Simple algorithm for generating random numbers with bigger/smaller probability

Question

I'm currently working on a game, with scrollable screen and I need to find a simple algorithm for placing obstacles in the game. I have a gameSpeed, that is increased in time (from 1 to 12, increased by 0.005 each 1/60s) and a range of available positions between 200 and 600 (ints). I'd like to achieve a bigger probability of receiving smaller number when the speed is bigger, but it's my 14th hour straight coding and I cannot come up with anything usable and not overcomplicated. I'd like to minimize Math and random functions so that the rendering loop won't take too long. Any help appreciated !

Answer 1

You could square or square-root the random number to shift the density in one direction. Math.random()*Math.random() will have a higher probability to produce smaller numbers (near 0) than higher ones (near 1).

Your formula could be like

var position = Math.pow(Math.random(), gameSpeed / 3) * 400 + 200;

Answer 2

The simplest answer I can think of is to create an array having more lower numbers compared to higher ones, for example, for producing random between [1,5] (both inclusive). So your array may look like [1,1,1,1,1,2,2,2,2,3,3,3,4,4,5]

And when you randomly pick an element from that array you will have a higher chance of picking up low number compared to high one.