C++ parameterized constructor makes code to stop working when large input's are passed

Question

void initialize(int arr[], int size[], int n)
{
    int i;
    for(i = 1; i <= n; i++) {
        arr[i] = i;
        size[i] = 1;
    }

}

class hell
{
    public:
    int edges;
    int vertices;
    pair<int , pair<int,int>> p[100000];
    int disjoint_set[10000];
    int cc_size[10000]; // size of connected components

    hell(int e, int v)
    {
      edges = e;
      vertices = v;

      initialize(disjoint_set, cc_size, vertices);
    }
};

In the following class when I create an object using vertices=100000 and edges=100000, the code stops working. But when we remove the initialize(disjoint_set, cc_size, vertices) it starts working. I don't have any clue to such behavior. Please guide me.

Array indices start at 0 and not 1, you have UB, change your loop bounds. Furthermore, if create a local hell variable, you are going to have huge arrays on the stack, this may not be a good idea (especially the 100k array of pair of pair... ). — Holt
– Holt, Commented Apr 24, 2018 at 14:00
Does it just hang? Throw an error? What about if you give it smaller values for verticies and edges? — Engineero
– Engineero, Commented Apr 24, 2018 at 14:00
Why not simply use a vector and allocate the size you actually needs instead of having a huge underlying array that you partially use? — Holt
– Holt, Commented Apr 24, 2018 at 14:04

Slava · Accepted Answer · 2018-04-24 14:25:54Z

3

Arrays in C++ are zero indexed, which means that valid index is in [0..n[ range. Your code does it wrong:

 for(i = 1; i <= n; i++) {
    arr[i] = i;
    size[i] = 1;
}

it should be:

 for(i = 0; i < n; i++) {
    arr[i] = i + 1;
    size[i] = 1 + 1;
}

or better use algo std::iota() and std::fill():

std::iota( arr, arr + n, 1 );
std::fill( size, size + n, 1 );

and you better use std::vector, which will adjust its size properly, rather than have huge array.

edited Apr 24, 2018 at 14:25

answered Apr 24, 2018 at 14:02

Slava

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3 Comments

Holt Over a year ago

You don't want std::iota for size, you want std::fill.

Slava Over a year ago

@Holt thanks, did not see that there is 1 not i, they are close.

Arijit Das Over a year ago

Thanks a lot !!! I have one more question, when I initialized the vertices = 10000 and edges = 100000, the code works but assigns hell.edge = 1 which seems weird. Is their any reason behind it ?

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