Task: Resistor mesh
C: #include <stdio.h>
#include <stdlib.h>
 
#define S 10
typedef struct { double v; int fixed; } node;
 
#define each(i, x) for(i = 0; i < x; i++)
node **alloc2(int w, int h)
{
	int i;
	node **a = calloc(1, sizeof(node*)*h + sizeof(node)*w*h);
	each(i, h) a[i] = i ? a[i-1] + w : (node*)(a + h);
	return a;
}
 
void set_boundary(node **m)
{
	m[1][1].fixed =  1; m[1][1].v =  1;
	m[6][7].fixed = -1; m[6][7].v = -1;
}
 
double calc_diff(node **m, node **d, int w, int h)
{
	int i, j, n;
	double v, total = 0;
	each(i, h) each(j, w) {
		v = 0; n = 0;
		if (i) v += m[i-1][j].v, n++;
		if (j) v += m[i][j-1].v, n++;
		if (i+1 < h) v += m[i+1][j].v, n++;
		if (j+1 < w) v += m[i][j+1].v, n++;
 
		d[i][j].v = v = m[i][j].v - v / n;
		if (!m[i][j].fixed) total += v * v;
	}
	return total;
}
 
double iter(node **m, int w, int h)
{
	node **d = alloc2(w, h);
	int i, j;
	double diff = 1e10;
	double cur[] = {0, 0, 0};

	while (diff > 1e-24) {
		set_boundary(m);
		diff = calc_diff(m, d, w, h);
		each(i,h) each(j, w) m[i][j].v -= d[i][j].v;
	}
 
	each(i, h) each(j, w)
		cur[ m[i][j].fixed + 1 ] += d[i][j].v *
				(!!i + !!j + (i < h-1) + (j < w -1));
 
	free(d);
	return (cur[2] - cur[0])/2;
}
 
int main()
{
	node **mesh = alloc2(S, S);
	printf("R = %g\n", 2 / iter(mesh, S, S));
	return 0;
}

C++: #include <iomanip>
#include <iostream>
#include <vector>

class Node {
private:
    double v;
    int fixed;

public:
    Node() : v(0.0), fixed(0) {
        
    }

    Node(double v, int fixed) : v(v), fixed(fixed) {
        
    }

    double getV() const {
        return v;
    }

    void setV(double nv) {
        v = nv;
    }

    int getFixed() const {
        return fixed;
    }

    void setFixed(int nf) {
        fixed = nf;
    }
};

void setBoundary(std::vector<std::vector<Node>>& m) {
    m[1][1].setV(1.0);
    m[1][1].setFixed(1);

    m[6][7].setV(-1.0);
    m[6][7].setFixed(-1);
}

double calculateDifference(const std::vector<std::vector<Node>>& m, std::vector<std::vector<Node>>& d, const int w, const int h) {
    double total = 0.0;
    for (int i = 0; i < h; ++i) {
        for (int j = 0; j < w; ++j) {
            double v = 0.0;
            int n = 0;
            if (i > 0) {
                v += m[i - 1][j].getV();
                n++;
            }
            if (j > 0) {
                v += m[i][j - 1].getV();
                n++;
            }
            if (i + 1 < h) {
                v += m[i + 1][j].getV();
                n++;
            }
            if (j + 1 < w) {
                v += m[i][j + 1].getV();
                n++;
            }
            v = m[i][j].getV() - v / n;
            d[i][j].setV(v);
            if (m[i][j].getFixed() == 0) {
                total += v * v;
            }
        }
    }
    return total;
}

double iter(std::vector<std::vector<Node>>& m, const int w, const int h) {
    using namespace std;
    vector<vector<Node>> d;
    for (int i = 0; i < h; ++i) {
        vector<Node> t(w);
        d.push_back(t);
    }

    double curr[] = { 0.0, 0.0, 0.0 };
    double diff = 1e10;

    while (diff > 1e-24) {
        setBoundary(m);
        diff = calculateDifference(m, d, w, h);
        for (int i = 0; i < h; ++i) {
            for (int j = 0; j < w; ++j) {
                m[i][j].setV(m[i][j].getV() - d[i][j].getV());
            }
        }
    }

    for (int i = 0; i < h; ++i) {
        for (int j = 0; j < w; ++j) {
            int k = 0;
            if (i != 0) ++k;
            if (j != 0) ++k;
            if (i < h - 1) ++k;
            if (j < w - 1) ++k;
            curr[m[i][j].getFixed() + 1] += d[i][j].getV()*k;
        }
    }

    return (curr[2] - curr[0]) / 2.0;
}

const int S = 10;
int main() {
    using namespace std;
    vector<vector<Node>> mesh;

    for (int i = 0; i < S; ++i) {
        vector<Node> t(S);
        mesh.push_back(t);
    }

    double r = 2.0 / iter(mesh, S, S);
    cout << "R = " << setprecision(15) << r << '\n';

    return 0;
}