There are three main issues in the posted code
a, b and ans are declared as variable length arrays (their sizes aren't known at compile time), a non-standard extension provided by some compilers. The OP could have used a standard container like std::vector or a user-defined class.
All the loops start at 1, while in C++ array indices are 0-based. The writer of this code seems to know this fact, but for some reason preferred to allocate extra (unused) space in all the arrays and to accomodate the termitation conditions accordingly.
The nested loop which tries to copy the values from b to ans is simply wrong and causes multiple accesses out of bounds.
int I = 1, J = 0; // ---> I = 0, keeping OP's convention
for (int i = 1; i <= n; ++i)
{
I += 2, J++; // ---> ++I, J = q
for (int j = 1; j <= p; ++j)
{
ans[I][J] = b[i][j];
I++; // ---> ++J
}
}
In the following snippet shows an alternative implementation where the matrices are modelled using a class and the merge function performs the wanted operation.
#include <algorithm>
#include <iomanip>
#include <iostream>
#include <numeric>
#include <stdexcept>
#include <vector>
template <class T>
class Matrix
{
size_t rows_{};
size_t cols_{};
std::vector<T> m_;
public:
Matrix() = default;
Matrix(size_t r, size_t c)
: rows_{r}, cols_{c}, m_(r * c)
{}
auto rows() const noexcept {
return rows_;
}
auto columns() const noexcept {
return cols_;
}
auto operator[] (size_t i) noexcept {
return m_.begin() + cols_ * i;
}
auto operator[] (size_t i) const noexcept {
return m_.cbegin() + cols_ * i;
}
auto begin() noexcept {
return m_.begin();
}
auto end() noexcept {
return m_.end();
}
};
template <class T>
std::ostream& operator<< (std::ostream& os, Matrix<T> const& m)
{
for (size_t i{}; i < m.rows(); ++i)
{
for (size_t j{}; j < m.columns(); ++j)
os << m[i][j] << ' ';
os << '
';
}
return os;
}
template<class T>
auto merge(Matrix<T> const& a, Matrix<T> const& b)
{
if (a.rows() != b.rows())
throw std::runtime_error{"Number of rows mismatch"};
Matrix<T> result(a.rows(), a.columns() + b.columns());
for (size_t i{}; i < a.rows(); ++i)
{
auto it = std::copy(a[i], a[i] + a.columns(), result[i]);
std::copy(b[i], b[i] + b.columns(), it);
}
return result;
}
int main()
{
int n, k;
std::cin >> n >> k;
int p = n - k + 1, q = n - k;
Matrix<int> a(n, q);
std::iota(a.begin(), a.end(), 1);
std::cout << a << '
';
Matrix<int> b(n, p);
std::iota(b.begin(), b.end(), n * q + 1);
std::cout << b << '
';
auto c = merge(a, b);
std::cout << c << '
';
}
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