# LeapfrogStream

Initializes a stream using the leapfrog method.

## Syntax

Fortran:

status = vslleapfrogstream( stream, k, nstreams )

C:

status = vslLeapfrogStream( stream, k, nstreams );

## Description

The function allows generating random numbers in a random stream with non-unit stride. This feature is particularly useful in distributing random numbers from original stream across nstreams buffers without generating the original random sequence with subsequent manual distribution. One of the important applications of the leapfrog method is splitting the original sequence into non-overlapping subsequences across nstreams computational nodes. The function initializes the original random stream (see Figure“Leapfrog Method”) to generate random numbers for the computational node `k, 0 ≤ k < nstreams`, where nstreams is the largest number of computational nodes used.

Leapfrog Method

The leapfrog method is supported only for those basic generators that allow splitting elements by the leapfrog method, which is more efficient than simply generating them by a generator with subsequent manual distribution across computational nodes. See VSL Notes for details.

For quasi-random basic generators the leapfrog method allows generating individual components of quasi-random vectors instead of whole quasi-random vectors. In this case nstreams parameter should be equal to the dimension of the quasi-random vector while k parameter should be the index of a component to be generated (`0 ≤ k < nstreams`). Other parameters values are not allowed.

The following code examples illustrate the initialization of three independent streams using the leapfrog method:

## FORTRAN Code for Leapfrog Method

...

TYPE(VSL_STREAM_STATE) ::stream1

TYPE(VSL_STREAM_STATE) ::stream2

TYPE(VSL_STREAM_STATE) ::stream3

! Creating 3 identical streams

status = vslnewstream(stream1, VSL_BRNG_MCG31, 174)

status = vslcopystream(stream2, stream1)

status = vslcopystream(stream3, stream1)

! Leapfrogging the streams

status = vslleapfrogstream(stream1, 0, 3)

status = vslleapfrogstream(stream2, 1, 3)

status = vslleapfrogstream(stream3, 2, 3)

! Generating random numbers

...

! Deleting the streams

status = vsldeletestream(stream1)

status = vsldeletestream(stream2)

status = vsldeletestream(stream3)

...

## C Code for Leapfrog Method

...

VSLStreamStatePtr stream1;

VSLStreamStatePtr stream2;

VSLStreamStatePtr stream3;

/* Creating 3 identical streams */

status = vslNewStream(&stream1, VSL_BRNG_MCG31, 174);

status = vslCopyStream(&stream2, stream1);

status = vslCopyStream(&stream3, stream1);

/* Leapfrogging the streams

*/

status = vslLeapfrogStream(stream1, 0, 3);

status = vslLeapfrogStream(stream2, 1, 3);

status = vslLeapfrogStream(stream3, 2, 3);

/* Generating random numbers

*/

...

/* Deleting the streams

*/

status = vslDeleteStream(&stream1);

status = vslDeleteStream(&stream2);

status = vslDeleteStream(&stream3);

...

## Input Parameters

Name | Type | Description
--- | --- | ---
stream | FORTRAN: TYPE(VSL_STREAM_STATE), INTENT(IN) C: VSLStreamStatePtr | FORTRAN: Descriptor of the stream to which leapfrog method is applied C: Pointer to the stream state structure to which leapfrog method is applied
k | FORTRAN: INTEGER, INTENT(IN) C: int | Index of the computational node, or stream number
nstreams | FORTRAN: INTEGER, INTENT(IN) C: int | Largest number of computational nodes, or stride

## Return Values

VSL_ERROR_OK, VSL_STATUS_OK
  Indicates no error, execution is successful.
VSL_ERROR_NULL_PTR
  stream is a NULL pointer.
VSL_ERROR_BAD_STREAM

  stream is not a valid random stream.

VSL_ERROR_LEAPFROG_UNSUPPORTED

  BRNG does not support Leapfrog method.

Parent topic: Service Routines