# Write a solver for systems of equations by using Lapack on Mac OS

Apple provides Accelerate library which includes linear algebra library (Lapack). In this post, we will go through writing a simple C++ program to use this library on Mac OS.

Let’s solve the following equations:

We can rewrite the equations as:

Now, let’s write our code:

``````#include <iostream>
#include <Accelerate/Accelerate.h>

int main()
{
int number_of_rows = 2;
int number_of_cols = 2;
int number_of_right_hand_side_cols = 1;
int LDA = 2;
int LDB = 2;
int IPIV[3];
int INFO=10;
char TRANS = 'N'; // Non transpose

double a[2*2] = {2, 1,
1, -1};
double b[2] = {0,
0};

dgetrf_(&number_of_rows, &number_of_cols, a, &LDA, IPIV, &INFO);
if (!INFO)
std::cout << "LU factorization executed without errors." << std::endl;

dgetrs_(&TRANS, &number_of_rows, &number_of_right_hand_side_cols,
a, &LDA, IPIV, b, &LDB, &INFO);

std::cout << "Result: x_0 = "<< *b <<", x_1 = "<<*(b+1) << std::endl;

if (!INFO)
std::cout << "Solver executed without errors." << std::endl;

return(0);
}``````

Now, run the code by using the following commands in a terminal:

`g++ -llapack lapack_test.cpp -o ltest.o./ltest.o`

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