Strange integer reading from file C++

Question

Really tried to look around, used search. Used Google, tried many ways, but I can't figure this out. (And I guess it's really easy for anyone who 'speaks fluent C++'.)

It's a really easy homework, but I have no idea why does it work this way.
Here is the code:

int temp;
string fname;
cout << "Kerem a fajlnevet: ";
cin >> fname;`

ifstream f;
f.open(fname.c_str());

int szam_madar, szam_helyseg;

f >> szam_madar;
f >> szam_helyseg;

int matrix[szam_helyseg * szam_madar];
for (int i = 1; i <= szam_helyseg; i++)
    {
        for(int j=1; j <= szam_madar; j++)
        {
            f >> matrix[i*szam_madar + j];
        }
    }

maximalisHelyseg(matrix,szam_helyseg,szam_madar);`

And the text I'm using at the read:
5 5 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1

It should create a 5x5 array, then fill it up in order.
Now... the result is quite different.
For the first few array member it gives something like 2686048, 4686232 and so on.

Answer 1

C/C++ arrays as 0 based. Their indices start from 0 and go to length-1. The first bug in your program is the indices of this loop. You should start from [0, szam_helyseg-1] and [0, szam_madar-1]

for (int i=1; i <= szam_helyseg; i++)
    {
        for(int j=1; j <= szam_madar; j++)
        {
            f >> matrix[i*szam_madar + j];
        }
    }

Also, since you are trying to represent a Matrix, it would beneficial to the clarity of the code if you use a 2D array. That will get rid of the index arithmetic (like i*szam_madar + j) which could also be a source of hard to find bugs.

Answer 2

you have stuff like:

for (int i=1; i <= szam_helyseg; i++)
{
    for(int j=1; j <= szam_madar; j++)
    {
        f >> matrix[i*szam_madar + j];
    }
}

But array indices count from 0, so the easiest way to do stuff is

for (int i=0; i < szam_helyseg; i++)
{
    for(int j=0; j < szam_madar; j++)
    {
        f >> matrix[i*szam_madar + j];
    }
}

etc. You can see that in the initial iteration, the index computed is 0, whereas in the original, it starts at szam_madar + 1 (which is a sign that something is wrong).

Also, variable length arrays are not standard C++ (referring to the line int matrix[szam_helyseg * szam_madar]; ). Use the array new operator, or (better), a vector<int> .