Garbage in array after calling function - c++

Question

The problem occurs in foo() (in the commented lines), and is that foo2() should return the result of a matrix multiplication repeated process in it's first parameter. It is working in the first case and failing right after.
B and B_tmp arrays should have the same values at the end of foo() and that's not happening
T is 1x6 matrix, A is 6x3 matrix, B is 200x3 matrix
foo3() multiplies TxA and store the result (1x3 matrix) at the end of B
What foo2() does at the beginning with B_t1_t2 is not relevant, it just prepares the 1x6 matrix, changing the order in some way
I must try to solve this without changing any function declaration
I'm new to c++ and have been searching for too long now, I'm desperated
#include <stdio.h>
#include <iostream>
#include <random>
#include <thread>
using namespace std;
double fRand(const double & min, const double & max) {
thread_local std::mt19937 generator(std::random_device{}());
std::uniform_real_distribution<double> distribution(min, max);
return distribution(generator);
}
int iRand(const int & min, const int & max) {
thread_local std::mt19937 generator(std::random_device{}());
std::uniform_int_distribution<int> distribution(min, max);
return distribution(generator);
}
void foo3(double T[6], double A[18], double *B)
{
for(int i = 0; i < 3; i++) {
double r = 0;
for(int j = 0; j < 6; j++) {
r += T[j] * A[i*3+j];
}
*B = r; B++;
}
}
void foo2(double *B, double *A, int from, int to)
{
for (int i=from; i < to; i++) { //This is not relevant but I leave it just in case
double B_t1_t2[6];
for (int x = 0; x < 3; x++)
B_t1_t2[x] = B[(i-1)*3 + x];
for (int x = 0; x < 3; x++)
B_t1_t2[x+3] = B[(i-2)*3 + x];
foo3(B_t1_t2, A, &B[i*3]);
}
}
void foo(double *A, double *B)
{
for (int i = 0; i < 18; i++)
A[i] = fRand(1, 2);
foo2(B, A, 2, 200);
cout << "\nB" << endl;
for (int i = 0; i < 600; i++)
cout << B[i] << " "; // HERE IS WORKING, B DOES NOT CONTAIN GARBAGE
cout << endl;
double B_tmp[600];
foo2(B_tmp, A, 2, 200);
cout << "\nB_tmp" << endl;
for (int i = 0; i < 600; i++)
cout << B_tmp[i] << " "; // WHY NOT WORKING HERE?
cout << endl;
}
int main()
{
double A[18], B[600];
for(int i = 0; i<6; i++)
B[i] = 1;
foo(A, B);
}
Why the second cout in foo() is showing garbage?
Also, if declarations must change, what would be the best way?
Im trying to use stack memory as much as I can.

Answer 1

Before calling foo(A, B); first 6 elements of B array were filled (all are set to 1). In foo function you call foo2 function twice. In first call you pass B array into foo2 function, and it works because B is filled. In second call of foo2 in foo you pass B_tmp array but all items of this array have garbage value, you didn't initialize them. So do
double B_tmp[600];
for (int i = 0; i < 6; ++i)
B_tmp[i] = 1;
foo2(B_tmp, A, 2, 200);