Why does math.random() call return a number in the range [0, 1) and not [0, 1]?

So math.random() (the call with no args) returns a number in the range [0, 1). Includes 0, excludes 1. But why does it exclude 1?

Not that I depend on it, and chances of even getting zero are already low enough.

And it seems to happen in most languages. In JavaScript, Math.random(), in C#, Random().Next(). Is there a reason for it then?

It could be for any number of reasons, and I don’t know if there’s any definitive one, however I can think of the following off the top of my head:

  • It makes it easy to scale your numbers by simply multiplying and/or rounding. math.random() * n is essentially range [0, n).
  • In languages where arrays start at 0, it makes it easy to choose an element in your array. my_arr_of_five[Floor(Random() * 5)] is never going out of bounds. Since Random never returns 1 we don’t hit the case of out-of-range index.
  • Maybe it’s intuitive, in Floor(Random() * 5) you have exactly 5 possible values, while in a [0, 1] range it’d be six.
  • Maybe someone very early on decided it would be standard, and so it became that.

These are just guesses, but regardless I personally like the arbitrary decision.

Some more reasons I found with a quick search: https://www.quora.com/Why-does-the-Math-random-function-return-a-double-in-the-range-0-1-instead-of-other-intervals-in-Java