# Random Numbers

Generate a list of random numbers using the roc-random package and the basic-cli platform.

## Random `Generators`

Some languages provide a function like JavaScriptâ€™s `Math.random()`

which can return a different number each time you call it. However, functions in Roc are guaranteed to return the same answer when given the same arguments, so something like `Math.random`

couldnâ€™t possibly be a valid Roc function! As such, we use a different approach to generate random numbers in Roc.

This example uses a `Generator`

which generates pseudorandom numbers using an initial seed value and the PCG algorithm. Note that if the same seed is provided, then the same number sequence will be generated every time! The appearance of randomness comes entirely from deterministic math being done on that initial seed. (The same is true of `Math.random()`

, except that `Math.random()`

silently chooses a seed for you at runtime.)

## Code

app "random-numbers" packages { pf: "https://github.com/roc-lang/basic-cli/releases/download/0.7.0/bkGby8jb0tmZYsy2hg1E_B2QrCgcSTxdUlHtETwm5m4.tar.br", rand: "https://github.com/lukewilliamboswell/roc-random/releases/download/0.0.1/x_XwrgehcQI4KukXligrAkWTavqDAdE5jGamURpaX-M.tar.br", } imports [ pf.Stdout, rand.Random, ] provides [main] to pf # Print a list of 10 random numbers in the range 25-75 inclusive. main = # Initialize "randomness" initialSeed = Random.seed 42 # Create a generator for values from 25-75 (inclusive) generator = Random.int 25 75 # Create a list of random numbers result = randomList initialSeed generator # Format as a string numbersListStr = result.numbers |> List.map Num.toStr |> Str.joinWith "," Stdout.line "Random numbers are: \(numbersListStr)" # Generate a list of numbers using the seed and generator provided # This is NOT cryptograhpically secure! randomList = \initialSeed, generator -> List.range { start: At 0, end: Before 10 } |> List.walk { seed: initialSeed, numbers: [] } \state, _ -> # Use the generator to get a new seed and value random = generator state.seed # Update seed so it can be used to generate the next value seed = random.state # Append the latest random value to the list of numbers numbers = List.append state.numbers random.value { seed, numbers }

## Output

Run this from the directory that has `main.roc`

in it:

$ roc run Random numbers are: 29,30,71,64,48,33,55,68,53,28