220 but i want to make sure there are never numbers that follow each other (ex: 77 and 78) and that the number doesn't appear twice

Is that clear ?

Answer 1

[the cyclist]

If you have the Statistics and Machine Learning Toolbox, you could do it like this:

x = sort(randsample(220,20,false));
while min(diff(x)) <= 1
    x = sort(randsample(220,20,false));
end

This is a little ugly and fairly inefficient. The algorithm generates the 20 numbers, then checks them for the constraint, and repeats the process as necessary.

I did a bit of testing, and only about 15% of the random sequences will obey the constraint, so the while loop will typically run several times. But maybe that is OK.

I'm guessing there is a more clever way that is more efficient, but it did not come to me immediately.

You can do the same thing with base MATLAB like this:

x = randperm(220);
x = sort(x(1:20));
while min(diff(x)) <= 1
    x = randperm(220);
    x = sort(x(1:20));
end