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.

Don't post your code in pastebin, post it here. If it's too big to be posted here, you need to post the smallest compilable program that demonstrates your problem. — Falmarri
– Falmarri, Commented Dec 13, 2010 at 17:02
Well.. can't really paste it here as you can see. So that qualifies my problem as invalid in an instant? :-/ — Apache
– Apache, Commented Dec 13, 2010 at 17:06
For one thing, you are creating a one-dimensional array rather than an actual matrix. While you can get away with that, it's really tricky to debug, as you have no doubt just experienced. — chrisaycock
– chrisaycock, Commented Dec 13, 2010 at 17:08

lijie · Accepted Answer · 2010-12-13 17:12:18Z

1

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>.

answered Dec 13, 2010 at 17:12

lijie

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2 Comments

Apache Over a year ago

This is the problem. Sorry. Thanks for the fast / clear answer. The "input by hand" was working by hand but I couldn't see where did I mess up. Thank you. | Also: Standard and C++ ... no such thing as far as I know. C ... fine. C++? No way. (Sadly.)

lijie Over a year ago

Standard C++ means C++ as defined by the ISO standard. But since this is homework, just use whatever works on the compiler -- things like indexing are more fundamental.

Sonny Saluja · Accepted Answer · 2010-12-13 17:08:46Z

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.

answered Dec 13, 2010 at 17:08

Sonny Saluja

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