Random Numbers
Ruby comes with a pretty nice random number generator. The method to get a randomly chosen number is rand. If you call rand just like that, you’ll get a float greater than or equal to 0.0 and less than 1.0. If you give it an integer parameter (by calling rand(5), for example), it will give you an integer greater than or equal to 0 and less than 5 (so five possible numbers, from 0 to 4).
Let’s see rand in action:
puts rand |
puts rand |
puts rand |
puts(rand(100)) |
puts(rand(100)) |
puts(rand(100)) |
puts(rand(1)) |
puts(rand(1)) |
puts(rand(1)) |
puts(rand(9999999999999999999999999999999999999)) |
puts('The weatherman said there is a') |
puts(rand(101).to_s + '% chance of rain,') |
puts('but you can never trust a weatherman.') |
0.780420251671991 |
0.27612531216364 |
0.0633567492235 |
21 |
12 |
99 |
0 |
0 |
0 |
3753289500611420937623882583890274685 |
The weatherman said there is a |
67% chance of rain, |
but you can never trust a weatherman. |
Note that for the weatherman example I used rand(101) to get numbers from 0 to 100 and that rand(1) always returns 0. Not understanding the range of possible return values is the biggest mistake I see people make with rand, even professional programmers, and even in finished products you can buy at the store. I once had a CD player that if set on Random Play, would play every song but the last one. (I wonder what would have happened if I had put in a CD with only one song on it.)
Sometimes you might want rand to return the same random numbers in the same sequence on two different runs of your program. (For example, I used randomly generated numbers to generate the worlds in Civilization III. If I found a world that I really liked, I’d save it, run tests on it, and so on.) In order to do this, you need to set the seed, which you can do with srand:
srand 1976 |
puts(rand(100)) |
puts(rand(100)) |
puts(rand(100)) |
puts(rand(100)) |
puts '' |
srand 1976 |
puts(rand(100)) |
puts(rand(100)) |
puts(rand(100)) |
puts(rand(100)) |
50 |
21 |
80 |
15 |
|
50 |
21 |
80 |
15 |
It will do the same thing every time you seed it with the same number. If you want to get different numbers again (like what happens if you never use srand), then just call srand, passing in no parameter. This seeds it with a really weird number, using (among other things) the current time on your computer, down to the millisecond.