How do I dereference multidimensional vector pointer? - c++

Question

I'm stuck trying to get the value of what A[i][j] is pointing to. double a = A[i][j];. How do I correctly do it? Could someone please explain?
// g++ jacobi.cpp -O0 -o jacobi && ./jacobi
#include <iostream>
#include <iomanip>
#include <vector>
#include <climits>
using namespace std;
void print_matrix(vector<vector<double>>& m) {
for (int i = 0; i < m.size(); i++) {
for (int j = 0; j < m[0].size(); j++) {
cout << setw(5) << fixed << setprecision(2) << m[i][j] << " ";
}
cout << endl;
}
cout << "==================================" << endl;
}
// calculate average temperature based on average of adjacent cells
double avg_temp_at(vector<vector<double>>& matrix, int i, int j) {
return (
matrix[i][j] +
(j-1 >= 0 ? matrix[i][j-1] : 0) +
(i-1 >= 0 ? matrix[i-1][j] : 0) +
(j+1 < matrix[0].size() ? matrix[i][j+1] : 0) +
(i+1 < matrix.size() ? matrix[i+1][j] : 0)
) / 5;
}
// sequential Jacobi algorithm
vector<vector<double>> jacobi_relaxation(vector<vector<double>>& matrix, int& threshold) {
vector<vector<double>> B (matrix.size(), vector<double>(matrix[0].size(), 0));
vector<vector<double>>* A = &matrix;
double max_delta = INT_MAX;
while (max_delta > threshold) {
max_delta = 0;
for (int i = 0; i < matrix.size(); i++) {
for (int j = 0; j < matrix[0].size(); j++) {
B[i][j] = avg_temp_at(*A, i, j);
double a = A[i][j];
double delta = abs(B[i][j] - a);
max_delta = max(max_delta, delta);
}
}
print_matrix(B);
A = &B;
}
return *A;
}
int main() {
int threshold = 1;
int n = 6;
vector<vector<double>> matrix (n, vector<double>(n, 0));
matrix[1][2] = 100;
matrix[2][2] = 100;
matrix[3][2] = 100;
print_matrix(matrix);
vector<vector<double>> x = jacobi_relaxation(matrix, threshold);
}

Answer 1

I tried your code and it gave me error on this line:
double a = A[i][j];
Change that line into this:
double a = (*A)[i][j];
and it will work.
Explanation:
It's basically the same trick as in line B[i][j] = avg_temp_at(*A, i, j);. A is a pointer, which is pointing to a vector. To accessing to pointers "real data" you must use the *.
Here you can find more info about pointers.
Hope it helps.