Études for Erlang

Étude 6-5: Random Numbers; Generating Lists of Lists

How do you think I got the numbers for the teeth in the preceding étude? Do you really think I made up and typed all 180 of them? No, of course not. Instead, I wrote an Erlang program to create the list of lists for me, and that’s what you’ll do in this étude.

In order to create the data for the teeth, I had to generate random numbers with Erlang’s random module. Try generating a random number uniformly distributed between 0 and 1.0 by typing this in erl:

1> random:uniform().
0.4435846174457203

Now, exit erl, restart, and type the same command again. You’ll get the same number. In order to ensure that you get different sets of random numbers, you have to seed the random number generator with a three-tuple. The easiest way to get a different seed every time you run the program is to use the now/0 built-in function, which returns a different three-tuple every time you call it.

1> now().
{1356,887000,432535}
2> now().
{1356,887002,15527}
3> now().
{1356,887003,831752}

Exit erl, restart, it and try these commands. Do this a couple of times to convince yourself that you really get different random numbers. Don’t worry about the undefined; that’s just Erlang’s way of telling you that the random number generator wasn’t seeded before.

1> random:seed(now()).
undefined
2> random:uniform().
0.27846009966109264

If you want to generate a random integer between 1 and N, use uniform/1; thus random:uniform(10) will generate a random integer from 1 to ...