Randomness

Randomness is frequently used in algorithmic music to introduce variation and unpredictability. However, in many compositional workflows, reproducibility is just as important. In bellplay~, there are two types of random functions:

• Unseeded: always produce different results.

  • Examples: rand, randn, random, scramble, choose, urn.

• Seeded: produce the same result each time when a non-zero seed is set. The name of most seeded functions are prefixed with an x.

  • Examples: xrand, xrandn, xchoose, xrandom, xscramble, urn (when seeded via setseed).

The seeded variants allow you to control randomness and make your scripts deterministic and repeatable.

---

rand vs xrand

The rand function produces a random float and is not seedable.

## will return a different result every time
print(rand(0, 1))

In contrast, xrand is a seeded version. The result will be the same every time you run the code:

## set the random seed
setseed(42);
## will return the same value every time the script is run
print(xrand(0, 1))

Default seed

If setseed is not called, the default seed for all seeded functions is 1.

---

choose vs xchoose

choose selects elements from a list, non-deterministically:

$files = 'flute.wav' 'viola.wav' 'guitar.wav';
print(choose(@choices $files))

With xchoose, the result becomes reproducible if a seed is set:

setseed(99);
$files = 'flute.wav' 'viola.wav' 'guitar.wav';
print(xchoose(@choices $files))

---

xscramble: seeded shuffling

The xscramble function randomly reorders a list, but deterministically with a seed.

setseed(@seed 7);
$items = 1 2 3 4 5;
print(xscramble(@llll $items))

---

urn (unique random number)

The urn function generates unique numbers without repetition. It can be seeded using setseed, enabling deterministic but non-repeating behavior.

setseed(@seed 2025);
initurn(
    @id 'myseq' 
    @size 4
    @useseed 1
);
for $i in (0...10) do (
    print(urn(@id 'myseq')) 
)

---

Putting it all together

The following script demonstrates how to use setseed to control different parameters.

randomness.bell

## set seed for deterministic random behavior (try 0 to disable it)
setseed(120);
## initialize onset time
$t = 0;
## total duration of the sequence in milliseconds
$totdur = 10000;
## define pitch class set (in symbolic pitch notation)
$chord = c5 d5 e5 g5 b5;
## generate events until reaching total duration
while $t < $totdur do (
    ## randomly select a pitch from the chord
    $pitch = xchoose($chord);
    ## choose a random octave transposition (±2 octaves, in midicents)
    $octave = xrandom(-2, 3) * 1200;
    ## randomly stereo panning (0 to 1)
    $pan = xrand();
    ## randomly choose duration between 1 and 5 seconds
    $dur = xrand(1000, 5000);
    ## randomly set gain (from soft to loud)
    $gain = xrand(0.25, 1);
    ## create a buffer using ezsampler with random pitch and duration
    $b = ezsampler(@pitch $pitch + $octave @duration $dur);
    ## transcribe the buffer with randomized parameters
    $b.transcribe(
        @onset $t
        @pan $pan
        @gain $gain
    );
    ## increment time by a small random step to control event density
    $incr = xrand(25, 300);
    $t += $incr
);
## trigger rendering
render( @play 1)

Summary

Function	Seeded?	Returns	Use Case
rand	❌	float	quick randomness
xrand	✅	float	reproducible randomness
choose	❌	any	random choice
xchoose	✅	any	repeatable random choice
xscramble	✅	list	deterministic shuffling
urn	✅ (optional)	int	deterministic non-repeating

Using setseed lets you design stochastic processes that behave consistently, which can be useful in certain contexts.

Result