CODEWITHSUNDEEP
matlabrandomrandom-seed
jeff
jeff

Reputation: 13653

Strange rand() behaviour in MATLAB

rand() does not seem to generate really random numbers. I have a simple program that returns a 6-digit number by calling :

for i=1:6
 r=rand(1,1)
end

so I ran this 4-5 times yesterday. And saved the output. Today I opened MATLAB again and called the same function again 4-5 times. The same numbers were returned.

Why is this happening?

Should I provide a random seed or any other fix?

Thanks for any help!

Upvotes: 1

Views: 840

Answers (2)

TroyHaskin
TroyHaskin

Reputation: 8401

To expand on @alexforrence's answer, rand and other related functions produce pseudo-random numbers (PRNs) that require an initial value to begin production. These numbers are not truly random since, following the initial seed, the numbers are produced via an algorithm, which is deterministic by its very nature.

However, being pseudo-random isn't necessarily a bad thing since models that use PRNs (e.g., Monte Carlo Methods) can generate portable, repeatable results across many users and platforms. Additionally, the seed can be changed to create sets of random numbers and results that are statistically independent but also produce repeatable results. For many scientific applications, this is very important. Also, "true" random numbers (next paragraph) have a tendency to "clump" together and not evenly spread over their range for a small sampling of the space, which will degrade the performance of some methods that rely on stochastic processes.

There are methods to create "true-er" random numbers by the introduction of randomness from various analogue sources (e.g., hardware noise). These types of numbers are extremely important for cryptographically secure PRNs, where non-repeatability is an important feature (in contrast to the scientific usage). True random number generators require special hardware that leverages natural noise (e.g., quantum effects). Although, it is important to remember that the total number of random numbers that can be generated and computationally used is limited by the precision of the numbers being used.

You can re-seed MATLAB with a pseudo-random seed using the rng function. However, "reseeding the generator too frequently within a session is not a good idea because the statistical properties of your random numbers can be adversely affected" [src].

Upvotes: 5

alexforrence
alexforrence

Reputation: 2744

From the Mathworks documentation, you can use

rng('shuffle');

before calling rand to set a "random" seed (based on the current time). Setting the seed manually (either by not changing the seed at startup, by resetting using rng('default'), or setting the seed manually by rng(number)) allows you to exactly repeat previous behavior.

Upvotes: 4

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