supercollider-common 1:3.6.3~repack-5 / usr / share / SuperCollider / HelpSource / Guides / Randomness.schelp

title:: Randomness
categories:: Random
summary:: Randomness in SC
related:: Reference/randomSeed

As in any computer program, there are no "truly random" number generators in SC.
They are pseudo-random, meaning they use very complex, but deterministic
algorithms to generate sequences of numbers that are long enough and complicated enough
to seem "random" for human beings. (i.e. the patterns are too complex for us to detect.)

If you start a random number generator algorithm with the same "seed" number
several times, you get the same sequence of random numbers.
(See example below, randomSeed)

section:: Create single random numbers

subsection:: Between zero and <number>
code::
5.rand          // evenly distributed.

1.0.linrand     // probability decreases linearly from 0 to <number>.
::

subsection:: Between -<number> and <number>
code::
5.0.rand2       // evenly distributed.

10.bilinrand    // probability is highest around 0,
                // decreases linearly toward +-<number>.

1.0.sum3rand    // quasi-gaussian, bell-shaped distribution.
::

subsection:: Within a given range
code::
rrand(24, 48)       // linear distribution in the given range.

exprand(0.01, 1)    // exponential distribution;
                    // both numbers must have the same sign.
                    // (Note that the distribution of numbers is not exactly an exponential distribution, 
                    // since that would be unbounded: we might call it a logarithmic uniform distribution.)
::

subsection:: Test them multiple times with a do loop
code::
20.do({ 5.rand.postln; });			// evenly distributed

20.do({ 1.0.linrand.postln; });		// probability decreases linearly from 0 to 1.0

20.do({ 5.0.rand2.postln; });		// even

20.do({ 10.bilinrand.postln; });		// probability is highest around 0,
							// decreases linearly toward +-<number>.

20.do({ 1.0.sum3rand.postln; });	// quasi-gaussian, bell-shaped.
::

subsection:: Collect the results in an array
code::
Array.fill(10, { 1000.linrand }).postln;

// or more compact:

{ 1.0.sum3rand }.dup(100)

// or:

({ 1.0.sum3rand } ! 100)
::

subsection:: Seeding
You can seed a random generator in order to repeat
the same sequence of random numbers:
code::
(
5.do({
	thisThread.randSeed = 4;
	Array.fill(10, { 1000.linrand}).postln;
});
)

// Just to check, no seeding:

(
5.do({ Array.fill(10, { 1000.linrand }).postln; });
)
::
See also link::Reference/randomSeed::.


subsection:: Histograms
Demonstrate the various statistical distributions visually, with histograms:

code::
Array.fill(500, {  1.0.rand }).plot("Sequence of 500x 1.0.rand");

Array.fill(500, {  1.0.linrand }).plot("Sequence of 500x 1.0.linrand");

Array.fill(500, {  1.0.sum3rand }).plot("Sequence of 500x 1.0.sum3rand");
::

Use a histogram to display how often each (integer)
occurs in a collection of random numbers, :
code::
(
var randomNumbers, histogram, maxValue = 500, numVals = 10000, numBins = 500;

randomNumbers = Array.fill(numVals, { maxValue.rand; });
histogram = randomNumbers.histo(numBins, 0, maxValue);
histogram.plot("histogram for rand 0 - " ++ maxValue);
)
::

A histogram for linrand:
code::
(
var randomNumbers, histogram, maxValue = 500.0, numVals = 10000, numBins = 500;

randomNumbers = Array.fill(numVals, { maxValue.linrand; });
histogram = randomNumbers.histo(numBins, 0, maxValue);
histogram.plot("histogram for linrand 0 - " ++ maxValue);
)
::
A histogram for bilinrand:
code::
(
var randomNumbers, histogram, minValue = -250, maxValue = 250, numVals = 10000, numBins = 500;

randomNumbers = Array.fill(numVals, { maxValue.bilinrand; });
histogram = randomNumbers.histo(numBins, minValue, maxValue);
histogram.plot("histogram for bilinrand" + minValue + "to" + maxValue);
)
::

