C++ Same array - different outputs - c++

Question

I'm trying to read from file (myfile.in) a 2D array. Rows and cols are given.
myfile>>n>>m; //rows and cols
for(int i = 0; i < n; i++) {
for(int j =0; j < m; j++) {
myfile>>tab[i][j];
cout<<tab[i][j]<<" ";
}
cout<<endl;
}
and the output on the screen is as it should be (as it is in file):
1 0 0 0 1 0 1
0 1 1 1 1 0 0
0 0 1 0 1 1 0
0 1 0 0 1 0 0
0 1 0 0 0 1 1
1 1 1 1 0 0 0
0 1 0 0 0 1 1
after that i tryied to print the array separately.
for(int i = 0; i < n; i++) {
for(int j =0; j < m; j++) {
cout<<tab[i][j]<<" ";
}
cout<<endl;
}
and the output is:
0 1 0 0 0 1 1
0 1 0 0 0 1 1
0 1 0 0 0 1 1
0 1 0 0 0 1 1
0 1 0 0 0 1 1
0 1 0 0 0 1 1
0 1 0 0 0 1 1
actually it's showing the last row, why?

Answer 1

According to your comment, you are actually initializing tab to be tab[0][0]. I don't know how come the compiler allows that, but the important thing is that you're writing outside your array bounds, triggering undefined behavior.
Try dynamically allocating your array after reading in n and m:
int n, m;
file >> n >> m;
int **tab = new int* [n];
for(size_t i = 0; i < n; ++i)
{
tab[i] = new int[m];
}
This way you'll always be sure to allocate only as much memory as you need.
Also, don't forget to delete the array when you're done:
for(size_t i = 0; i < n; ++i) delete[] tab[i];
delete[] tab;
As you can see this method tends to add a bit too much unnecessary complexity. An elegant alternative would be using a container such as a std::vector<std::vector<int>>:
using namespace std;
vector<vector<int>> tab;
for(int i = 0; i < n; ++i) {
vector<int> current_row;
for(int j = 0; j < m; ++j) {
int buff;
file >> buff;
current_row.push_back(buff);
}
tab.push_back(current_row);
}

Answer 2

int n=0, m=0; int tab[n][m];
This is not legal C++ for two reasons:
Dimensions of an array must be constant expressions. n and m are not.
You are creating an array of 0 size.
If the constant-expression (5.19) is present, [...] its value shall be greater than zero.
Your compiler is accepting it because it has extensions that accept both of these. Nonetheless, your array has size 0 and so it has no elements. Anything you attempt to write to it will be outside the bounds of the array.
Reading myfile>>n>>m; doesn't automatically change the size of the array. You already declared it as being size 0. Nothing you do will change that.
Instead, you'd be much better off using a standard library container, such as a std::vector, which can change size at run-time. Consider:
myfile >> n >> m;
std::vector<std::vector<int>> tab(n, std::vector<int>(m));
You can then use this tab object in exactly the same way as you have above.