Initialilze an array of vectors with zeros in C++

Question

I want a array of vectors with 0 as a single element in all the individual vectors. Is there a much more efficient way? does this work?

#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;
int main(){

        int lastAnswer = 0;
        int n,q;
        cin >> n >> q;
        vector<int> seqArr[n];
        for(int i=0;i<n;i++)
        {
                fill(seqArr[i].begin(),seqArr[i].end(),0);
        }
return 0;
}

Indeed, since the vector’s size is 0. You can use seqArr[i].push_back(0); instead. — Fabien Bouleau, Dec 01 '17 at 06:22
`vector seqArr[n];` require Variable-Length-Array (VLA) which is a C11 feature not available in C++ — Mihayl, Dec 01 '17 at 06:23
Possible duplicate of [Initializing a two dimensional std::vector](https://stackoverflow.com/questions/17663186/initializing-a-two-dimensional-stdvector) — Mihayl, Mar 26 '18 at 19:10

Mihayl · Accepted Answer · 2018-03-26T19:09:48.787

7

You should use a vector if you want an array with a variable length:

vector<vector<int>> seqArr(n);
for (int i = 0; i < n; ++i)
{
    seqArr[i].push_back(0);
}

or simply

vector<vector<int>> seqArr(n, vector<int>(1, 0));   // n vectors with one element 0

Variable-Length-Array (VLA) is a C99 feature that is not available in C++. In C++, the size of an array must be known at compile time.

edited Mar 26 '18 at 19:09

answered Dec 01 '17 at 06:25

Mihayl

3,821
2
13
32

MikeMB · Answer 2 · 2017-12-01T08:56:46.270

2

As variable length arrays are not a part of c++, I'd recommend using a vector of vectors, which also solved your initialization problem:

 vector<vector<int>> sequArray(n,{0}); //vector of n vectors, each containing a single 0

edited Dec 01 '17 at 08:56

answered Dec 01 '17 at 07:27

MikeMB

20,029
9
57
102

Initialilze an array of vectors with zeros in C++

2 Answers2