CODEWITHSUNDEEP
c++vector
Anmol Sood
Anmol Sood

Reputation: 73

Why is this code on vectors giving runtime error?

What is wrong with this code , why is it giving runtime error ?

    #include <iostream>
    #include <vector>
    using namespace std;

    int main() {

        vector < int > a[2];
        a[0][0]=1;
        a[1][0]=2;
        cout << a[0][0];
        cout << a[1][0];
        return 0;
    }

Also please correct it .

Upvotes: 0

Views: 2223

Answers (4)

4pie0
4pie0

Reputation: 29724

why is it giving runtime error ?

vector < int > a[2];

After this declaration vectors are empty yet. So this is undefined behavior trying to acces their elements. This is the cause of runtime error. You need to initialize them first:

vector < int > a[2];
a[0].push_back(1);
a[1].push_back(2);

Declaration

std::vector<int>(10);

specify that this vector has size of 10. If you want to specify the size of vector in container you can do it:

std::vector<int> va[2];
va[0] = std::vector<int>(10);

However this might be better:

va[0].reserve(100);  // but this just pre-allocates memory, doesn't initialize

Upvotes: 0

user1508519
user1508519

Reputation:

operator[] provides no bounds checking. If you used at instead, you would get a nice exception telling you what the problem is. It is undefined behavior to provide an index n if the container size is not greater than n.

a[0].at(0) =1;
a[1].at(0) =2;
terminate called after throwing an instance of 'std::out_of_range'

  what():  vector::_M_range_check

Upvotes: 0

Marius Bancila
Marius Bancila

Reputation: 16318

Because a is an array of two empty vectors. You are trying to access elements that don't exist.

Upvotes: 3

Mike Seymour
Mike Seymour

Reputation: 254431

why is it giving runtime error ?

Because the two vectors are empty, so you can't access their elements using [].

Also please correct it .

    vector < int > a[2];
    a[0].push_back(1);
    a[1].push_back(2);

Upvotes: 4

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