Generating uniform random numbers in Lua

I am programming a Markov chain in Lua, and this requires me to uniformly generate random numbers. Here is a simplified example to illustrate my question:

pickRandomArrayItem = function(x)
    local r = math.random(1,10)
    print(r)
    return x[r]
end

exampleArray = {"a","b","c","d","e","f","g","h","i","j"}

print(pickRandomArrayItem(exampleArray))

My issue is that, when I re-run this program multiple times, the exact same random number is generated resulting in the example function selecting the exact same array element. However, if I include many calls to the example function within the single program by repeating the print line at the end, I get suitable random results.

This is not my intention as a proper Markov pseudo-random text generator should be able to run the same program with the same inputs multiple times and output different pseudo-random text every time. I have tried resetting the seed using math.randomseed(os.time()) and this makes it so the random number distribution is no longer uniform. My goal is to be able to re-run the above program and receive a randomly selected array element every time.

Solution

You need to run math.randomseed() once before using math.random(), like this:

math.randomseed(os.time())

From your comment that you saw the first number is still the same. This is caused by the implementation of random generator in some platforms.

The solution is to pop some random numbers before using them for real:

math.randomseed(os.time())
math.random(); math.random(); math.random()

Note that the standard C library random() is usually not so uniformly random, a better solution is to use a better random generator if your platform provides one.

Reference: Lua Math Library