NAG CL Interface
d01xbc (multid_quad_monte_carlo_1)
1
Purpose
d01xbc evaluates an approximation to the integral of a function over a hyper-rectangular region, using a Monte–Carlo method. An approximate relative error estimate is also returned. This function is suitable for low accuracy work.
2
Specification
void |
d01xbc (Integer ndim,
double |
(*f)(Integer ndim,
const double x[],
Nag_User *comm),
|
|
Nag_MCMethod method,
Nag_Start cont,
const double a[],
const double b[],
Integer *mincls,
Integer maxcls,
double eps,
double *finest,
double *acc,
double **comm_arr,
Nag_User *comm,
NagError *fail) |
|
The function may be called by the names: d01xbc, nag_quad_multid_quad_monte_carlo_1 or nag_multid_quad_monte_carlo_1.
3
Description
d01xbc uses an adaptive Monte–Carlo method based on the algorithm described by
Lautrup (1971). It is implemented for integrals of the form:
Upon entry, unless the argument
, the function subdivides the integration region into a number of equal volume subregions. Inside each subregion the integral and the variance are estimated by means of pseudorandom sampling. All contributions are added together to produce an estimate for the whole integral and total variance. The variance along each coordinate axis is determined and the function uses this information to increase the density and change the widths of the sub-intervals along each axis, so as to reduce the total variance. The total number of subregions is then increased by a factor of two and the program recycles for another iteration. The program stops when a desired accuracy has been reached or too many integral evaluations are needed for the next cycle.
4
References
Lautrup B (1971) An adaptive multi-dimensional integration procedure Proc. 2nd Coll. Advanced Methods in Theoretical Physics, Marseille
5
Arguments
-
1:
– Integer
Input
-
On entry: the number of dimensions of the integral, .
Constraint:
.
-
2:
– function, supplied by the user
External Function
-
f must return the value of the integrand
at a given point.
The specification of
f is:
double |
f (Integer ndim,
const double x[],
Nag_User *comm)
|
|
-
1:
– Integer
Input
-
On entry: the number of dimensions of the integral.
-
2:
– const double
Input
-
On entry: the coordinates of the point at which the integrand must be evaluated.
-
3:
– Nag_User *
-
Pointer to a structure of type Nag_User with the following member:
- p – Pointer
-
On entry/exit: the pointer
should be cast to the required type, e.g.,
struct user *s = (struct user *)comm → p, to obtain the original object's address with appropriate type. (See the argument
comm below.)
Note: f should not return floating-point NaN (Not a Number) or infinity values, since these are not handled by
d01xbc. If your code inadvertently
does return any NaNs or infinities,
d01xbc is likely to produce unexpected results.
-
3:
– Nag_MCMethod
Input
-
On entry: the method to be used.
- The function uses only one iteration of a crude Monte–Carlo method with maxcls sample points.
- The function subdivides the integration region into a number of equal volume subregions.
Constraint:
or .
-
4:
– Nag_Start
Input
-
On entry: the continuation state of the evaluation of the integrand.
- Indicates that this is the first call to the function with the current integrand and arguments ndim, a and b.
- Indicates that a previous call has been made with the same arguments ndim, a and b with the same integrand. Please note that method must not be changed.
- Indicates that a previous call has been made with the same arguments ndim, a and b but that the integrand is new. Please note that method must not be changed.
Constraint:
, or .
-
5:
– const double
Input
-
On entry: the lower limits of integration, , for .
-
6:
– const double
Input
-
On entry: the upper limits of integration, , for .
-
7:
– Integer *
Input/Output
-
On entry:
mincls must be set to the minimum number of integrand evaluations to be allowed.
Constraint:
.
On exit:
mincls contains the total number of integrand evaluations actually used by
d01xbc.
-
8:
– Integer
Input
-
On entry: the maximum number of integrand evaluations to be allowed. In the continuation case this is the number of new integrand evaluations to be allowed. These counts do not include zero integrand values.
Constraints:
- ;
- .
-
9:
– double
Input
-
On entry: the relative accuracy required.
Constraint:
.
