push_back上的STL列表分段错误

Question

I'm having a problem when using STL's list. I'm loading a digraph in format

#of_tests
#of_vertexes #of_edges
#vertex1 > #vertex2
#vertex3 > #vertex4
...

But I get a SIGSEGV on first call of push_back on my adjacency lists, and I'm really baffled - the array of lists is initialized, so I don't call it on null.

I've checked the tests, and I'm within bounds (I never call a method beyond allocated array).

Here's the code

#include <iostream>
#include <list>

using namespace std;

int * deg_in;
list<int> * edge;
int n;

int main()
{
    int z;
    cin >> z;

    deg_in = new int[n]();
    edge = new list<int>[n]();

    while(z--)
    {
        int m;
        cin >> n >> m;

        while(m--)
        {
            int a, b;
            char trash;
            cin >> a >> trash >> b;

            /// Vertexes are given 1 .. n, but stored 0 .. n - 1
            a--;
            b--;

            edge[a].push_back(b);   /// code fails here
            deg_in[b]++;
        }
/// do somethig with graph


    delete [] deg_in;
    delete [] edge;
}


    return 0;
}

Any help is appreciated.

Answer 1

You're deleting your data within the loop, so you'll crash on the second iteration. It's not obvious because the code is badly formatted. Change:

/// do somethig with graph


    delete [] deg_in;
    delete [] edge;
}

to:

/// do somethig with graph

    }
    delete [] deg_in;
    delete [] edge;

Alternatively you can allocate and de-allocate within the loop instead.

Take-home message: always take care to format your code properly, otherwise you're more likely to make simple hard-to-spot mistakes like this.

Answer 2

Your code allocates the deg_in and edge arrays before inputting n . Since n is declared in global scope, it is initialized to zero and thus the arrays are of length 0. The SIGSEGV thus arises because the program tries to access an unallocated part of memory.

In addition, you delete the arrays immediately after trying to process the first test case and the arrays are not re-allocated for every test case.

From the context, it seems likely that the deg_in and edge arrays are meant to be per-testcase. In that case, the code should be:

while (z--)
{
    int m;
    cin >> n >> m;

    deg_in = new int[n]();
    edge = new list<int>[n]();

    // input graph

    delete [] deg_in;
    delete [] edge;
}

Like Paul R said, formatting your code consistently help reduce the chance of simple mistakes. Cheers.