Segmentation fault while taking input in a 2d array in C++

Question

The computer is giving segmentation fault whenever I try to read input into this. My input looks like:

Basically, I want to store the above input directly into a 2D array. The first column is X-coordinates, 2nd column Y-coordinate and finally the value needed to store in (X,Y) COORDINATE of a 2D array.

long leastTimeToInterview(int n, int k, int m) {
    int a[m+1][m+1];
    int i=0,x=0,y=0;

    for (i=1;i<=m;i++){
        scanf("%d %d",&x,&y);
        scanf("%d",&a[x][y]);
        a[y][x]=a[x][y];

    } 
    return 11; 
}

Does the segfault appear after the first or the second `scanf `? — Detonar, Jun 21 '18 at 08:25
You're getting a Stack Overflow. That's why variable length arrays are a bad idea. — rustyx, Jun 21 '18 at 08:27
If most of the fields aren't used you might also use a `std::map` — Detonar, Jun 21 '18 at 08:29
a[y][x]=a[x][y]; most probably error is coming from this statement.. what if y > m+1 or x > m+1 — udit043, Jun 21 '18 at 09:03
If `m` is 10^5, the size of `a` is around 40 gigabytes. You’re almost certainly not expected to use an array here. — molbdnilo, Jun 21 '18 at 18:39

Detonar · Answer 1 · 2018-06-21T08:59:04.243

Like rustyx mentioned, this error could be a result of a stack overflow.

The easiest aproach would be to reduce the size of your array or use another data structure which allocates memory dynamically like std::vector, std::list or std::map.

In your example it also looks as if most of your array is empty. In this case i sugest using std::map. Some other optimisations in regard of memory usage are discussed in this question: Any optimization for random access on a very big array when the value in 95% of cases is either 0 or 1?

score 0 · Answer 2 · answered Jun 21 '18 at 08:59

There is no such thing as dynamically sized array in C++'s type system so you need to change it to somthing else like std::vector. The other problem is that even if your compiler supports dynamically sized arrays you still get stack overflow (at least it's very likely). So I suggest that you change your function to something like this:

long leastTimeToInterview(int n, int k, int m) {
    std::vector<std::vector<int>> a(m + 1, std::vector<int>(m + 1));
    int i = 0, x = 0, y = 0;

    for(i = 1; i <= m; i++) {
        scanf("%d %d", &x, &y);
        scanf("%d", &a[x][y]);
        a[y][x] = a[x][y];
    }
    return 11;
}

There still might be an error in the logic of your application but your example and explanation don't give enough information to tell that.

Segmentation fault while taking input in a 2d array in C++

2 Answers2