-
10:
– double *
Output
-
On exit: the best estimate obtained for the integral.
-
11:
– double *
Output
-
On exit: the estimated relative accuracy of
finest.
-
12:
– double **
Input/Output
-
On entry: if
or
, the memory pointed to and allocated by a previous call of
d01xbc must be unchanged.
If then appropriate memory is allocated internally by d01xbc.
On exit:
comm_arr contains information about the current sub-interval structure which could be used in later calls of
d01xbc. In particular,
gives the number of sub-intervals used along the
th coordinate axis.
When this information is no longer useful, or before a subsequent call to
d01xbc with
is made, you should free the storage contained in this pointer using the NAG macro
NAG_FREE. Note this memory will have been allocated and needs to be freed only if the error exit NE_NOERROR or
NE_QUAD_MAX_INTEGRAND_EVAL occurs. Otherwise, no memory needs to be freed.
-
13:
– Nag_User *
-
Pointer to a structure of type Nag_User with the following member:
- p – Pointer
-
On entry/exit: the pointer
, of type Pointer, allows you to communicate information to and from
f(). An object of the required type should be declared, e.g., a structure, and its address assigned to the pointer
by means of a cast to Pointer in the calling program, e.g.,
comm.p = (Pointer)&s. The type Pointer is
void *.
-
14:
– NagError *
Input/Output
-
The NAG error argument (see
Section 7 in the Introduction to the NAG Library CL Interface).
6
Error Indicators and Warnings
- NE_2_INT_ARG_GE
-
On entry, while . These arguments must satisfy .
- NE_2_INT_ARG_LT
-
On entry, while . These arguments must satisfy .
- NE_ALLOC_FAIL
-
Dynamic memory allocation failed.
- NE_BAD_PARAM
-
On entry, argument
cont had an illegal value.
On entry, argument
method had an illegal value.
- NE_INT_ARG_LE
-
On entry, .
Constraint: .
- NE_INT_ARG_LT
-
On entry, .
Constraint: .
- NE_QUAD_MAX_INTEGRAND_EVAL
-
maxcls was too small to obtain the required accuracy.
In this case
d01xbc returns a value of
finest with estimated relative error
acc, but
acc will be greater than
eps. This error exit may be taken before
maxcls nonzero integrand evaluations have actually occurred, if the function calculates that the current estimates could not be improved before
maxcls was exceeded.
- NE_REAL_ARG_LT
-
On entry,
eps must not be less than 0.0:
.
7
Accuracy
A relative error estimate is output through the argument
acc. The confidence factor is set so that the actual error should be less than
acc % of the time. If you desire a higher confidence level then a smaller value of
eps should be used.
8
Parallelism and Performance
d01xbc is not threaded in any implementation.
The running time for
d01xbc will usually be dominated by the time used to evaluate the integrand
f, so the maximum time that could be used is approximately proportional to
maxcls.
For some integrands, particularly those that are poorly behaved in a small part of the integration region, this function may terminate with a value of
acc which is significantly smaller than the actual relative error. This should be suspected if the returned value of
mincls is small relative to the expected difficulty of the integral. Where this occurs,
d01xbc should be called again, but with a higher entry value of
mincls (e.g., twice the returned value) and the results compared with those from the previous call.
9.1
Additional Information
The exact values of
finest and
acc on return will depend (within statistical limits) on the sequence of random numbers generated within this function.
If desired, you may ensure the identity or difference between runs of the results returned by this function by calling
g05cbc or
g05ccc immediately prior to calling this function.
g05cbc has the prototype
void g05cbc(Integer seed)
where
seed is a scalar value used to initialize the underlying random number generator. Using the same value for
seed will ensure that the same sequence of random values are generated and hence that the same result from this function will be obtained.
g05ccc has the prototype
void g05ccc()
Each time
g05ccc is called the underlying random number generator will be reinitialized using a random seed, ensuring a different sequence of values being used. Consequently this function may return different numerical results.
10
Example
This example calculates the integral
10.1
Program Text
10.2
Program Data
None.
10.3
Program Results