Segmentation fault error C++

Question

Im writing a simple program that alternates two vectors and push_backs's them into a 3rd vector however i get a segmentation fault error (core dumped). I did some research and it seems to be I'm accessing memory that either isn't there or I'm not supposed to. I know this a simple fix but I'm pretty new to C++ so anything would be appreciated.

vector<int> alternate(vector<int> a, vector<int> b)
{
    int n = a.size();
    int m = b.size();

    vector<int> c(n + m);

    int i;

    for(i = 0; i < a.size() and i < b.size (); i++)
    {
        c.push_back(a[i]);
        c.push_back(b[i]);
    }

    return c;
}

int main () {

    vector<int> a,b,c;
    int temp1;
    int temp2;

    while (temp1 != -1) {
        cin >> temp1;
        if(temp1 == -1) {
            break;
        }
        a.push_back(temp1);
    }

    while (temp2 != -1) {
        cin >> temp2;
        if(temp2 == -1) {
            break;
        }
        b.push_back(temp2);
    }

    c = alternate(a,b);

    int i;
    for(i = 0; i < c.size(); i++) {
        cout << c[i] << endl;
    }
}

Answer 1

As far as I can see you have a two problems here:

1. You used temp1 and temp2 in you conditions and you didn't initialize them, that's a UB. Try int temp1 = 0; int temp2 = 0; int temp1 = 0; int temp2 = 0; .
2. In function alternate you are initializing your returning vector with the sum of your two input vectors like this vector<int> c(n + m); , then you use push_back to add elements of these inputs. this way you will have n+m zeros at start of your returning vector then elements of your input. I'm pretty sure you don't want this. you don't need to give default size to you vector, just use push_back or if you insist on using default size then assign values to indexes of your vector.

Answer 2

There are a few problems with your code, some of which are mentioned above. The most important problems are the following:

1. temp1 and temp2 are uninitialized. Depending on your compiler, when accessed in the while loop they may be either 0 or some random number.
2. The final vector will be too big. When you create c you are creating it with a size of n+m hence it already has that many elements, each with a value of 0. Then you are adding the elements of the other vectors resulting in a final vector that is roughly (n+m)*2 in size.
3. I said "roughly" above because another problem is that unless n==m your final vector will be missing the last elements from the larger of a and b. You need to iterate to the larger of n and m, checking in the while loop to ensure you can access items safely.

Other lesser problems are the following:

1. You are making a copy of a and b when you pass them into alternate. By using a const reference you can avoid the copy.
2. Although unlikely, it is possible for a or b to hold more items than can be referenced by an "int". Use a size_t to be safe. Or even better use "auto" to allow the compiler to determine the correct type automatically.
3. Limit the visibility of your temporary variables by bringing them into the "for" and "while" loops.
4. Use the newer vector for loops to avoid the need for indices when possible (it isn't possible in alternate, but it is possible when writing out the final results.

After all that, the result should look something like the following:

#include <iostream>
#include <vector>

using namespace std;

vector<int> alternate(const vector<int>& a, const vector<int>& b)
{
    const auto n = a.size();
    const auto m = b.size();
    const auto N = max(n, m);

    vector<int> c;
    c.reserve(n+m);

    for(size_t i = 0; i < N; ++i)
    {
        if (i < n) c.push_back(a[i]);
        if (i < m) c.push_back(b[i]);
    }

    return c;
}

int main () {

    vector<int> a,b,c;

    while (true) {
        int temp;
        cin >> temp;
        if(temp == -1) {
            break;
        }
        a.push_back(temp);
    }

    while (true) {
        int temp;
        cin >> temp;
        if(temp == -1) {
            break;
        }
        b.push_back(temp);
    }

    c = alternate(a,b);

    for (auto val : c) {
        cout << val << endl;
    }
}

Answer 3

正如两个答案所指出的，未初始化的变量以及使我的“ c”向量太大是罪魁祸首