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
– Fabien Bouleau, Commented Dec 1, 2017 at 6:22
vector<int> seqArr[n]; require Variable-Length-Array (VLA) which is a C11 feature not available in C++ — Mihayl
– Mihayl, Commented Dec 1, 2017 at 6:23

Mihayl · Accepted Answer · 2018-03-26 19:09:48Z

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, 2018 at 19:09

answered Dec 1, 2017 at 6:25

Mihayl

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Comments

MikeMB · Accepted Answer · 2017-12-01 08:56:46Z

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 1, 2017 at 8:56

answered Dec 1, 2017 at 7:27

MikeMB

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