Question

I would like to generate a random floating point number between 2 values. What is the best way to do this in C#?

Answer 1

The only thing I’d add to Eric‘s response is an explanation; I feel that knowledge of why code works is better than knowing what code works.

The explanation is this: let’s say you want a number between 2.5 and 4.5. The range is 2.0 (4.5 – 2.5). NextDouble only returns a number between 0 and 1.0, but if you multiply this by the range you will get a number between 0 and range.

So, this would give us random doubles between 0.0 and 2.0:

rng.NextDouble() * 2.0

But, we want them between 2.5 and 4.5! How do we do this? Add the smallest number, 2.5:

2.5 + rng.NextDouble() * 2.0

Now, we get a number between 0.0 and 2.0; if you add 2.5 to each of these values we see that the range is now between 2.5 and 4.5.

At first I thought that it mattered if b > a or a > b, but if you work it out both ways you’ll find it works out identically so long as you don’t mess up the order of the variables used. I like to express it with longer variable names so I don’t get mixed up:

double NextDouble(Random rng, double min, double max) {     return min + (rng.NextDouble() * (max - min)); }