我正在尝试在 for 循环中添加向量，但在 2 次迭代后我得到了错误的答案 (C)

Question

I am trying to implement the iterative formula Xn+1 = T*Xn + C, for a given number of iterations i. Here X and C are vectors and T is a 3x3 matrix. However, if I go beyond 2 iterations (fori=0;i<2;i++) the formula starts giving an incorrect result and I am not sure why. Any help is appreciated.

#include <stdio.h>
int main(void) {
  double T[3][3] = {{0.0,-0.5,-0.75},
                   {0.6,0.0,0.4},
                   {0.25,-0.375,0.0}};

  double C[3] = {2.0,2.8,3.375};
  double x_n[3] = {0.0,0.0,0.0};
  double x_n_plus_1[3] = {0.0,0.0,0.0};
  double Tx[3] = {0.0,0.0,0.0};
  int i, row, col, ctr;
  for(i=0;i<3;i++) {
    for(row=0;row<3;row++) {
      for(col=0;col<3;col++){
        Tx[row] += T[row][col] * x_n[col]; //Tx_n = T*x_n
      }
    }
    for(ctr=0;ctr<3;ctr++) {
      x_n_plus_1[ctr] = Tx[ctr] + C[ctr];//x_n+1 = Tx_n + C
    }
    for(ctr=0;ctr<3;ctr++) {
      x_n[ctr] = x_n_plus_1[ctr];
    }
  }
  for(ctr=0;ctr<3;ctr++) {
    printf("%lf\n",x_n[ctr]);
  }
  return 0;
}

Answer 1

As pmg proposed: "You need to reset the elements of Tx to zero at the beginning (or end) of each i loop". For each iteration of i, you need to re-initialize the Tx. If you go beyond 2 iterations (fori=0;i<2;i++) the formula starts giving an incorrect result because, staring second iteration, Tx is not still equal to {0, 0, 0}

#include <stdio.h>
int main(void) {
  double T[3][3] = {{0.0,-0.5,-0.75},
                   {0.6,0.0,0.4},
                   {0.25,-0.375,0.0}};

  double C[3] = {2.0,2.8,3.375};
  double x_n[3] = {0.0,0.0,0.0};
  double x_n_plus_1[3] = {0.0,0.0,0.0};

  int i, row, col, ctr;
  for(i=0;i<3;i++) {
    double Tx[3] = {0.0, 0.0, 0.0};
    for(row=0;row<3;row++) {
      for(col=0;col<3;col++){
        Tx[row] += T[row][col] * x_n[col]; //Tx_n = T*x_n
      }
    }
    for(ctr=0;ctr<3;ctr++) {
      x_n_plus_1[ctr] = Tx[ctr] + C[ctr];//x_n+1 = Tx_n + C
    }
    for(ctr=0;ctr<3;ctr++) {
      x_n[ctr] = x_n_plus_1[ctr];
    }
  }
  for(ctr=0;ctr<3;ctr++) {
    printf("%lf\n",x_n[ctr]);
  }
  return 0;
}