Parallel Trapezoidal Rule. Builds a derived type * for use with the distribution of the input data.

/* get_data3.c -- Parallel Trapezoidal Rule. Builds a derived type
* for use with the distribution of the input data.
*
* Input:
* a, b: limits of integration.
* n: number of trapezoids.
* Output: Estimate of the integral from a to b of f(x)
* using the trapezoidal rule and n trapezoids.
*
* Notes:
* 1. f(x) is hardwired.
* 2. the number of processes (p) should evenly divide
* the number of trapezoids (n).
*
* See Chap 6, pp. 90 & ff in PPMPI
*/
#include

/* We'll be using MPI routines, definitions, etc. */
#include "mpi.h"

void Build_derived_type(
float* a_ptr /* in */,
float* b_ptr /* in */,
int* n_ptr /* in */,
MPI_Datatype* mesg_mpi_t_ptr /* out */);

main(int argc, char** argv) {
int my_rank; /* My process rank */
int p; /* The number of processes */
float a; /* Left endpoint */
float b; /* Right endpoint */
int n; /* Number of trapezoids */
float h; /* Trapezoid base length */
float local_a; /* Left endpoint my process */
float local_b; /* Right endpoint my process */
int local_n; /* Number of trapezoids for */
/* my calculation */
float integral; /* Integral over my interval */
float total; /* Total integral */
int source; /* Process sending integral */
int dest = 0; /* All messages go to 0 */
int tag = 0;
MPI_Status status;

void Get_data3(float* a_ptr, float* b_ptr, int* n_ptr, int my_rank);
float Trap(float local_a, float local_b, int local_n,
float h); /* Calculate local integral */

/* Let the system do what it needs to start up MPI */
MPI_Init(&argc, &argv);

/* Get my process rank */
MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);

/* Find out how many processes are being used */
MPI_Comm_size(MPI_COMM_WORLD, &p);

Get_data3(&a, &b, &n, my_rank);

h = (b-a)/n; /* h is the same for all processes */
local_n = n/p; /* So is the number of trapezoids */

/* Length of each process' interval of
* integration = local_n*h. So my interval
* starts at: */
local_a = a + my_rank*local_n*h;
local_b = local_a + local_n*h;
integral = Trap(local_a, local_b, local_n, h);

/* Add up the integrals calculated by each process */
MPI_Reduce(&integral, &total, 1, MPI_FLOAT,
MPI_SUM, 0, MPI_COMM_WORLD);

/* Print the result */
if (my_rank == 0) {
printf("With n = %d trapezoids, our estimate\n",
n);
printf("of the integral from %f to %f = %f\n",
a, b, total);
}

/* Shut down MPI */
MPI_Finalize();
} /* main */


/********************************************************************/
void Build_derived_type(
float* a_ptr /* in */,
float* b_ptr /* in */,
int* n_ptr /* in */,
MPI_Datatype* mesg_mpi_t_ptr /* out */) {
/* pointer to new MPI type */

/* The number of elements in each "block" of the */
/* new type. For us, 1 each. */
int block_lengths[3];

/* Displacement of each element from start of new */
/* type. The "d_i's." */
/* MPI_Aint ("address int") is an MPI defined C */
/* type. Usually an int. */
MPI_Aint displacements[3];

/* MPI types of the elements. The "t_i's." */
MPI_Datatype typelist[3];

/* Use for calculating displacements */
MPI_Aint start_address;
MPI_Aint address;

block_lengths[0] = block_lengths[1]
= block_lengths[2] = 1;

/* Build a derived datatype consisting of */
/* two floats and an int */
typelist[0] = MPI_FLOAT;
typelist[1] = MPI_FLOAT;
typelist[2] = MPI_INT;

/* First element, a, is at displacement 0 */
displacements[0] = 0;

/* Calculate other displacements relative to a */
MPI_Address(a_ptr, &start_address);

/* Find address of b and displacement from a */
MPI_Address(b_ptr, &address);
displacements[1] = address - start_address;

/* Find address of n and displacement from a */
MPI_Address(n_ptr, &address);
displacements[2] = address - start_address;

/* Build the derived datatype */
MPI_Type_struct(3, block_lengths, displacements,
typelist, mesg_mpi_t_ptr);

/* Commit it -- tell system we'll be using it for */
/* communication. */
MPI_Type_commit(mesg_mpi_t_ptr);
} /* Build_derived_type */


/********************************************************************/
void Get_data3(
float* a_ptr /* out */,
float* b_ptr /* out */,
int* n_ptr /* out */,
int my_rank /* in */) {
MPI_Datatype mesg_mpi_t; /* MPI type corresponding */
/* to 3 floats and an int */

if (my_rank == 0){
printf("Enter a, b, and n\n");
scanf("%f %f %d", a_ptr, b_ptr, n_ptr);
}

Build_derived_type(a_ptr, b_ptr, n_ptr, &mesg_mpi_t);
MPI_Bcast(a_ptr, 1, mesg_mpi_t, 0, MPI_COMM_WORLD);
} /* Get_data3 */


/********************************************************************/
float Trap(
float local_a /* in */,
float local_b /* in */,
int local_n /* in */,
float h /* in */) {

float integral; /* Store result in integral */
float x;
int i;

float f(float x); /* function we're integrating */

integral = (f(local_a) + f(local_b))/2.0;
x = local_a;
for (i = 1; i <= local_n-1; i++) {
x = x + h;
integral = integral + f(x);
}
integral = integral*h;
return integral;
} /* Trap */


/********************************************************************/
float f(float x) {
float return_val;
/* Calculate f(x). */
/* Store calculation in return_val. */
return_val = x*x;
return return_val;
} /* f */