A histogram for exprand:
code::
(
var randomNumbers, histogram, minValue = 5.0, maxValue = 500, numVals = 10000, numBins = 500;

randomNumbers = Array.fill(numVals, { exprand(minValue, maxValue); });
histogram = randomNumbers.histo(numBins, minValue, maxValue);
histogram.plot("histogram for exprand: " ++ minValue ++ " to " ++ maxValue);
)
::

And for sum3rand (cheap quasi-gaussian):
code::
(
var randomNumbers, histogram, minValue = -250, maxValue = 250, numVals = 10000, numBins = 500;

randomNumbers = Array.fill(numVals, { maxValue.sum3rand; });
histogram = randomNumbers.histo(numBins, minValue, maxValue);
histogram.plot("histogram for sum3rand " ++ minValue ++ " to " ++ maxValue);
)
::


subsection:: on Collections
All of the single-number methods also work for (Sequenceable)Collections,
simply by applying the given random message to each element of the collection:
code::
[ 1.0, 10, 100.0, \aSymbol ].rand.postln;		// note: Symbols are left as they are.
List[ 10, -3.0, \aSymbol ].sum3rand.postln;
::

subsection:: Arbitrary random distributions

An integral table can be used to create an arbitrary random distribution quite efficiently. The table
building is expensive though. The more points there are in the random table, the more accurate the
distribution.
code::
(
var randomNumbers, histogram, distribution, randomTable, randTableSize=200;
var minValue = -250, maxValue = 250, numVals = 10000, numBins = 500;

// create some random distribution with values between 0 and 1
distribution = Array.fill(randTableSize,
	{ arg i; (i/ randTableSize * 35).sin.max(0) * (i / randTableSize) }
);

// render a randomTable
randomTable = distribution.asRandomTable;

// get random numbers, scale them

randomNumbers = Array.fill(numVals, { randomTable.tableRand * (maxValue - minValue) + minValue; });
histogram = randomNumbers.histo(numBins, minValue, maxValue);


histogram.plot("this is the histogram we got");
distribution.plot("this was the histogram we wanted");
)
::

section:: Random decisions

code:: coin :: simulates a coin toss and results in true or false.
1.0 is always true, 0.0 is always false, 0.5 is 50:50 chance.
code::
20.do({ 0.5.coin.postln });
::
biased random decision can be simulated bygenerating a single value
and check against a threshhold:
code::
20.do({ (1.0.linrand > 0.5).postln });
20.do({ (exprand(0.05, 1.0) > 0.5).postln });
::

section:: Generating Collections of random numbers
code::
		// size, minVal, maxVal
Array.rand(7, 0.0, 1.0).postln;

// is short for:

Array.fill(7, { rrand(0.0, 1.0) }).postln;
::
code::
		// size, minVal, maxVal
List.linrand(7, 10.0, 15.0).postln;

// is short for:

List.fill(7, { 10 + 5.0.linrand }).postln;
::

code::
Signal.exprand(10, 0.1, 1);

Signal.rand2(10, 1.0);
::

section:: Random choice from Collections

code::choose:: : equal chance for each element.
code::
10.do({ [ 1, 2, 3 ].choose.postln });
::

Weighted choice:

code::wchoose(weights):: : An array of weights sets the chance for each element.
code::
10.do({ [ 1, 2, 3 ].wchoose([0.1, 0.2, 0.7]).postln });
::

section:: Randomize the order of a Collection

code::
List[ 1, 2, 3, 4, 5 ].scramble.postln;
::

section:: Generate random numbers without duplicates
code::
f = { |n=8, min=0, max=7| (min..max).scramble.keep(n) };
f.value(8, 0, 7)
::

section:: Randomly group a Collection
code::
curdle(probability)
::
The probability argument sets the chance that two adjacent elements will be separated.
code::
[ 1, 2, 3, 4, 5, 6, 7, 8 ].curdle(0.2).postln;	// big groups

