Matrix-Vector Multiplication on MPI - ERROR Compiled code - c++

Question

I need assistance to resolve an error in the following code:
#include <iostream>
#include <mpi.h>
using namespace std;
//matrix in two dimension in memory!!
int main(int argc, char** argv)
{
const int WIDTH = 100;
const int HEIGHT = 100;
int id, P;
double tempValue = 0;
MPI_Init(&argc, &argv);
MPI_Comm_size(MPI_COMM_WORLD, &P);
MPI_Comm_rank(MPI_COMM_WORLD, &id);
double A[WIDTH][HEIGHT];
double x[HEIGHT], b[WIDTH];
int upperBound, lowerBound = 0;
// Master controls worksharing..
if (id == 0)
{
// Init A & x
for (int i = 0; i < WIDTH; i++)
for (int j = 0; j < HEIGHT; j++)
A[i][j] = 1;
for (int j = 0; j < HEIGHT; j++)
x[j] = 2;
// Send to each node its portion of A to be processed
int portionSize = WIDTH / P;
for (int i = 0; i < P; i++)
{
lowerBound = i * portionSize;
upperBound = (i + 1) * portionSize;
// let the last node process the remainder
if (i == (P - 1))
upperBound += (HEIGHT - portionSize * P);
if (i > 0)// Do not send to master node!!
{
// Send to node i the lower & upper bounds the A portion
//and complete vector x
MPI_Send(&lowerBound, 1, MPI_INT, i, 0, MPI_COMM_WORLD);
MPI_Send(&upperBound, 1, MPI_INT, i, 0, MPI_COMM_WORLD);
MPI_Send(&A[lowerBound][0], (upperBound - lowerBound) * HEIGHT,
MPI_DOUBLE, i, 0, MPI_COMM_WORLD);
MPI_Send(&x[0], HEIGHT, MPI_DOUBLE, i, 0, MPI_COMM_WORLD);
}
}
// master perform part of the job...
for (int i = 0; i < portionSize; i++)
{
tempValue = 0;
for (int j = 0; j < HEIGHT; j++)
tempValue += A[i][j] * x[j];
b[i] = tempValue;
}
//Get the results in order, each node would send their boundaries and data part
for (int i = 1; i < P; i++)
{
MPI_Recv(&lowerBound, 1, MPI_INT, i, 0, MPI_COMM_WORLD, MPI_STATUSES_IGNORE);
MPI_Recv(&upperBound, 1, MPI_INT, i, 0, MPI_COMM_WORLD, MPI_STATUSES_IGNORE);
MPI_Recv(&P[lowerBound], (upperBound - lowerBound), MPI_DOUBLE, i, 0,
MPI_COMM_WORLD, MPI_STATUSES_IGNORE);
}
// Print the first 2 values to check..
cout << "b[0]=" << b[0] << " b[Width-1]=" << b[WIDTH - 1] << endl;
}
else // the rest of the workers do their parts
{
//Receive the inputs
MPI_Recv(&lowerBound, 1, MPI_INT, 0, 0, MPI_COMM_WORLD, MPI_STATUSES_IGNORE);
MPI_Recv(&upperBound, 1, MPI_INT, 0, 0, MPI_COMM_WORLD, MPI_STATUSES_IGNORE);
MPI_Recv(&A[lowerBound][0], (upperBound - lowerBound) * WIDTH, MPI_DOUBLE, 0, 0,
MPI_COMM_WORLD, MPI_STATUSES_IGNORE);
MPI_Recv(&x, HEIGHT, MPI_DOUBLE, 0, 0, MPI_COMM_WORLD, MPI_STATUSES_IGNORE);
cout << "Node:" << id << " Received from:" << lowerBound << " to " << upperBound - 1
<< endl;
double* result = new double[upperBound - lowerBound];
//Do the job
for (int i = lowerBound, resultCounter = 0; i < upperBound; i++, resultCounter++)
{
tempValue = 0;
for (int j = 0; j < HEIGHT; j++)
tempValue += A[i][j] * x[j];
result[resultCounter] = tempValue;
}
//send the results
MPI_Send(&lowerBound, 1, MPI_INT, 0, 0, MPI_COMM_WORLD);
MPI_Send(&upperBound, 1, MPI_INT, 0, 0, MPI_COMM_WORLD);
MPI_Send(&result[0], upperBound - lowerBound, MPI_DOUBLE, 0, 0, MPI_COMM_WORLD);
delete[] result;
}
MPI_Finalize();
return 0;
}
When I compile the code in Microsoft Visual Studio 2019, I get this error message:
Error (active) E0142 expression must have pointer-to-object type ConsoleApplication9 C:\Users\m_swe\Desktop\Assignments\Assignments\PrjP2P\MatMPI\MatMPI\Source.cpp 59
Error C2109 subscript requires array or pointer type ConsoleApplication9 C:\Users\m_swe\Desktop\Assignments\Assignments\PrjP2P\MatMPI\MatMPI\Source.cpp 59

Answer 1

I think the problem is on line: 59
MPI_Recv(&P[lowerBound], (upperBound - lowerBound), MPI_DOUBLE, i, 0,
MPI_Recv takes in a pointer to a buffer (the first argument) where you are going to receive and store the incoming data. In this case it could be in some variable which you can define inside the for loop, as:
int receivedValues[ WIDTH * HEIGHT ];
for (int i = 1; i < P; i++)
{
MPI_Recv(&lowerBound, 1, MPI_INT, i, 0, MPI_COMM_WORLD, MPI_STATUSES_IGNORE);
MPI_Recv(&upperBound, 1, MPI_INT, i, 0, MPI_COMM_WORLD, MPI_STATUSES_IGNORE);
MPI_Recv(&receivedValues[0], (upperBound - lowerBound), MPI_DOUBLE, i, 0,
MPI_COMM_WORLD, MPI_STATUSES_IGNORE);
// do your computation here with receivedValues
}
}