rand

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Synopsis

rand [string|int] float|vector [float|vector]

rand is NOT undoable, NOT queryable, and NOT editable.

Multiple random number streams:
The three stream-based variants of rand accept a stream name parameter which specifies the name of the random number stream to use when generating random numbers. For details on random number streams please refer to the section "An overview of MEL's multiple random number stream support" later in this document. The behaviour of the function is identical to the non-stream versions with the exception that random number generation is performed from the specified rather than the default random number stream. The first two stream-based variants take a string which specifies the stream name. The third variant takes an integer as the stream name (this is a convenience function which internally converts the integer to a string in the event the use wants to work with numeric-only stream names).

An overview of MEL's multiple random number stream support:
The random numbers returned by rand, sphrand and gauss all follow a fixed sequence (called a random number stream). Prior to the addition of multiple random number streams to Maya, the only way to ensure repeatability was to explicitely reseed the generator using the seed function. There are, however, cases where random calls occur in arbitrary order and reseeding is either impossible or impractical.

To resolve this issue, MEL supports an arbitrary number of user-defined random number streams. Each MEL random call (rand, sphrand, gauss, etc) allows a stream name to be provided which identifies the random stream to be used during random number generation. Streams are created implicitely: each MEL random function checks first if the stream exists and if not, it is created and seeded with an internal default. The internal default is identical so that the default and user-defined streams always generate the same sequence of numbers when generated via an identical set of random calls. (This repeatability can be avoided by manually reseeding the stream via the seed function). Stream names may be any user-defined string not containing a "*" as a character. The stream named "default" is pre-defined and refers to the standard stream used by the non-stream-based MEL random calls. The intent with random number streams is that the user can target specific areas of MEL random calls to use dedicated random number streams and thus ensure repeatability of the random numbers which are generated.

Internally, streams are managed through an efficient database, so it is not unreasonable to allocate large stream counts (e.g. 100,000 streams). Streams may be deleted via the delrandstr function. The state of a random number stream may be queried via the randstate function.

Return value

Related

MEL examples

   rand 42;
   // Result: 39.1327 //
   rand 42;
   // Result: 23.8585 //
   rand -80 42;
   // Result: -12.1565 //
   rand <<1,1,1>>;
   // Result: <<0.531557, 0.920261, 0.515431>>  //
   rand <<1,1,1>> <<100, 200, 300>>;
   // Result: <<81.2325, 38.4956, 266.008>>  //

// <b>Multiple random number stream examples:</b><br>
   string $s1 = "stream1";
   // Result: stream1 //
   rand $s1 42;
   // Result: 39.1327 //
   rand $s1 42;
   // Result: 23.8585 //
   string $s2 = "stream2";
   // Result: s2 //
   rand $s2 42;
   // Result: 39.1327 //
   rand $s2 42;
   // Result: 23.8585 //

   // Example using the convenience method which allows an integer to be used
   // as the stream name:
   // Assume `particleID' is an integer... //
   rand particleID -80 42;
   // Result: 33.6711 //