CODEWITHSUNDEEP
c++vectorconstants
Pippi
Pippi

Reputation: 2571

Const correctness - how to access the elements of a vector of vectors in C++?

I have a vector of vectors of integers, and the vectors have unique lengths. I want to assign an index - an integer i to the integers. For example, if the vector V contains 2 vectors, and the first vector V.at(0) is length 2, and the second vector V.at(1) is length 4, an index 3 will correspond to the first element of the second vector, V.at(1).at(0).

I got errors which I suspect is related to const correctness. How can I fix it? 

Class A{
...
}

   A func(int i) const {
    int j = 0;
    int sum = 0;
    int u = 0;

    // stop when pass by the j-th vector which i is found
    while (sum < i){
        sum = sum + V.at(j).size();
        ++j;
    }

    // return the position
    u = V.at(j).at(i-sum);
    return A(std::make_pair<j, u>);   
  }

Error message: 

error: the value of 'j' is not usable in a constant expression
     return A(std::make_pair<j, u>);   
                                      ^
note: 'int j' is not const
     int j = 0;
               ^
error: the value of 'u' is not usable in a constant expression
     return A(std::make_pair<j, u>);   
                                         ^
                                         ^
note: 'int uid' is not const
     int u = V.at(j).at(i-sum);

Upvotes: 2

Views: 1064

Answers (1)

John Kugelman
John Kugelman

Reputation: 361565

Should be parentheses, not angle brackets:

return A(std::make_pair(j, u));

Angle brackets are for template arguments which can only be types and constants. That's why the compiler complains about the variables not being constants.

Upvotes: 4

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