[ 1, 2, 3, 4, 5, 6, 7, 8 ].curdle(0.75).postln;	// small groups
::

section:: Random signal generators, i.e. UGens
list::
## link::Classes/PinkNoise::
## link::Classes/WhiteNoise::
## link::Classes/GrayNoise::
## link::Classes/BrownNoise::
## link::Classes/ClipNoise::
## link::Classes/LFNoise0::
## link::Classes/LFNoise1::
## link::Classes/LFNoise2::
## link::Classes/LFClipNoise::
## link::Classes/LFDNoise0::
## link::Classes/LFDNoise1::
## link::Classes/LFDNoise3::
## link::Classes/LFDClipNoise::
## link::Classes/Dust::
## link::Classes/Dust2::
## link::Classes/Crackle::
## link::Classes/TWChoose::
::

Also see UGens>Generators>Stochastic in the link::Browse#UGens>Generators>Stochastic:: page.

subsection:: UGens that generate random numbers once, or on trigger:

definitionlist::
## link::Classes/Rand:: || uniform distribution of float between (lo, hi), as for numbers.
## link::Classes/IRand:: || uniform distribution of integer numbers.
## link::Classes/TRand:: || uniform distribution of float numbers, triggered
## link::Classes/TIRand:: || uniform distribution of integer numbers, triggered
## link::Classes/LinRand:: || skewed distribution of float numbers, triggered
## link::Classes/NRand:: || sum of n uniform distributions, approximates gaussian distr. with higher n.
## link::Classes/ExpRand:: || exponential distribution
## link::Classes/TExpRand:: || exponential distribution, triggered
## link::Classes/CoinGate:: || statistical gate for a trigger
## link::Classes/TWindex:: || triggered weighted choice between a list.
::

subsection:: Seeding
Like using randSeed to set the random generatorsfor each thread in sclang,
you can choose which of several random generators on the server to use,
and you can reset (seed) these random generators:
list::
## link::Classes/RandID::
## link::Classes/RandSeed::
::

subsection:: UGens that generate random numbers on demand
("Demand UGens")
list::
## link::Classes/Dwhite::
## link::Classes/Dbrown::
## link::Classes/Diwhite::
## link::Classes/Dibrown::
## link::Classes/Drand::
## link::Classes/Dxrand::
::

see random patterns with analogous names

section:: Random Patterns
definitionlist::
## link::Classes/Prand:: || choose randomly one from a list ( list, numRepeats)
## link::Classes/Pxrand:: || choose one element from a list, no repeat of previous choice
## link::Classes/Pwhite:: || within range [<hi>, <lo>], choose a random value.
## link::Classes/Pbrown:: || within range [<hi>, <lo>], do a random walk with a maximum <step> to the next value.
## link::Classes/Pgbrown:: || geometric brownian motion

## link::Classes/Plprand::
## link::Classes/Phprand::
## link::Classes/Pmeanrand::
## link::Classes/Pbeta::
## link::Classes/Pcauchy::
## link::Classes/Pgauss::
## link::Classes/Ppoisson::
## link::Classes/Pexprand::

## link::Classes/Pwrand:: || choose from a list, probabilities by weights
code::
Pwrand([ 1, 2, 3 ], [0.1, 0.3, 0.6], 20);
::

## link::Classes/Pshuf:: || scramble the list, then repeat that order <repeats> times.

## link::Classes/Pwalk:: || code::Pwalk( (0 .. 10), Prand([ -2,-1, 1, 2], inf));:: random walk.

## link::Classes/Pfsm:: || random finite state machine pattern, see its help file. see also MarkovSet in MathLib quark

## link::Classes/Pseed:: || sets the random seed for that stream.
::

some basic examples
code::
(
Pbind(\note, Prand([ 0, 2, 4 ], inf),
	\dur, 0.2
).play;
)

(
Pbind(
	\note, Pxrand([ 0, 2, 4 ], inf),
	\dur, 0.2
).play;
)

(
Pbind(
	\note, Pwrand([ 0, 2, 4 ], [0.1, 0.3, 0.6], inf),
	\dur, 0.2
).play;
)

(
Pbind(
	\midinote, Pwhite(48, 72, inf),
	\dur, 0.2
).play;
)

(
Pbind(
	\midinote, Pbrown(48, 72, 5, inf),
	\dur, 0.2
).play;
)

(
Pbind(
	\midinote, Pgbrown(48, 72, 0.5, inf).round,
	\dur, 0.2
).play;
)
::