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I am using an array of vectors. I have to count number of rows in the array. I am using the technique used for a 2D array. But it does not give desired output. 

int main()
{    
    int i,p=7;
    vector<float> *mat=new vector<float>[p];
    int k=5;
    for(i=0;i<p;i++){
        mat[i].assign(k,0);
    }
    cout<<sizeof(mat)/sizeof(mat[0])<<endl;

return 0;
}

Why is this resulting in 0 ? Shouldn't the output be 7? How to calculate number of rows ?

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  • 5
    First of all, sizeof(mat) give you the size of the pointer, not the size of whatever memory it point so. Secondly, sizeof(mat[0]) give you the size of the std::vector object itself, not the number of elements in the vector (that would be e.g. mat[0].length()). And why don't you use a vector of vectors (i.e. vector<vector<float>>) instead (or if k is a compile-time constant, a vector of arrays, as in vector<array<float, 5>>)? Commented May 28, 2018 at 9:33
  • Possible duplicate of Pointer array and sizeof confusion Commented May 28, 2018 at 9:35
  • @remi sizeof is a computed at compile time, you can't do mat[0].length() Commented May 28, 2018 at 9:40
  • @Tyker Ah, right. Will remove that comment, thanks. Commented May 28, 2018 at 9:41
  • @Tyker I guess it was .size() I meant. Wouldn't that work? Commented May 28, 2018 at 9:52
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