Why does this crash?

[tendavola]

This code crashes, unless the indicated line is commented out, then it works beautifully. I cannot understand this because it is the 3rd in a series of 4 repetitions of the same thing. Any help is appreciated. Thanks

int x=0;
    int y=0;
    int lp=0;
    for(lp=1;lp<=4;lp++)
    {
        y=0;
        x++;
        y++;
        A[x][y]=rand() % 9 + 0;
        cout << x << "-" << y<< " A=" <<A[x][y] << " " << "\n";
        y++;
        A[x][y]=rand() % 9 + 0;
        cout << x << "-" << y<< " A=" <<A[x][y] << " " << "\n";
        y++;
        A[x][y]=rand() % 9 + 0;//comment out this line and it works
        cout << x << "-" << y<< " A=" <<A[x][y] << " " << "\n";
        y++;
        A[x][y]=rand() % 9 + 0;
        cout << x << "-" << y<< " A=" <<A[x][y] << " " << "\n";

Expected output

1-1 A=6
1-2 A=6
1-3 A=5
1-4 A=3
2-1 A=3
2-2 A=7
2-3 A=8
2-4 A=0
3-1 A=4
3-2 A=4
3-3 A=8
3-4 A=2
4-1 A=7
4-2 A=2
4-3 A=6
4-4 A=5

[mrnutty]

my guess is that you are going out of the array bounds. Whats the size of the
array?

[tendavola]

heres the full code

int main()
{
    srand(time(NULL));
     int A[4][4];


    int x=0;
    int y=0;
    int lp=0;
    for(lp=1;lp<=4;lp++)
    {
        y=0;
        x++;
        y++;
        A[x][y]=rand() % 9 + 0;
        cout << x << "-" << y<< " A=" <<A[x][y] << " " << "\n";
        y++;
        A[x][y]=rand() % 9 + 0;
        cout << x << "-" << y<< " A=" <<A[x][y] << " " << "\n";
        y++;
        A[x][y]=rand() % 9 + 0;//comment out this line and it works
        cout << x << "-" << y<< " A=" <<A[x][y] << " " << "\n";
        y++;
        A[x][y]=rand() % 9 + 0;
        cout << x << "-" << y<< " A=" <<A[x][y] << " " << "\n";
    }

    return 0;
}

[tendavola]

..what truly baffles me, is that if

cout << "Fine till here";

is added on line 27, it will print that then fail...... I dont understand what is failing..

[tendavola]

Ok, so I figured it out with the help from the post above about array bounds.

i changed

int A[4][4];
to
int A[5][5];

But i dont understand why it works, could anyone explain?

[NP-complete]

may i know which compiler you are using?
You surely have array out of bounds in line 24.

Also may i know the headers you are including??

[amrith92]

HmmMMMmmm... IMHO, I think this can be done in a simpler way, by using two loops instead of one, that iterates over the entire range of your 2D array. If you have worked with matrices in C++, then you'll know what I'm talking about. Now in C++, array indices start from 0, not 1, so you'll need to fix your loop or your array dimensions. Try this:

for(int i = 0; i < 4; ++i) { // To traverse the rows like so: ((0,0), (1,0)... etc
    for(int j = 0; j < 4; ++j) { // To traverse a column like so: (0, 0), (0,1)...etc
        A[i][j] = rand() % 9;
        cout << i+1 << "-" << j+1 << " A=" <<A[i][j] << " " << "\n"; // I guess its cheating, but you get your output :)
    }
}

(For this code, you wouldn't have to change your array's dimensions)

Hope this helps!

[amrith92]

@NP-complete
From the functions the OP has used, I should think that he has used the following headers:

#include <iostream>
#include <cstdlib>
#include <ctime>

[NP-complete]

@tendavola
glad that you figured it out.

Explanation:
An array ALWAYS starts from 0. So if you have something like int A[4] the first element will be at A[0]. Then A[1],A[2] and A[3]. Note we have already reached 4 elements as we wanted (0-3) hence A[4] is meaningless and when you try to assign something to A[4] we say that you are going out of bounds.

Hope that helps.

[NP-complete]

@amrith
Yep he should include all the three.

His main problem i guess was the out of bounds thing.

The above explanation was for tendavola.

[NP-complete]

@tendavola:

You must also take a deeper look into what amrith92 posted. his code is generally the way you should do these things. And if you follow, his loops iterates from 0 to
"less than" 4. That is, it effectively goes 0,1,2 and 3.