C++理解全局变量在调用函数中的应用

#include<iostream>
using namespace std;
const int n=3;
void jacobi();
double A

={{10,-1.0,-2.0},{-1.0,10.0,-2.0},{-1.0,-1.0,5.0}};///在  main() 函数前写,不然在调用jacobi()时会出错
double B
={72,83,42};
int main()
{
jacobi();

}
void jacobi()
{
double X
={0,0,0,};
for (int k=0;k<10;k++)
{
for(int i=0;i<n;i++)
{
double sum=0;
for(int j=0;j<n;j++)
{
if(j==i) continue;
sum+=A[i][j]*X[j];
}
X[i]=(B[i]-sum)/A[i][i];
}
}
for (int i=0;i<n;i++)
cout<<X[i]<<'\t';
cout<<endl;
}