Generating Random Numbers

Problem

You need to generate a random number in a certain range.

Solution

Use JavaScript’s Math.random() to get floating-point numbers from 0 <= x < 1.0. Use multiplication and Math.floor to get a number in a certain range.

probability = Math.random()
0.0 <= probability < 1.0
# => true

# Note that percentile does NOT ever reach 100. A full range of 0 to 100 is actually a span of 101.
percentile = Math.floor(Math.random() * 100)
0 <= percentile < 100
# => true

dice = Math.floor(Math.random() * 6) + 1
1 <= dice <= 6
# => true

max = 42
min = -13
range = Math.random() * (max - min) + min
-13 <= range < 42
# => true

Discussion

This is a straight lift from JavaScript.

Note that JavaScript’s Math.random() does not allow you to seed the random number generator to force certain values. See Generating Predictable Random Numbers for that.

To generate a number from 0 up to (but not including) n, multiply by n. To generate a number from 1 to n (inclusive), multiply by n and add 1.