#include "mpi.h"
#include <iostream>
#include <vector>
#include <cmath>
using namespace std;
int main(int argc, char** argv) {
    MPI_Init(&argc, &argv);
    int rank, size;
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
    MPI_Comm_size(MPI_COMM_WORLD, &size);
    // coefficient matrix
    vector<vector<double>> C = {
        {0.05, -0.06, -0.12, 0.14},
        {0.04, -0.12, 0.08, 0.11},
        {0.34, 0.08, -0.06, 0.14},
        {0.11, 0.12, 0, -0.03}
    };
    // constant vector
    vector<double> d = {-2.17, 1.4, -2.1, -0.8};
    // initial approximation (vector of zeros)
    vector<double> x = {0, 0, 0, 0};
    // desired accuracy
    double eps = 1e-6;
    // iteration counter
    int k = 0;
    // get the number of rows per process
    int rows_per_proc = 4 / size;
    // scatter the coefficient matrix and constant vector
    vector<double> local_C(rows_per_proc * 4);
    vector<double> local_d(rows_per_proc);
    MPI_Scatter(C.data(), rows_per_proc * 4, MPI_DOUBLE, local_C.data(), rows_per_proc * 4, MPI_DOUBLE, 0, MPI_COMM_WORLD);
    MPI_Scatter(d.data(), rows_per_proc, MPI_DOUBLE, local_d.data(), rows_per_proc, MPI_DOUBLE, 0, MPI_COMM_WORLD);
    // iteration process
    while (true) {
        // compute new approximation locally
        vector<double> x_new(rows_per_proc);
        for (int i = 0; i < rows_per_proc; i++) {
            double s = 0;
            for (int j = 0; j < 4; j++) {
                s += local_C[i * 4 + j] * x[j];
            }
            x_new[i] = s + local_d[i];
        }
        // gather new approximation from all processes
        vector<double> global_x(4);
        MPI_Allgather(x_new.data(), rows_per_proc, MPI_DOUBLE, global_x.data(), rows_per_proc, MPI_DOUBLE, MPI_COMM_WORLD);
        // check for accuracy on root process
        if (rank == 0) {
            double norm = 0;
            for (int i = 0; i < 4; i++) {
                norm += pow(global_x[i] - x[i], 2);
            }
            if (sqrt(norm) < eps) {
                break;
            }
        }
        // broadcast updated approximation to all processes
        MPI_Bcast(global_x.data(), 4, MPI_DOUBLE, 0, MPI_COMM_WORLD);
        x = global_x;
        // increment counter on all processes
        k++;   }
    // print solution and number of iterations on root process
    if (rank == 0) {
        cout << "Solution: ";
        for (int i = 0; i < 4; i++) {
            cout << x[i] << " ";}
        cout << endl;
        cout << "Number of iterations: " << k << endl;   }
    MPI_Finalize();
    return 0;}

C++ Online Compiler

Write, Run & Share C++ code online using OneCompiler's C++ online compiler for free. It's one of the robust, feature-rich online compilers for C++ language, running on the latest version 17. Getting started with the OneCompiler's C++ compiler is simple and pretty fast. The editor shows sample boilerplate code when you choose language as C++ and start coding!

Read inputs from stdin

OneCompiler's C++ online compiler supports stdin and users can give inputs to programs using the STDIN textbox under the I/O tab. Following is a sample program which takes name as input and print your name with hello.

#include <iostream>
#include <string>
using namespace std;

int main() 
{
    string name;
    cout << "Enter name:";
    getline (cin, name);
    cout << "Hello " << name;
    return 0;
}

About C++

C++ is a widely used middle-level programming language.

  • Supports different platforms like Windows, various Linux flavours, MacOS etc
  • C++ supports OOPS concepts like Inheritance, Polymorphism, Encapsulation and Abstraction.
  • Case-sensitive
  • C++ is a compiler based language
  • C++ supports structured programming language
  • C++ provides alot of inbuilt functions and also supports dynamic memory allocation.
  • Like C, C++ also allows you to play with memory using Pointers.

Syntax help

Loops

1. If-Else:

When ever you want to perform a set of operations based on a condition If-Else is used.

if(conditional-expression) {
   //code
}
else {
   //code
}

You can also use if-else for nested Ifs and If-Else-If ladder when multiple conditions are to be performed on a single variable.

2. Switch:

Switch is an alternative to If-Else-If ladder.

switch(conditional-expression){    
case value1:    
 // code    
 break;  // optional  
case value2:    
 // code    
 break;  // optional  
......    
    
default:     
 code to be executed when all the above cases are not matched;    
}

3. For:

For loop is used to iterate a set of statements based on a condition.

for(Initialization; Condition; Increment/decrement){  
  //code  
}

4. While:

While is also used to iterate a set of statements based on a condition. Usually while is preferred when number of iterations are not known in advance.

while (condition) {  
// code 
}

5. Do-While:

Do-while is also used to iterate a set of statements based on a condition. It is mostly used when you need to execute the statements atleast once.

do {  
 // code 
} while (condition);

Functions

Function is a sub-routine which contains set of statements. Usually functions are written when multiple calls are required to same set of statements which increases re-usuability and modularity. Function gets run only when it is called.

How to declare a Function:

return_type function_name(parameters);

How to call a Function:

function_name (parameters)

How to define a Function:

return_type function_name(parameters) {  
 // code
}